* General
* Suspension Air Bags
* Rebuilding
* Differential Codes and Usage
* Gear Recommendations for the Street
* Swapping

Submit corrections and additions to this information to The Olds FAQ Compiler.


Gear Ratio Terminology

The terms higher and lower can get a little confusing when reffering to rearend ratios. If someone changes to a higher gear ratio they are actually changing to a numerically lower ratio. If you were to swap a 2.93:1 gearset with a 3.42:1 gearset you would be switching to a lower gear even though the ratio of driveshaft turns to turns of the wheel is higher. If you swap a 3.42:1 with a 2.93:1 you are switching to a higher ratio. I don't how high ended up meaning low numerically or low ended up meaning high numerically, but they do.

A higher gear ratio would improve mileage and lessen wear on the engine somewhat but at the cost of less acceleration. A car is also capable of a higher top speed with a higher gear. Cars that are meant to do a lot of interstate cruising have higher gears, generally in the 2.48 - 3.08:1 range. Cars meant for acceleration have lower gears, usually something numerically above 3.08:1.

A numerically higher ratio will cause the same tire travel but at higher RPM. For example, a 3.42:1 ratio will cause the same tire travel but at higher RPM than a 2.93.

A lower gear (3.42:1) will require a lesser amount of torque than a higher gear (2.93:1) to accelerate at a given rate. A higher gear will require more torque to accelerate at a given rate.

An engine that puts out a small amount of torque will generally be better off with a lower gear ratio. An engine that puts out a lot of torque will accelerate quite well with a higher gear ratio. That's why the 4500lb 1970 Delta 88 with the police package could run 0-60 in something like 6.9 seconds despite having a 2.93:1 rearend ratio. It had the 455 with 500 ft/lb of torque and didn't need a very low gear to get up to speed quickly. (it also had an estimated top speed of 140). Those of us who just want insane acceleration put a low gear behind a big 'ol 455 in a light car and then spend the rest of their days trying to figure out how to eliminate tire spin!

[ Thanks to Greg Pruett for this information ]

Determining Gear Ratio

You will have to put the rear up on jackstands, put the transmission in neutral. Make sure that the car is quite stable, get under the car, and TURN THE DRIVESHAFT. Count the number of turns of the driveshaft it takes to turn one wheel one time. If you turn a wheel, instead of turning the driveshaft, you will only turn the other wheel (not the driveshaft).

It also works better (more accurate) if you turn the driveshaft enough times to turn one wheel ten times, and count the number of times you turn the driveshaft. This is because it may be difficult to tell if the driveshaft goes around, say, 2.56 or 2.78 times for one turn of a tire. But if you turn the driveshaft enough times to turn a wheel ten times, you will have 25.6, or 27.8, or 41.1 (or whatever) turns of the driveshaft. Then just divide by ten to get the ratio of the rear.

Note also that there are two different ring gear carriers, one for 3-series ratio gears and one for 4-series gears. The difference is the dimension from the ring gear mounting flange to the pinion centerline, due to the larger diameter pinion required for the lower numerical gear ratios. A set of 4-series gears can be installed in the 3-series carrier with a spacer and longer bolts (not desireable due to reduced strength), but the reverse is not true.


These formulas are for giving you a good idea of what to expect not 100% accurate, but pretty close. Some things you need to remember is that if your tire is 25" standing still it will be taller at speed. How much depends on tire construction, rubber compound, inflaltion. It will be shorter on acceleration depending on engine power, wheel spin. Then you have tranny slippage and probably other small variables.

To figure rpm: 168 * gear ratio * mph / tire radius
To figure mph: tire radius / 168 * rpm / gear ratio
To figure gear ratio: tire radius * rpm / 168 / mph
To figure tire radius: 168 * mph * gear ratio / rpm
[ Thanks to Daren for this information ]

N-bolt Terminology

The tern "n-bolt", as pertaining to the rear end of the car, refers to the number of bolts retaining the ring gear to the carrier inside the differential. Fortunately (or unfortunately), this number usually corresponds to the number of bolts holding the differential cover, so you can usually tell whether you have a "10-bolt" or "12-bolt" rear from the number of bolts on the outside of the casing. This is independent of the rear end gear ratio.

The Type O (Oldsmobile) 12-bolt rear ends use a smaller ring gear than the Type C (Chevy) 12-bolt, resulting in lower strength. The good news it that the Type O units retain the axle shafts at their outboard ends, while the Type C units use the infamous c-clips at the imboard end. This is why you see C-Clip Eliminator kits available for the Chevy axles. Also the Chevy rear has a "scalloped" cover where the Olds is smooth.

N-bolt Differences

An Olds 12 bolt rear end is not a "true" 12-bolt. It may have 12 bolts on the outside cover, but it is a 10 bolt gear on the inside. The more common type "C" is very much different than the type "O".

A "true 12-bolt" has come to mean a Chevy c-clip 12-bolt rear. Oldsmobile 12 bolt rear ends differ from Chevys internally. In other words, the guts of a Chevy rear end will not fit in an Olds rear end housing, and vice versa. The phrase, "more like a 10-bolt", is usually talking about the size of the ring gear and/or strength of the carrier. The guts of a 10 bolt would not work or fit in a 12 bolt, and vice versa. There are differences between the 10 bolt and the 12 bolt both internally and externally.

Note also that there are two different ring gear carriers, one for 3-series ratio gears and one for 4-series gears. 3 series and 4 series carriers are for the 12 bolt Chevys. The difference is the dimension from the ring gear mounting flange to the pinion centerline, due to the larger diameter pinion required for the lower numerical gear ratios. A set of 4-series gears can be installed in the 3-series carrier with a spacer and longer bolts (not desireable due to reduced strength), but the reverse is not true.

The ring gear size is the size of the largest gear, parallel to the wheels. A 12 bolt Olds has a 10 bolt carrier that measures 8.3", whereas a 12 bolt Chevy has a 12 bolt carrier that measures 8.8". A 10 bolt Olds out of a 1971 has a 10 bolt that measures 8.5". 1968 and 1969 10 bolts might have an 8.2" ring gear. Starting in 1970, 10 bolts might be 8.5". Newer 10 bolts found under the last RWD Cutlass and Chevy are little 7.5".

Oldsmobile bolted the axles from the inside of the housing, whereas Chevy used c-clips. As for Buick and Pontiac, they used different types of posi actuating items called "cones" and "spools", whereas 12 bolt Chevys and Olds used what are called "clutch packs".

Type "O" Oldsmobile Rear Ends

The biggest weak spots in Olds rears are the wheel bearings and the availability of parts. Positive axle retention is good, but put in the new "updated" style wheel bearings for more strength.

Gears for the Olds 12 bolt rear are hard to find. The "O" was only manufactured for 2-3 years. So there isn't enough call for someone to start casting and machining new cases. While posi cases are not available (as far as I know) rebuild kits are. Unless the case you have is outright cracked or otherwise junk you should be able to rebuild it.

Type "C" Chevy Rear Ends

This rear end retains the axle shafts with "C" clips. These do break from time to time, and the result is the axle shaft works its way out of the axle tube. A hint that a clip might be broken is a faint moan from the rear axle when backing up (mine made no noise going forward). Moral of the story: stick with your Olds rear.

The "C" rear can be modified with "a C-clip eliminator kit". That solves that problem of relatively fragile broken "C" clips.

If you want/need a stronger more available rear, the "C" 12 bolt, will bolt in with no changes to your car. It came in the C*evelle after all. The rear U-joint will need to be changed to a hybrid one. NAPA and others carry such a U-joint.

Given the "C" 12-bolt is almost a 9" ring gear, parts are as common as sand on the beach, and almost as cheap. It has some redeeming values once modified for performance. NHRA won't allow the faster cars to run one without the eliminator.

Type "B" Buick Rear Ends

Type "P" Pontiac Rear Ends

Ford 9" Rear End

The 9 inch Ford is the setup these days for many of the same reasons as the Chevy; plus you can change gears by yanking the axels and swapping in the "pumpkin". But getting one setup to bolt into the "A" or "G" bodies is much more expensive at the outset.

The best will have a large "N" cast into the front of the case, between the webs. This indicates it is made of nodular iron, as opposed to slightly weaker regular iron.

Get a 9" out of the boneyard for the axles and the center section, and buy a custom housing from Moser Engineering or Currie Enterprises with your exact suspension system mounts. You could also use the boneyard housing and narrow it, but, the two places I mentioned do it all for you.

Pay particular attention to the brakes and wheel bolt pattern on the 9" you pull out. You could need a redrilled bolt pattern to match your wheels. You can weld brackets onto the narrowed rear yourself, but I think this is something worth farming out, unless you are/know a good welder.

[ Thanks to Cliff Feiler, Brian Kennedy for this information ]

Differential Usage

Actually, 1968 to 1970 Olds 442's (and I believe all Olds for that matter) built in the US plants all had the Type O rear axle. All of the 442's built in the Oshawa, Ontario plant had the Type C (Chevy) axle. In 1971 they went to a GM corporate 10 bolt.

The ring gear size in the 1985 to 1987 442's is the 8.5". Very desireable. The same is used in the Grand National. The Monte SS has the 7.5/7.626" rear. Not nearly as hot as the 8.5" rear.

There were a number of differing 10 bolt differentials made during the 1970's by GM. And all the internals are different - nothing swaps.

Posi or Open Carrier

With both rear wheels off the ground (or front wheels for you Toro-heads!), turn one wheel. If both wheels turn in the same direction, then it is a posi. If they turn in opposite directions, then it is an open carrier or blown posi.

A posi differential will have a metal tag attached to a lower right hand cover bolt warning about the type of diff. oil to use. Open diffs usually have no tags, except Chevy diffs usually have a code and ratio tag attached to a lower left cover bolt.

Posi (short for posi-traction) is an option that is not specific to amount of bolts (ie. 10 bolt or 12 bolt), but was a factory option. You can change a single track car to posi by putting in a positraction differential. In other words you can put a posi unit in a 10 or 12 bolt, as long as you have the right posi unit. There are posi units for 10 bolts, and posi units for 12 bolts (not interchangeable).

Clutch and Cone Anti-Spin

The cone units have advantages and disadvantages versus the clutch type units. The most immediate advantage is that they seem to work fine with most any gear lube. Friction discs and clutch plates are not used. Resistance is achieved by the steel cone (which has a series of raised spiral groves) rubbing on the smooth steel case. Under acceleration the cone is forced into the case which results in limited slip. I have never had to use lubes specifically formulated for posi units in my cone posi's. Even though the car came with a "limited slip lube" tag on the housing. Mobil synthetic works fine, as is. But doesn't work fine in the "C" rear without additive.

When a cone posi wears out it is because the cone has bottomed out in the case. While you can't make it good as new, you can restore it to like new or better limited slip action by grinding the lip off the bottom of the cones and deepening the recess in the case. Olds/Pontiac cone posi's use a split case. Open it up by removing the eight (I think) grade 8 bolts which hold the two sections together. Grinding a ¼ total separation between the case and cone is more than enough. I then use side bearing shims to take up the slack in the gears. These are the same shims you need to use to set backlash. You put them between the splined cone and splined driven sections on each side. Installation is a lot easier if you use an axel shaft to keep the splines aligned. If you over pack the shims, the cone unit will actually act like a locker style rear. Been there, done that and changed it to a more reasonable preload.

You can also shim, modify or replace the triple coil springs which serve as preload. I've used an old exhaust valve to increase the preload once. Increased it a LOT. The case halves should go together with some preload, just don't over do it.

I don't know how long one restored this way will last, I'm still using one after several years, lots of street use and some drag racing. It still works great.

All the ruined cone type posi's I have seen were severly abused. They should last quite a while if not abused. I still have the _original_ cone posi that shipped with my '67 in the car. It has never worn out. The restored unit cost me $10 bucks from a Pontiac guy in Hemmings.

If I had access to either cone or friction plate posi's I would go with the cone, first because the original poster of this question stated he was told they are cheaper. Next, in my experience, the cone works as well or better than the more common design. And, given the smaller ring gear, they are stonger than the other designs (four pinion gears is much more than double the strength of two). The case is also almost full circle, there is no need for a large opening through which the plates and frictions are installed. Only enough space to ensure full lubrication is needed.

FWIW, this is the type of posi Olds used for the 66-67 W-30's. As noted above, I've never broken it.

In the final analysis, both designs work and will last a long time if not severely abused.

There are also other designs such as locking, Torsen (sp?), air lockers and others I am sure. I have no experience with these units but they receive rave reviews in the 4X4 mags. Just remember that magazines have two main functions. To show you good stuff so you'll buy the mag again and to sell the goods their advertises pay them to push. The latter usually prevails.

The 3.55 posi is almost certainly a two pinion posi, while good, it is not extremely strong. It is also a cone type posi unit. If that is shot it cannot be rebuilt in the same fashion as the plate/friction type. They can however be made functional with a little work. The 4-pinion posi case is pretty good, and a lot stronger. You can not, without swapping parts around, install anything lower than 3.55 gears. The case changes for the 3.90 and lower gears (higher numerically).

[ Thanks to Bob Handren for this information ]

Locker Differentials

So you want a locker. Don't do it, you don't need it. Just buy the Ford Motorsport 4 pinion traction-lok diff. Its about $100 cheaper, and is much better suited for a mostly street and some strip car. A "Detroit Locker" is streetable but only for true weekend warrior street/strip cars. The are noisy, and tend to cause the car to plow in corners (cause the axles are "locked" most of the time) unless you are taking a real sharp turn.


The 1965 thru early 1967 axels are weaker than later axles.

In the late '70s and early '80s, GM converted axle shaft retention from bolted in, to being held in with c-clips in the differential housing. There was a problem with some axles. The groove in the axle shaft that the c-clip held on to was machined too wide. This allowed excess endplay in the axle shaft. Over time, the clip and the groove would wear due to the excessive side movement and the clip would fall out. Then the axle shaft comes right out of the axle carrier.

[ Thanks to Bob Handren, Steve Ochs for this information ]

Suspension Air Bags

Air bags are also called air springs. They are essentially big rubber balloons that fit inside the rear springs (or on top, for the leaf spring vehicles). They preload the suspension similar to air shocks, but are more controllable. You can load the right side harder to plant that tire down firmer, while leaving the left nearly unassisted, to provide fairly even traction left-to-right.

They sit right inside the rear coils and air up to assist the springs. (actually, they resist the springs, making them harder to compress). They act similarly to air shocks, but they leave those mounting points free to put useful shocks in their place. Let's face it, air shocks leave a lot to be desired for control and ride. The air bags can be inflated independently, and allow for preloading the right side of the axle. This effectively puts more weight on the drive tire, making it harder to break loose. Result: instant posi.

Plus, without the air shocks, you can have more axle control and less chance of wheel hop. This is an aspect that makes them desirable even with existing posi's. They install literally in minutes. The most time consuming part is running the air lines. Mount the air valve beind the license plate - some models already have a mounting hole (probably for an air shock valve). A couple of 5/16" holes does the trick. Or you can take out the rubber stops on the license plate pocket and stick the valves there. I've got an air guage clipped to the back of the license plate to make checking them a breeze.

The bags just squish up and you squeeze them in through the expanded coils of the springs. It takes a little effort when they are almost in. Just wiggle them around a little. You let them go when the last part of the bag has cleared the coil, and they pop back into shape, and then you simply plug them in to the air lines. Run the air lines through the frame. I'd recommend putting some 5/16" fuel line around the lines to protect them from rubbing. Some models come with heat shields - a good idea to use them to protect your investment. The whole thing can be done with a jack and a pair of pliers. The jack is probably optional. Well worth $60.

They work awesome. There are different versions available. The day I put them on, I took a friend out and tried for hours to prove that I had not changed the open rear to a posi! Gravel, pavement, water, didn't matter, I couldn't break loose! I finally had to unload one of the bags and do a bit of a smokeshow, and that finally convinced him. Very impressive items.

And if you already have an anti-spin or posi differential, definately get a set! I'd actually recommend them highly. In addition to controlling axle movement with the shocks, you can preload the suspension to give equal traction characteristics and control/prevent/reduce wheel hop (i.e. put more power to the ground).

Posi-traction is great, but the ability to load each wheel separately was invaluable. I don't see why it wouldn't be fine on a street car. The driving and ride are not compromised at all. Control of the rear is what you are getting here. Different air pressures and combinations give you different traction results.

[ Thanks to Charley Buehner, Bill Culp for this information ]

64/65 A-body Owners:

My 64 f-85 is back on the road after a 6 week hiatus on jack stands. As you may recall, the right rear axle bearing seized and ran down the bear journal on the axle, thus ruining the axle. After finding 2 other axles out of the only other 1964 and 1965 Olds that I've been able to find in N. GA (at least that had a rearend), and seeing that they both had the same problem, I took one of my bad axles to a machine shop and paid $90 for a journal repair and bearing install. Fixed, no problem (high price, I know, but when there is only one machine shop in town that will do it, and he knows it).

BEWARE. I pulled a 1966 Tempest axle and it had the same problem. Watch those rear bearings. This was a premature failure, as this bearing had no more that 40k miles on it when it siezed (it was a high quality bearing, too).

1964 and 1965 Olds owners: Olds axles are interchangable with 64/65 Buick A-bodies (and that's it!)

Gear Lube

An anti-spin or posi differential requires an appropriate dose of special anti-spin or pose additive. Otherwise, you will hear a clanking coming from the rear of the car when turning. With some synthetic gear lubes, the differential performed fine with out the additive.

Mobil 1 in a "C" posi rear currently made the most horrible banging noises you could imagine. This would occur after running on the straight highway and then stopping and making a turn. After a loud "clang" it would loosen up and be OK until you did the same routine again. I added the GM additive and it went away. So my experience is that Mobil 1 needs the GM additive for use in posi rears.

[ Thanks to Bob Handren, Jason Labey for this information ]
[ Thanks to Mark Prince, Bob Valentine, Jeff Zeilmann, Bill Culp, Dave Paulison, Bob Handren, Nick DiGiovanni, Joe Padavano, Luke, Bruce D. Brumm, Bob Barry, Doug Ahern, Scott Kozhill, Robert Lavey for this information ]

Differential Codes and Usage

[ Notice: ]Please refer to the Differential Identification section for the location of external identification!

Identification for GM Differentials

Stamped Codes:
The two letter code is approx. 3/8" in height and is stamped into the axle tube on the inboard, passenger side (toward passenger compartment). Sometimes they are also found outboard on the passenger side (towards rear of car). There are usually more than two letters or numbers, because other manufacturing codes were stamped there.

It's normal to have to wire brush the heck out of the area before you will be able to see it, but it will be there. Typically it is covered/rusted over.

What you are looking for exactly is the two or three adjacent letters, preceded by one or more numbers, of the code that is stamped (NOT RAISED) into the axle tube.

The letters themselves can be very fine and very faint. Further, there can be flaws in the stamping dies or the application of same. What may look like one letter is actually another, so look carefully.

I recently measured the distance the code was from the right side backing plate which was 13". This one happened to be on the outboard side of the axle.

Axle ratios are identified by letter or number code. On F85, Toronado and Jetstar 88, the ratio letter codes are followed by numbers giving the date of production, identified as follows:

Full size models except Jetstar 88:
1959-64 Symbol X (circled). If no symbol, axle is non-locking.
1965-69 "L" followed by number code.
1966-69 Toronado Letter code.

F-85 and Jetstar 88:
1961-64 Symbol X (circled). If no symbol, axle is non-locking.
1965-69 Letter codes identify both locking and non-locking.

In 1964 the F85 had another ratio marking (example: P3.08L). It is on the right rear inboard side of axle houlsing tube. "P" is vendor; 3.08 is ratio; "L" means locking axle. Axle ratio codes are located as follows:

Full size models:
1959-63 On carrier casting lower locating boss.
1964 exc Jetstar 88 On carrier casting lower locating boss.
1965-69 exc Toronado Stamped on right rear inboard side of axle housing tube.
1966-67 Toronado Ratio and date code stamped on the flange below fill tube boss.
1968-69 Toronado Ratio and date code stamped on flange near RH spreader hole.

F-85 and Jetstar 88:
1961-64 On right side of housing cover.
1965-69 On right rear inboard side of axle housing tube.

Ribs on Differential:
Look at the rib(s) cast into the side of the center section. If there is only one rib it is a "B" Buick rear. If there are two converging (towards the front of the car) ribs it is a "P" Pontiac rear. The single rib rears are much weaker than the two rib versions. Among other things, the number and size of the pinion gear splines are smaller.

On each GM differential, there is a code on the ring gear which shows the number of teeth, and therefore the ratio. Remove the rear end cover and read it, then use the table above to figure the ratio. Or do the long division.

[ Notice: ]Please refer to the Differential Codes section for internal numbers!

[ Thanks to Glenn, Karl Aune, Bill Culp, Bob Handren for this information ]

Type O Axle Codes

1959 - 1964:
Model Ratio Teeth Open Anti-spin
All except Jetstar 88 2.69 43-16 2
2.56 41-16 3
3.90 39-10 4
3.64 40-11 6
2.87 43-15 7
3.42 41-12 8
3.23 42-13 9
3.08 40-12 0
Model Ratio Teeth Open Anti-spin
F-85 3.08 40-13 A
3.23 42-13 B
3.36 37-11 C
Model Ratio Teeth Open Anti-spin
F-85 3.08 40-13 A
3.23 42-13 B
3.36 37-11 C
Model Ratio Teeth Open Anti-spin
F-85 3.08 40-13 A
3.23 42-13 B
3.36 37-11 C
Model Ratio Teeth Open Anti-spin
F-85 3.08 40-13 A
3.23 42-13 B
2.93 39-14 C
3.55 39-11 D
3.36 37-11 E
3.90 39-10 F
Jetstar 88 3.08 40-13 A
3.23 42-13 B
2.78 39-14 C
3.55 39-11 D
3.36 37-11 E
3.90 39-10 F
Model Ratio Teeth Open Anti-spin
Dynamic 88, Delta 88, Starfire, except Police 2.73 41-15 QA QB
2.93 41-14 QC QD
3.08 40-13 QE QF
3.23 42-13 QG QH
3.42 41-12 QI QJ
except Vista Cruiser 2.78 39-14 SA SB
3.08 40-13 SC SD
3.23 42-13 SE SF
3.23 39-11 SI SJ
Vista Cruiser 3.08 37-12 SM SN
3.23 42-13 SO SP
3.55 39-11 SU SY
except Vista Cruiser and Police 2.78 39-14 SA SB
3.08 40-13 SC SD
3.23 42-13 SE SF
3.55 39-11 SI SJ
Vista Cruiser except Police 3.08 37-12 SM SN
3.23 42-13 SO SP
3.56 39-11 SU SY
Jetstar 88 3.08 40-13 RC RD
3.23 42-13 RE RF
3.55 39-11 RG RH
Jetstar 1, Dynamic 88, Delta 88, Starfire 2.73 41-15 QA QB
2.93 41-14 QC QD
3.08 40-13 QE QF
3.23 42-13 QG QH
3.42 41-12 QI QJ
98 2.73 41-15 QK QL
3.08 40-13 QM QN
3.23 42-13 QO QP
3.42 41-12 QQ QR
Model Ratio Teeth Open Anti-spin
Toronado 3.21 AB
Jetstar 88 2.78 39:14 RA RB
3.08 40:13 RC RD
3.23 42:13 RE RF
Dynamic, Delta 88 2.73 41:15 QA QB
2.93 41:14 QC QD
3.08 40:13 QE QF
3.23 42:13 QG QH
3.42 41:12 QI QJ
98 2.73 41:15 QK QL
3.08 40:13 QM QN
3.23 42:13 QO QP
3.42 41:12 QQ QR
Jetstar 88, except Police 2.78 39-14 RA RB
3.08 40-13 RC RD
3.23 42-13 RE RF
3.55 39-11 RG RH
Vista Cruiser except Police 3.08 40-13 SM SN
3.23 42-13 SO SP
3.55 39-11 SU SY
Note: Codings not available on '66 4.11-1 (37-9), and 4.33-1 (39-9).
except Vista Cruiser and Police 2.78 39-14 SA SB
3.08 40-13 SC SD
3.23 42-13 SE SF
3.55 39-11 SI SJ
3.90 39-10 TG TH
Note: Codings not available on '66 4.11-1 (37-9), and 4.33-1 (39-9).
Jetstar 1, Dynamic 88, Delta 88, Starfire 2.73 41-15 QA QB
2.93 41-14 QC QD
3.08 40-13 QE QF
3.23 42-13 QG QH
3.42 41-12 QI QJ
3.42 41-12 QS• QT•
• With heavy duty 15" wheels & hubs & drums.
Model Ratio Teeth Open Anti-spin
98 2.73 41-15 QK, RO QL, RP
3.08 40-13 QM, RS QN, RT
3.23 42-13 QO, RU QP, RY
3.42 41-12 QQ, RW QR, RX
Delmont 88 w/330 2.78 39-14 RA, QV RB, QU
3.08 40-13 RC, RI RD, RJ
3.23 42-13 RE, RK RF, RL
Delmont 88 w/425, Delta 2.73 41-15 QA, RO QB, RP
2.93 41-14 QC, RQ QD, RR
3.08 40-13 QE, QY, RM, RS QF, QE, RN, RT
3.23 42-13 QG, QW, RG, RU QH, QX, RH, RV
3.42 41-12 QI, QS, RW, RY QJ, QT, RX, RZ
Toronado 3.21 AB
except Vista Cruiser 2.78 39-14 SA, S2, SK SB, S3, SL
3.08 40-13 SC, S4, SQ SD, S5, SR
3.23 42-13 SE, S6, SV SF, S7, SW
2.41 41-17 SS, SU, ST TA, TS, TB
3.42 41-12 TL, S8, TP TM, S9, TQ
3.91 43-11 TN, T2, TU TO, T3, TV
Vista Cruiser 3.08 40-13 SM, T4, SG SN, T5, SH
3.23 42-13 SO, T6, TC SP, T7, TD
Model Ratio Teeth Open Anti-spin
Type "O" differential
except Vista Cruiser and 442 2.56 41-16 R2, R6 R3, R7
2.78 39-14 SA, S2 SB, S3
3.08 40-13 SC, S4 SD, S5
3.23 42-13 SE, S6 SF, S7
3.42 41-12 TL, S8 TM, S9
3.91 42-11 TN, T2 TO, T3
Type "C" differential
2.56 41-16 R8, TR R9, TW, TH
2.73 41-15 RA, TT RB, TJ
3.08 43-14 RC, T4 RD, T5
3.31 43-13 RE, T6 RF, T7
Vista Cruiser w/400, 442 4.33 39-9 TW, TY
Vista Cruiser w/350 3.08 40-13 SM SN
3.23 42-13 SO SP
2.78 39-14 TA TB
Model Ratio Teeth Open Anti-spin
Type "O" differential
except Vista Cruiser 2.56• 41-16 R2, R8 R3, R9
2.56¤ 41-16 R6 R7
2.78¤ 39-14 S2 S3
3.08¤ 40-13 S4 S5
3.23¤ 42-13 S6 S7
3.42¤ 41-12 S9
2.78• 39-14 SA SB
3.08• 40-13 SC SD
3.23• 42-13 SE SF
3.91¤ 43-11 T3
3.42• 41-12 TM
3.81• 43-11 TO
4.33• 39-9 TY
Type "C" differential
2.78• 39-14 RA RB
3.08• 40-13 RC RD
3.23• 42-13 RE RF
3.07¤ 43-14 T4 T5
3.31¤ 43-13 T6 T7
3.55• 39-11 TL
3.73• 41-11 TN
2.56¤ 41-16 TR TH
2.73¤ 41-15 TT TJ
¤ HD brake (Police) or Disc.
• Std brake
Delmont 88 w/350 2.78 39-14 RA RB
3.08 40-13 RC RD
3.23 42-13 RE RF
Delmont 88 w/455, Delta 2.73 41-15 QA, QK, QS QB, QL, QT
2.56 41-16 QC, QQ QD, QR
3.08 40-13 QE, QM, QY QF, QN, QZ
98 2.73 41-15 QA, QK QB, QL
2.56 41-16 QC, QQ QD, QR
3.08 40-13 QE, QM QF, QN
Toronado 3.07 43-14 T
Vista Cruiser 3.08 40-13 SM SN
3.23 42-13 SO SP
2.78 39-14 TA TB
Delta 88 w/350 2.78 39-14 RA RB
3.08 40-13 RC RD
3.23 42-13 RE RF
All Delta w/455, std. brakes, 98 optional 2.56 41-16 QC QD
2.93 41-14 QI QJ
All Delta w/455, h.d. brakes, Police or disc brakes, 98 standard 2.56 41-16 QQ QR
2.93 41-14 QU QV

1969 - 1970
All 1969 O axles use a sealed ball bearing as you describe. The 70 O axle uses the bearing/seal as you describe. They are not interchangeable. The Vista Cruisers use a larger bearing and are not interchangeable with your Cutlass O axle. There was a design change from 68 to 69 VC axle bearings - hence the note in the chassis manual. To replace the axle from a 69 SA code (2.78) Cutlass, you need to find an axle from a 67-69 Cutlass series (Not V/C or Canadian built). With a ratio between 2.56 and 3.23.
1978 - 1988
First, ALL GM 1981 to 1988 G-Bodies used a 7.5" ring gear rear axle EXCEPT: Monte Carlo SS - 7-5/8" 3.73, Olds 442 - 8.5" 3.73, T-Type and Grand National - 8.5" 3.42 or 3.73. The 8.5" axle is the same used in the B-body (full sized) except a shorter axle tube length. Second, the 7-5/8" axle is actually used in the mid 80's F-bodies Camaro & Firebird. This axle is the one used in the Monte Carlo SS. Third, the '80s S and T trucks use a 7.5" ring gear axle. A short trip to the service manuals on these vehicles is all it takes to check this information out.

The 442 and Regal axles are hard to find because 442s are rare and most T-types and Grand Nationals are not junked yet. For older Cutlasses, the B-body 8.5" axle might fit better than the G-body axle.

             RPO           |--------Axle Code--------|
Ring Gear   Code   Ratio   Conventional   Limited Slip
  7.5"       G72    2.14       2AK            2BK   (aluminum brake drums)
  7.5"       GH2    2.29       2AH            2BH
  7.5"       GU1    2.41     2AJ,2AZ        2BJ,2BZ
  7.5"       GM8    2.56       2AA            2BA
  7.5"       GU2    2.73     2AB,2AX        2BB,2BX
  7.5"       GU4    3.08     2AC,2AV        2BC,2BV
  7.5"       GU5    3.23       2AD            2BD
  7.625"     GT4    3.73       2TF            2TH   (Monte Carlo SS)
  8.5"       GU6    3.42       3TP            3TJ   (T-Type, GN Std.)
  8.5"       GT4    3.73     3TG,3TE        3TR,3TX (442, GN Opt.)

  8.5"       GU2    2.73       8YB            8YP   (B-Body)
  8.5"       GU4    3.08       8YD            8YR   (B-Body)
  8.5"       GU5    3.23       8YE            8YS   (B-Body)

If you don't have the codes with you to indentify if it is an 8.5" or not, I found that if you measure the distance between the very bottom bolt for the cover and the next adjacent one (either right of left), it will be 3 ¾" for a 8.5" and 3 1/4" for a 7.5".

All 1978 and up Olds came with 7.5" 10 bolts, no 12 bolts (Chevy or Olds) whatsoever. So guys with 1978 and up G bodies had a few options: beef up the 7.5" with better axles and a good posi unit, or find an 8.5" out of an 83-84 HO, 85-85 442, or 84 and up T-Type or GN, or go with a Ford 9" from Currie or Moser, or buy the control arms from Southside traction and install an older 1968 or so Chevy 12 bolt or 10 bolt.

A friend of mine just old me that the 1964 to 1967 A-Body cars ("intermediates") rear end will bolt into a G-body Cutlass with minor love taps to the upper trailing arms. He said it is ½ inch wider on both sides, but other than that, it is the 8.2 inch 10 bolt. Much stronger than the 7.5in. And no matter what rear end I find it will have better gears than my 2.14's!

The 1968 to 1972 A-body differential will bolt up. Summit sells a control arm set that will properly adapt them.

Be sure to get the driveshaft with the rear end, since it is well - 1/2 inch shorter. Also get the rear anti-sway bar that links the lower control arms, to help your 60 ft. times. Posi-traction units are available from Auburn Gear works for about $295 for clutch type and $395 for the better cone type units, in case you get find a reasonable priced differential with open gears. A number of places sell 8 1/2 in. gear sets for changing ratios.

The 3.73 should have been available in the 442's, (hopefully the 442 guys can substianiate this, and give you differential numbers to look for) and the Turbo Regals had 8.5 inch rear ends with 3.42 gears standard and 3.73 optional from 84 through 87. Earlier years had reported optional higher ratios in 7 1/2 inch units (sometimes the information in the American Musclecar Publications should be taken with a grain of salt). On the front of the right axle tube the second and third codes should be TJ (for open rear end) and TP (for limited slip differentials). There may be a little tag on one of the bolts on the cover indicating the need for limited differential fluid, but do not rely on that.

The 7 1/2 inch units look similiar to the 8 1/2 inch units from the outside, but most publications show a more rounded cover for the 8 1/2 inch unit. Having had the units side by side I could barely tell the difference. They both are 10 bolt covers. The mounting points for the upper control arms hang out more on the smaller units, maybe a half inch !

[ Thanks to Chris Fair, Bill Culp, Steve Ochs, Tom Millard, Loyd Bonecutter, Thomas Martin, Charley Buehner for this information ]

Gear Recommendations for the Street

Gearing is much less than an exact science. I definitely don't have any concrete answers for you here. I can only give my thoughts and assumptions, and the directions I'd consider. What you end up doing is up to you only, and either way you go, it should end up fast ('cuz it's an Olds, and that's inherent in the design, right?). The gearing and RPM range of the motor will just dictate when that fastness is most noticed.

What do you want? Good street performance or 1/8 or1/4 mile performance? If you want good street performance gear it so you keep the rpm's in the optimum power range of your motor at your typical cruise mph. If you want straight line performance gear it so that the motor is at the top of the power band at the end of the track. You also want to make sure that your tires can handle the extra torque that a steeper gear will create at the tires.

Running 3.23's, I think it's too steep for a BB and the highway. For racing, though, I'd go a bit steeper. 3.50's might be fine, or 3.73's might be where to shoot for. It will probably all boil down to your engine's redline. If your engine can stay at 8,000 RPM for a while without the mechanical rev limiter (aka rod) taking over, then the steeper gears will be great. The 455's usually don't get that high, though, (without BIG$$$) which should leave you topped before 6000, I'd assume. With 4.11's and that range, I'd guess you'd likely be topped out before the end of the quarter, I'm not sure. My GS with the 3.42's goes through at just over six, usually, but its RPM characteristics are markedly different than the Olds was for me. OTOH, with the really steep gears, you'll top out faster, but you'll have alot more speed keeping you going. You'll only go 100 MPH, but you'll be there alot sooner than the other guy, and he'll still have trouble keeping up, even if he goes through the traps at 120 MPH.

I guess in my eyes, it will depend on what RPM range your motor produces it's torque. If you'll have monster low end torque, but limited high end, then a lower (numerical) gear would probably be better. If you have a lot of power at lower RPM's, you may be happy with a 3.23 to 3.55 gear, and still maintain highway drivability.

If you have nothing but high end power, than the motor will need all the help it can get to get the sled moving, so the 3.73's and higher (numerically) would be a better choice. You need a 3.73 - 3.90 gear in order to get the RPM's up high fast. With 3.90's and 245R60/15's you are turning 3500 RPM at 60 MPH.

If you plan on streeting it, you'd want to modify those numbers a bit, too. I'd have a hard time recommending anything steeper than 3.73's for the street, unless you get an overdrive in there. It'll drive ya nuts on the highway. It looks like with the proper selection of tires, you'll have no problem locking them onto the pavement. The stall and gears will like that.

The rear end gears you pick depend on what you want the car to do. The Dart's 3.55's and high RPM HP isn't too practical for around town (35mph-45mph speed limits), because to enjoy it, it has to be wound out too much, which results in going too fast when really enjoying it. 3.73's or 3.90's would be better suited for this car for power, but it will kill all highway drivability.

I recommend 3.23's highly. I like the good combination of torque at lower rpms, and gas mileage that is acceptable.

If you're running a tall tire (30"+), then you need to take that into account in your gear selection. That 3.50 gear and a 27" tire might be the equivalent of a 3.00 gear with a much taller tire. Figure what RPMs you can live with at highway speed, and then figure out what gears will put you there with your tire and tranny selection (you might have to go with 4.56's to get you there with a tall tire).

Some examples:

1970 Buick GS Stage 1 with a 3:23 posi. Used for 3 years as a family car. The RPMs were not objectionable at all; my wife never complained. The car was always quiet & smooth. I believe that I was turning a little under 3000 RPM (27-2800?) at 70 MPH. Gas mileage was 14 MPG on the highway.

1973 Vista Cruiser w/455 & a 3.23 posi was always very quiet at 70 MPH & (after modifications) got 14 MPG on the highway. The Vista Cruiser was owned for 13 years with 130K miles on it. It was used often for trailering long distances.

1962 Jetfire with a 3.31 diff is just fine as far as highway engine noise. The 1968 442 has a 3.91 posi which is far too noisy at highway speeds. I am trying a 3.42 posi diff in my current 1970 GS, hoping that I will like it. It may be borderline though.

1964 Olds (330/4V) is a 3.36 diff and 215R70/14's: the 3.36's are a good mix of matching the 330's mid range torque and high RPM HP with a drivable car on the freeway. It turns 3,000 at 60 MPH; good passing, 40-70 MPH (kick down 2nd gear) acceleration is a lot of fun.

1969 Dart (built 318/4V) with a 3.55 diff and 225R70/14 tires. The engine is built for high RPM HP, it has nothing below 3000 RPM. The torque comes in at 3,000 RPM, and the the HP come in at 4,000. It turns 3,200 RPM at 60 MPH.

1974 Plymouth Satelitte (440/4V) with a 3.23 diff and 255R60/15 tires: tons of low end torque match the 3.23's at 35 MPH to 90 MPH (with 2 gear changes) quite nicely. The 3.23's allows a kick down to second at 50 MPH. This thing is a monster between 45 MPH and 70 MPH. (ex-cop car).

1970 4-4-2 with the standard 455 (mildly modified) and 3.23 diff. At 60 MPH, I could punch it and instantly pass anything I ever met. Yet the RPM's were comfortably in the 2500-3000 range.

Cutlass with 3.42's and 25 in. tall 235/60/14 tires. It turns about 2700 RPM @ 60 MPH. I like the 3.42's for all around performance and driving ease.

Additional Alternatives

Why compromise? Go with the overdrive transmission and keep the 3.73s! Alternatively, get a Ford 9" and two center sections, one with 3.91's and one with 3.08's. Switch gears for your upcoming driving needs.

I have 3:91's in my H/O and, while the engine tachs out on the freeway, I don't mind the noise (I'm on the far end of that tolerance scale).

Actually, have you really considered the cheapest way out? Get a set of tall rear tires for highway cruising. This will effectively give you a lower numerical rear end for improved highway mileage and reduced rpms. Just swap the shorter tires for strip terrorizing. The only problem is that your speedo will be wrong with at least one of these configurations!

BFG makes some T/As for street trucks which are pretty tall (up to 35" diameter, which obviously won't fit in your wheel well). These are not the off-road tires which most stores stock, so you may have to order them.

[ Thanks to Doug Ahern, Charley Buehner, Bill Culp, Joe Padavano, Dave Wyatt, Bob Barry, Daren for this information ]


And Anti-spin Swap

With the replacement rear end OUT of the car: Unbolt the retainers on the ends of the housing. Get an axle puller (Auto Zone will loan one to you) and jerk out the axles. Clean those nasty axles, press off the old bearings and retainers and press on the new bearings and retainers. Then go put those axles in the corner. It will be a while before you need them. But first go try to stick them into the posi carrier you have to make sure the axle is the right diameter and has the correct number of splines.

Next remove the old axle seals and replace them with new ones. You will need a seal puller or a slide hammer with an attachment. The seals are pressed in the end of the axle housing. The best way to press them back in is to find a socket that is rhe right diameter and hammer them back into the axle tubes.

Next unbolt the cover plate and drain out all that old fluid. Next you have to pull out the old carrier. Clean off the caps that hold on the carrier with a good degreaser like alcohol. Then before removing the caps be sure to mark them as right and left or whatever. Be sure also to know which end is top and bottom. I used fingernail polish. Markings is necessary because these bastards are line bored after assembly. This is line bore thing is important to remember. Now take out that greasy carrier. You may have to pry it out. It also helps if you turn the pinion gear it will help lift the carrier out of the housing.

Now it is time to take out the pinion gear. The hardest thing will be getting the pinion nut off. I would soak it down with WD 40 or Liquid Wrench. Then get an impact wrench to get the pinion nut off. Of course you will have to find some way of keeping the pinion from turning. Once the nut is off, get a big hammer and smack the pinon gear. It will eventually come out of the housing. Do not screw up and smack the yoke.

You mihgt want to get a piece of wood and put it on the pinion and hit the wood instead of trying to hit the rear of the pinion while trying to miss the yoke. Once the pinion comes out, pry out the pinion seal. You can do this with a dull chisel or screw driver.

Next you will have to remove all of the old bearing races from the housing. There will be a notch in the housing at the rear of the race where you can get a punch in there to knock out the race. Actually there will be two notches. Now press the new bearing races into place. For this I found some really big sockets and an extension. I put the socket on the race and proceeded to hammer away. This was a major PITA for me because the rear end was still in the car. There are two bearing races in the carrier. The rear and front pinion bearings. Now get your new posi carrier and press the bearings onto the carrier. Go ahead and put your ring gear onthe posi carrier if it is not already on there. Be sure everything is clean. Be sure to put in the new pinion oil seal.

Now the fun begins. If you have a richmond gear there will be a check distance engraved on the face of the pinion gear and a backlash measurement engraved on the ring gear. These are CRITICAL numbers. If not you will have to use some general numbers. I think factory backlash is .005. My Richmond gear backlash was to be .008. If you have too much backlash you will know in a hurry. When you drive down the road and let up onthe accelerator you will get one loud whine.

The check distance is the distance from the centerline of the ring gear to the face of the pinion gear. This is one MF to measure. Technically you cannot do what you think is right. That is: lay a flat piece of metal across the face of the two caps and measure down to the face of the pinion. The reason this is not accurate is because the saddles are line bored. The caps are not exactly split in 1/2. Unless you have a pinion depth setter, you may just have to use the above method and hope for the best. However, if you do not have a check distance dimension you will just have to do it like this. I am going to ASSUME that you do not have a check distance dimension. In that case do the following:

If a shim came off of the pinon gear, put it back on the new pinion gear. Then press the rear pinion bearing up on the pinion shaft untill it is tight against the rear face of the pinion and shim. Then put the crush sleeve on then put the front pinion bearing on. The front pinion bearing is not pressed into place. It is held in place by the yoke. Then slide this mess back into the housing, put the yoke on and tighten it up slightly. You tighten up the yoke untillit takes about 15 INCH pounds to turn the pinion gear. OK assume this is done.

Now it is time to put the new posi carrier in place. Try to hold the races on the bearings and put it in the saddles. The ring gear should engage the pinion nicely. Now comes the trial and error BS. You put shim packs on each side of the carrier untill you get the proper backlash on the ring gear and the proper paint pattern. Once you THINK you are close, get some gear marking compound and apply it to the ring gear. Be sure to put your caps back on and torque them to about 65 FOOT pounds. Double check that torque I am doing this from memory. Once you have everything in place and painted up turn the pinion gear and see what pattern you get on gears. You are looking for a rectangular pattern that is even across the drive face of the ring gear. I am sure it will not be right the first time. Now you get to move shims from side to side untill the pattern is correct. Then check the backlash. I kinda did mime backwards. I got the backlash right and then checked the paint pattern. It turned out to be perfect when the backlash was correct. I also was able to measure my check distance before I ever put in the carrier. But I am assuming you can't.

Once you have the paint pattern anb backlash correct, you are good to go. Put everything back together and drive off. And listen. If you have strange noises. TRY AGAIN. Be sure to use a good lube and put the limited slip additive in there or you will get chatter and grabbing and such. Do not use synthetic gear lube!

The most critical part in all of this is getting the correct gear teeth mesh. Take your time with those shim packs. Of course if you have crappy luck like me and your pinion depth is wrong, you will be screwed. You will have to take the pinion gear back out and put in the right shim to adjust the depth of the pinion in and out.

[ Thanks to John Harris for this information ]

Clutch Plate Anti-Spin

I can see no reason why a rebuilt clutch plate type posi can't be rebuilt good as new. This assumes no obvious mechanical failures which would otherwise render the unit useless. It is/was common practice among drag racers to take a brand new unit and tighten it up as much as possible by using thicker plates and other means. If the car is street driven, you must use gear lube formulated for posi units.

[ Thanks to Bob Handren for this information ]

Cone Plate Anti-Spin

It is also a cone type posi unit. If that is shot it cannot be rebuilt in the same fashion as the plate/friction type. They can however be made functional with a little work.

[ Thanks to Bob Handren for this information ]

Parts Sources

General: The prices you were quoted for gears are about twice the going rate it you buy direct. Usual practice is to double the cost if you ask someone else to order 'em for you.

For most applications, gear and carriers are available, but cases are not.

1967-1970 "O" Axle Overhaul Kit:
Until now, many items required to properly overhaul an O axle were unavailable (Chevy 12 bolt parts do not interchange). Which meant reusing the old parts. This often results in premature bearing wear and shortened ring & pinion life. Our new kit includes the following items required for a complete differential overhaul and/or ring & pinion change: carrier bearings/races, pinion bearings/races, ring gear bolts, our new crush sleeve, case side shims, pinion lock nut, marking compound, Loctite pinion seal, cover gasket and sealer.

Available for all 1967 thru 1970 (A body) O axle 12 bolts. Open or Anti-Spin. Specify year and ratio. Complete kit priced at $159.95. Available only from the mechanical parts specialists at Supercars Unlimited. Where Cutlass/442 is ALL we do!

Supercars Unlimited
8029 A SW 17th
Portland, OR 97219
PH: (503) 244-8249
FX: (503) 244-9639

"O" Axle Parts:
Check with the gear suppliers that advertise in just about every worthwhile auto magazine. Ring and pinion gears are now available also. Either Perfection American or Zoom. A great dealer I have used it Drive Train Specilaties (DTS). They (as well as others) even have the shims, bearings, seals and gaskets in kit form so the rear can be set up properly.

28 Spline Axles:
If the differential has 28-spline axles, Reider Racing (313/946-9231) can fix you up.

31 Spline Axles:
If the diffential has 31-spline axles, you'll have to find an original unit and have it rebuilt.

[ Thanks to Brad Nicholson, Bob Handren for this information ]


Gears are available for most every differential. But buying and installing a bunch of parts can get expensive - fast. It would probably be a bunch cheaper in the long run to find the rear with the gears and anti-spin that you want.

Interchange Guide

Year From Year To Model(s) Comments
64-66 64-66 GM A-body - Cutlass, Chevelle, LeMans, Skylark Bolt in.
67 64-66 GM A-body Lower spring perch is different, requiring the use of the 67 springs.
68-72 64-66 GM A-body Axles are about an inch wider than the earlier ones and also have the control arm mounting locations in a slightly different place; the axle will physically bolt up to the early car but the pinion angle will be wrong.
This table works in the opposite direction ("year to" to "year from") as well.

[ Thanks to Joe Padavano for this information ]


To upgrade an open differential to a posi, you must change the carrier to a posi type. That is, just the "guts" inside the rear end. You can even keep your existing gear set. A new or rebuilt unit should be about the same in durability. If you have a regular "C" type rear end then parts for this are very easy to find. However, if you have a "O" type rear end used in 1967 to 1970 models then a posi carrier is very difficult to find. Best bet is to get a rear end specialist to do the work.

An entire rear end from another GM product is a bolt-in fit as long as you're doing it in the same body style. Minor differences such as brake lines and U-joints are the only trouble spots. Buick, Olds, and Pontiac rear ends are interchangable if they are the same frame size (an "A" body GTO rear will fit in a Cutlass).

In regards to swapping 1964 to 1967 A-body 10 bolt rear axles, with 1968 to 1972 A-body axles, the entire housings will swap. First of all, the 67 and 68 Type-O axle housings carry exactly the same part number (don't know about Type-C axles). Regarding the 1968 and up axle being wider, I first heard about this width difference (which was more like an inch, not 1/4") in Car Craft about 20 years ago. I sometimes wonder if this isn't in the same catagory as the notorious "offset engine mounts" allegedly in the Supremes.

A couple of things you do need to look out for, however. First, the early axles (1964-66) may have slightly different mounting points for the control arms. Installing these in a later car with the late control arms could result in an incorrect pinion angle and the attendant U-joint wear, unless you use the control arms that go with the axle. Second, some axles (for example the 1971-up corporate 10-bolt) have a different dimension from the u-joint flange to the axle centerline. This will sometimes require a slightly different driveshaft length. I ran into this problem when I installed the axle from a 1972 442 into a 1968 Malibu. The Chebby driveshaft was about an inch too long.

As a general rule, most of the axle shafts will swap as long as they have the same spline count. This excludes 10 bolt "C" axle shafts (as in Ch*vy) which are unique in that they have an internal "C" clip to retain the axle in the housing and use a roller bearing in the tube end. This is as opposed to a bearing which is pressed onto the axle and then bolted to the tube end as most other sane manufacturers do.

Most GM diffs carriers carry ring and pinion sets up to 3:42 ratio, from 3:73 and up you have to change the carrier. The reason is that as the pinion gets smaller (with the lower gear ratio), the deeper the crown gear has to be. Instead of making extemely thick crown gears, GM made two types of carriers instead. Some after market suppliers have a spacer which allows you to put a 3:73 gear set on a 3:23 carrier. I have done this before on several cars than run both on the street and the strip, and don't want to change the carrier.

As for gear combo, say if you had just a posi carrier with say a 2.73 gear, you could put a 4:10 gear or even 4:33. Consult the chassis manual or the gear manufacturers catalog for compatible gearing. The only thing you will need to do is buy a shim kit because when you put higher gears on the carrier, the carrier shifts relative to the pinion. These shims go between the carrier bearings and the start of the axle tubes in the housing.

The service manuals from 1967 and 1969, and also the illustration section of part listings from that era, have pictures and diagrams of the center section castings (using the ribs to ID the rear) and rear covers (Chevy rears have that goofy wrinkle).


If you have an old bearing handy, it has the # on the race that can be used to get a replacement. This # is a standard that can be found in any bearing parts shop or even at the usual auto parts place that you frequent.

[ Thanks to Bill Culp, Joe Padavano for this information ]

Differential Internals

The big question for swapping the internals is of course, just what did the new internals come out of. Many cars being very close in model year used different internals, even though the cover on the differential has the same number of bolts. Identify what you have first.

You need a few tools to do the swap correctly:

If you don't have these tools, you can buy a set of ring & pinion installation tools for $150-$200, and an impact gun would run you around $90-$120 for a cheap one (of course, that is assuming you have an air compressor).

Now if you have or can borrow all the tools, the critical things to measure are the pinion depth, pinion bearing preload, carrier side clearance, and gear backlash. These are all outlined in the shop manual (which you should refer to when doing this job).

[ Thanks to Bob Barry for this information ]

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