403 CID Engine Detail

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General Information

Note: The information contained in all of the "Engine Detail" sections should be read before proceeding with modifications, etc., because some information that applies to all engines, or all small blocks or all big blocks, might not be duplicated in every section.

The 403 was made from 1977 to 1979 and used across the Buick, Olds, Pontiac and Cadillac vehicle lines. It was also used in other applications like motor homes, boats and some industrial applications. It was sort of meant to replace the previous use of big block engines in the BOP line. Stock specs on a 1977-79 403 are: 185hp@3600 RPM, 320ft/lbs torque @2200 RPM, 4.351 x 3.385 bore/stroke, 8.0:1 compression ratio.

Big pistons and a short stroke are an excellent combination for a high-RPM engine, but here the other inherent 403 weakness comes into play: Windowed main webs, and to a lesser extent, siamesed cylinders. The bottom end of this block is not suitable for extended use at high rpm with very high cylinder pressure.

Sources For 403s
1977 Cutlass, Vista Cruiser, Delta 88, Custom Cruiser, 98, Toronado, Electra 225, Bonneville
1978 Delta 88, Custom Cruiser, 98, Toronado
1979 Custom Cruiser, 98

[ Thanks to Fred Nissen, Jason Adcock, Bob Barry for this information. ]

Streen Machine
If you are looking for a super reliable street engine that will squeeze the absolute maximum torque into an 87 octane, cheap to buy, pollution legal till 1979, minimum weight package that is a direct bolt in (including weight) for any Olds small block, try a 403.

Here are my results: Stock 403 from 1978 Toro, new timing chain, Jacobs ignition, diesel sized 4" tall K & N air filter (extra low Toro bottom half, tall diesel top half), dual 2 1/2" pipes with crossover and no cats, switch pitch transmission with electronic controller, 2.41:1 axle and a 4100 lb RWD car. Measured many times with my stopwatch and also with VC2000 computer: 0-60 7.49sec; 1/4 mi at 91.2 mph in 15.86 sec. The slow time reflects the axle ratio; the car has gobs of power between 70 and 100 mph, as I have enjoyed demonstrating to many cars that were more nearly matched with their 4 speeds at 0-60 including a certain Mercedes sports car who thought he could top end me after I blew him away at the line. I estimate 270 horsepower by modern measurements by comparing it to the published times and weight of many modern cars. Is this a race car; NO. It is a very practical street car which has seen 20mpg (NOT with me driving) and the only car I have ever owned to get to 300,000 miles without once seeing a tow truck It's not quite pollution legal with no cats, but it would pass our test here that way.

[ Thanks to Bruce Roe for this information. ]

Main Bearing Webs
The main bearings are supported in/ connected to the block with webs of cast iron. Later motors, after 1976, including the 403's [most, anyhow] had not solid webs but webs with holes. Thus, there were essentially 3 fingers of cast iron holding each main bearing.

Good luck finding a Solid Main Web 403 motor. Rumor has it that some 403 blocks were made w/o holes in the webs- Solid Main Web 403's. Supposedly 403 solid main web blocks are found in the Pontiac Bonneville with the towing package, and in some 1979 Pontiac Trans Ams with a towing package.

Then again, for all but 600HP race motors, it's not necessary; the metallurgy of the 403's is quite well done.

[ Thanks to Chris Witt for this information. ]
Siamesed Cylinders
The design does have some limitations due to the siamesed cylinders and windowed webs. You must insure the cooling system is large enough and in good order. I would not recommend one for an ultimate dragstrip build up, and I would avoid doing anything that greatly raised the compression especially if the block was overbored. Head gaskets blowing would be the concern here.
[ Thanks to Fred Nissen for this information. ]
RPM Maximum
The 403 is a mediocre all out performance engine. The main bearing webs are the weak link and the engines can usually accept only a .040 overbore. They are thin. Yes you can bolt the mounts off the 350 right to the 403 and drop it in. As for supercharging you need to look to a different engine assembly. A 350 or 455 is a much better choice. Danny Lattimore has a stock class 403 in a Trans Am that turns 7500 RPM. So things can be done but at a major cost.

Sure, you could build a 7,500 RPM motor, but it'll cost you a good chunk of change. In the U.S., you'd have to dip to the hip for likely two or three thousand dollars (and that's at the *cheap* end of the spectrum). Seven, maybe seventy-five hundred is possible with a windowed main web engine, and even then it's under serious stress. Once again, why do you want an engine that can rev this high? Yes, NASCAR mechanics can build a 9,000 rpm motor, but they have to be rebuilt every 500 miles.

First of all, you'd need a forged crank, which should be good up to seven or seventy-two hundred. After that, you'd need forged pistons, and they're pretty costly here in North America as it is. What are prices like in the Netherlands, anyway?

Add in a wicked cam, large valves, strengthened valve-train and pushrods, and a set of *really* stiff valve springs, otherwise they'll float off of the pushrod, and your power will be flowing out your exhaust pipe. (not to mention that the valvetrain would fly apart after a 1/4 mile at 8,000!)

There are a few different ways you can boost up the compression, which would help with a wild cam: you can shave the heads, which makes the combustion chamber closer to the piston head. Or, you can buy pistons that can kick up the compression. Most pistons have two reliefs cut into them, which eliminates the possibility of having a piston slamming into a valve. You can buy pistons with smaller cuts in them, which raises the compression. Just make sure your cam timing is perfect, and be sure that a valve won't stick open. Otherwise your piston and your exhaust valve will get married, and you'll be buying them one expensive present!

OK, ok, theoretically speaking I suppose it could be done, since the basic geometry of the 403 is pretty decent. You could probably get 8000rpm out of a 403 if you:

  1. Start with a solid-main web block (if they in fact exist), completely blueprinted, and filled the lower water passages with block hardener and use a full block girdle. O-rings would be a definite *must*
  2. Used *Ultra* lightweight forged pistons, and absolutely used aluminum rods
  3. A billet crankshaft would be called for, though if you like living on the edge, a factory 330 crank could be used, though I'd want to magnaflux it about every 10-15 passes.
  4. Batten heads, *heavily* ported, with the biggest possible valves, might just get you to 8000rpm.
  5. Fabricated sheetmetal intake, with at least 1400 cfm sitting up there.
  6. Huge tube headers
  7. Crank-triggered ignition
  8. .700"+ lift solid cam, aluminum rockers, titanium valves
  9. External oil pump, perhaps dry sump, or at least a stock system with the standard mods and external drain-back lines. It would be good to have more than 8 quarts of oil circulating in there.

The basic geometry of the 403 is very similar to the Ford or Chevy 302's, which are capable of being high-winding motors, so whatever you do to them should work on the 403. The big problem would be finding a solid-main block, without which this buildup would simply be a kamikazee run (i.e. you'd see 8000rpm with this motor... once).

If you could find a suitable block, though, you could probably put together the above motor for $20,000. It might produce 800-900hp, and have an effective power band of 5500-8500rpm (I'm just guessing here; somebody here might want to run these numbers through one of those dragstrip Jr. type programs to see what it really would do). Wouldn't be that much fun to drive a car with that engine on the street, though.

Once again, the compression can be boosted a lot, by small-dish pistons and/or smaller chamber earlier small block heads. Open up the breathing with free flowing xst, big valves, port the heads, nice intake & carb... Rowdier cam, much stiffer valve springs, the usual tricks. Improve the oiling system. Match the timing curve to the new attributes of compression, cam, and fuel used.

On a side note, I remember Dave Smith building a 403-based engine to try out a trick main girdle to strengthen the bottom end on those lightweight blocks. It cross-bolted the custom main caps, supplemented with studs at the original 2 bolt locations. He brought it out to Orange County on a test day and had Tom Chelbana run it a time or two. That's all it got -- the girdle worked, but the webs inside the block broke. I don't think Dave ever tried another 403 that way, the diesel block's just too much better and his main caps for it are the only way to fly.

[ Thanks to S. Dave Ward, Ken Snyder, Jim Chermack, Chris Witt, Bob Barry for this information. ]


Actually, it seems to me that the 403 should be a kick-@$$ motor. Besides just slipping in over the mandatory 400 cu in limit ;-), the bore/stroke ratio of this thing is incredible. The thing has a bigger bore than a big block Chebby. If you relocated the valve centerlines in the heads, you could run some impressive valve diameters, with the result that you should be able to flow an incredible amount of air. Think about it: many on this list sing the praises of the short-stroke 400 motors - here is essentially the same motor with a better bore/stroke and a smaller/lighter block. Why aren't there more of these out there kicking butt? The ultimate Olds motor might just be a 403 (yeah, good luck finding a SMW block - anybody thought about welding up the windows and doing an align bore?) with a forged 330 crank and Edelbrock heads with relocated enormous valves. The smaller bearing diameters would also reduce bearing speed and thus friction. I may just have to build one of these.

The 403 responds well to a bit more camshaft and compression. I used earlier 350 heads ('68-'72 will do it, but the '72's are the best) to raise the compression, and an Edelbrock Performer-grind cam (these cams are available from other sources than Edelbrock, who I suspect purchases them from another supplier).

A true-dual exhaust really livens up the motor; the factory crossmember from a pre-'75 (year of mandatory catalytic converters) allows true duals without excessive bending, though you'll have to check with emissions laws regarding keeping or discarding your catalytic converter.

The next upgrade on my list is a change from that 2.41 rear axle ratio; while my engine improvements improved bottom-end power (it can roast the tires at will off the line), the real improvements come into play above 3500 rpm, which means my car really starts to take off at about 70mph in second gear. I plan on moving to a 3.23 rear gear, though moving to a beefed 4-spd automatic with Overdrive would allow me to move to a 3.73 rear gear. I don't make enough hp to justify a Gear Vendors overdrive ($2800!).

The 2400rpm stall converter in front of a TH-400 improves the launches of my car.

[ Thanks to Joe Padavano, Bob Barry for this information. ]

Block Boring:

An Olds 403 can only be bored out 0.040" before problems with cooling can occur. The siamesed cylinder layout (no cooling jacket between cylinders), and the minimal iron between cylinders is the cause of this limitation.

Well, after a 300+ mile journey with my often-hot running 403, a 403 should not, at least for a car that sees any kind of traffic or distance driving, be bored .060 over.

After watching my temp gauge hover at 250 for over 10 miles of uphill driving at 70MPH, (and the oil pressure fall to 15PSI at that temp, from the normal 30) I am somewhat disheartened, but not really surprised. Going up those hills, I had plenty of opportunity to "play around" by flooring it (to see if richening the mix helped at all..actually got no hotter or colder) and watching the temp, but the temp was pretty constant depending on terrain. 220 on the flats after the hill climbing was about the norm. Very rarely did it drop back down to the 195 the T-stat is rated for. Coasting at high speed (neutral) did drop the temp very quickly, so I know the radiator is working real good.

Another person has overheated a stock 403 (stuck thermostat) three times in two days, shutting the engine down right after each occurrance on a 70,000 mile engine. Now 13 years later and at over 215,000 miles, the stock 403 is still running strong.

With a bored 403, or even stock 403, keep the cooling system in A-1 condition. Drain cooling system (radiator, block drain plugs, etc) at least every 3-4 years. Use a cooling system additive that promotes the thermal transfer of heat from the engine to the coolant (Water Wetter, Sy-Cool, etc). I would avoid using any form of stop leak! Use a 6 or 7 blade fan, radiator shroud, 3 row or more radiator, and keep the lower air dam in place. If you can get away with it in your climate, use a 30/70 ratio of coolant to water to enhance the performance of the cooling system additive. A thermostat rated at lower than the stock 195° unit might help as well. But the problem is really with the ability and capacity to transfer heat than overall temp. The ability and capacity to transfer heat directly affects overall temp.

[ Thanks to Dorian Yeager, David Brown for this information. ]

Short Block:


A stock 403 Olds engine has tons of torque. Carbs aren't the problem. The problem is the heads. Their combustion chambers are too large, around 83ccs. This results in a compression ratio around 8.0 to 1. Try to find a set of 350 heads from 1973 or before. This will raise compression enough to even think about HP. Before you think about a carb I would think about an intake manifold, if your's is stock.

If you add 72 or earlier 350 cylinder heads, your compression will be abound 9.5:1-10:1. It will increase your HP considerably, though you'll probably want to add a cam that can take advantage of that high compression. If matched with an Edelbrock intake, I'm guessing around 300 hp, though that's a really rought estimate and depends on the parts that you use. Most 350 heads will boost compression, not much boost from the common #8 heads.

Yeah, what he said. My calculations say that unmachined 64 cc heads such as '68 to '72 350 heads would boost the stock 403 to 9.50:1 compression. Mill the heads 0.030" and you have just over 10:1 CRatio. This assumes 0.040" head gasket, and a few other assumptions.

This is my recommendation also. Get '72 350 heads [hard xst seats], install the 403's bigger 2.000 valves or even larger W30 2.072" valves, and limit your expenses. That motor will kick ass w/ the appropriate cam, intake, exhaust. As for those who doubt the ability of the 403 to handle power with its open main webs, you might call a racer who uses one, such as:

The stock 403 heads have 4 additional coolant holes drilled for additional cooling (because of the siamesed bores), compared to 350 heads. Drill these additional holes in any 350 heads you might use for additional cooling potential and additional protection from blowing head gaskets. This advice comes from a number of experienced rebuilders and Oldsmoholics.

A high compression 403 will not shatter a stock piston, unless you are running nitrous. The "7a" heads only give you about 9.5:1 compression, and the factory high-compression 350's came with 10.5:1 and cast pistons, so you're pretty safe. Just have it balanced, and keep her under 6000rpm.

As far as heads are concerned, my personal choice would be a set of big block heads with big valves. Given that the 403 small block obviously moves as much air as the 400 big block, why strangle it with small block heads? Adapting the big block heads to the 403 requires a couple of things:

Edelbrock aluminum heads would be great, but that's an additional $1500. Stock big block heads with 2.07/1.625 valves will be more than adequate for this motor, especially if you can do a simple cleanup on the exhaust ports to remove the A.I.R. bumps. Also, have the exhaust flanges milled or welded and milled (as required) to ensure that the divider between the center two exhaust ports goes all the way to the flange surface. You will also need to run the numbers on true compression ratio with your pistons, head gaskets, and head chamber volume. Most stock big block heads run about 80 cc in the combustion chamber, which will be a little large for your 403. Of course, milling the heads will require you to check pushrod height and possibly run a non-stock pushrod length. If you're planning to do some work to the heads (milling and exhaust port cleanup), the smog-motor J heads will be fine. These have hardened valve seats and if the exhausts are ported will flow the same as the harder to find (and priced accordingly) C heads.

When you're done, you should have a small block Olds which will run as well as the vaunted short stroke 400 motor of 66-67.

[ Thanks to Ian M. C. Dixon, DrkSole, Chris Witt, Bob Barry. Joe Padavano for this information. ]

Head Gaskets
I blew another head gasket in the 403 (other side this time, so it's got all new gaskets) in the same manner as I did the other side last year; I had driven on a hot day, parked it, and revved the motor after it had sat for a little while.

It seems that the 403's need a constant flow of coolant to keep everything nice and square; when you let it sit, the nice cool (relatively speaking) radiator fluid keeps that thin bit of iron between the siamesed bores somewhat straight, but when it's shut down, the heat from the engine evens out, and the cylinder walls perhaps distort. Then, when you give it full-throttle shortly after starting the engine when it's hot, the high cylinder pressure (I've got 175psi static pressure, plus 16' initial timing) can blow out the gasket between the cylinders.

I neglected to pay attention to a rule I made for myself, to not rev the motor for at least five minutes driving time after starting it hot. I might add in a recommendation to O-ring any overbored 403's with small-chambered high-compression heads when you're doing other machine work, unless you don't mind an annual head-gasket change.

Oh, and the fact that I've got properly-torqued ARP head bolts doesn't seem to make any difference in this. Those big pistons are part of the 403's weakness; the large bores leave little room for the head gasket, which limits the amount of cylinder pressure you can develop before blowing a head gasket.

[ Thanks to Bob Barry for this information. ]



[ Thanks to for this information ]
403 With a 425 Crank
If I win the lottery (which I don't play), I'd tool up for some 403 castings with a diesel bottom-end to take the 425's crank. I'm still scheming about a 425 crank with cut-down journals being dropped into a 403 block; I _want_ a 500+ci SMALL BLOCK! Of course, I'd have to put 260 stickers on it... :)
[ Thanks to Bob Barry for this information. ]

Buildup Examples

'87 Cutlass
I have a 403 in my 87 Cutlass, and it performs very well. The 403, stock, is only about 185 hp, but 320 ft-lbs of torque. This is what it has in it now:

• .030 over bore
• 72 350 heads
• Edelbrock performer RPM intake
• Crane cam (.480"/284 degrees (intake), .496"/292 degrees (exhaust) advertised (at 0 degrees))
• Crane lifters
• Crane valve springs
• Summit True roller timing chain-cast piston 9.5:1

With these not-so-expensive upgrades, my car runs mid 13's.

'79 403

• '79 403 block bored .030 over => 409 CID
• 403 crank ground .010
• stock 403 rods
• Arias(?) forged aluminum pistons, 10.5:1
• Crane Cam .480/.496; (280-290 adv.? cam card missing!, maybe 292 or was it 272, dual pattern though, both with some high 200's duration (advertised)
• Crane lifters, pushrods and springs
• '72 350 (7a) heads
• Edelbrock 7111 performer RPM intake
• Carter 750cfm AFB
• Mallory Unilite distibutor w/ Cccel supercoil
• TH-350, stock converter, shift kit
• 3.08 posi

Recently smoked a 96 Vette (since we fortunately didn't reach the point where he'd be in 6th gear!). 5.0's are effortless, generally, non-blown ones at least.

[ Thanks to Ray Costanzo for this information. ]

1979 H/O

1979 Hurst/Olds
Built Racing Head Service 403,
Mondello JM 22-25 cam
W-31 heads
Edelbrock Performer intake,
rebuilt Carb Shop quadrajet
Holley electric fuel pump
less than 100 miles on rebuilt Turbo 350 & B&M 11" converter
3.23 Auburn posi unit
Hellwig swaybars
new dual exhaust
solid body
fiberglass cowl induction hood
Hurst dual gate shifter
original Hurst wheels w/ good BFG tires
Runs VERY strong at 13.1@ 105 mph 1/4 mile
[ Thanks to Todd Evans for this information. ]

'80 Cutlass
I had a 1980 Cutlass with a 403 stuffed in it. It ran 13.60s stock with street tires, well semi stock.

Stock rebuild
moly rings
Edlebrock intake 3711
Comp Cam's cam (I believe the one with the 433 lift and 262 duration)
650 dual feed double pumper Holley carb
dual 2.5" Flowmasters
TH-350 trans with shift kit and no stall (with stall I would burn up in the gate)
3.73 rear end

With that combo I beat almost every Chevy on the west side of Chicago and Ford Mustangs couldn't stay there either. Believe me with a light body and the right gear, you can't lose with a 403. It came stock with 185 horses and 320 lb. of torque. They called me the green machine from Ogden and Kosner.

[ Thanks to Samuel Johnson for this information. ]


403 on $500
Well, for that price, you'll have to stick with your stock shortblock, and the stock intake, carb and ignition will have to serve. Under that budget, however, you might be able to address the two main weaknesses of the 403: the cam and the compression.

A set of 7a heads from any '72 350, with your 403's stock 2.00" intake valves, will raise the engine's compression and improve its exhaust flow. The purchase price and machine work will cost you around $400.

A decent cam like the Mondello JM18-20, the Edelbrock Performer or even the '68 Hurst/Olds cam with new lifters (and springs, if you can afford them) and a new timing chain, will make a noticeable difference as well.

In all the above things, the details (such as degreeing the cam, port-matching the heads and manifolds, etc) are the most important thing, and will maximize the gains you realize from these modifications. Needless to say, the above price estimates will not include labor, and neither of these mods are "bolt-ons", but they will be the most effective.

Any other mods above and beyond the heads and cam would be geared toward improving the durability of the engine, but wouldn't necessarily contribute toward power output in any dramatic way. Those kinds of mods will go way beyond $500, though (things such as forged pistons, oiling system mods, rod and crank preparation, balancing, etc.).

[ Thanks to Bob Barry for this information. ]
Consider swapping a set of earlier 64cc heads onto the engine. See Buildup above.

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