Pistons, Connecting Rods

* 1949 - 1964 * 1964 - 1990 * 1995 - Present

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




Pistons, Connecting Rods 1949 - 1964

Identification


Piston Dish Volumes


Strengthening/Machining



Pistons, Connecting Rods 1964 - 1990

Identification

Pistons

The notches in the top of the piston are for identification. In the "good Olds days", there were different pistons available for different compression ratios, and since almost all big blocks had some sort of dish, the different notches made it easier to tell which pistons were in an engine.

High compression pistons will have a sharp angled notch cut into the piston crown. Low compression pistons have a round cut in their crown. Pistons with both are probably special (eg. '70 W-31 pistons).

CID Comp/Pin Height Skirt Length Bore
330 1.615" 3.9385"
350 1.615" 4.057"
Early 400 1.615" 4.000"
425 1.615" 4.125"
260 1.615"
307 1.615"
403 1.615"
455 1.74"
Late 400

All '64-'67 Olds V-8's (330, 400, 425, excluding the 394) and the 350, had a pin height of 1.615", so theoretically the pistons would interchange if you could bore the block out, and this is a reasonable amount, unlike the 260 and 307.

So, a .0615" overbore (pretty reasonable) would allow a 330 to use a 400ci piston to make a 340ci motor, but it would need a .1865" (!) to use a 425ci piston, which would result in a 362ci motor.

350 and 425 share the same piston pin height. A set of 425 pistons would make a nice high-compression 350, since the 425's 4.125" bore is a possible overbore from the stock 350 4.057" bore.

But...an Olds 350 normally uses the same 1.615" pin height, and begins with a 4.057" bore, so it would need only a .068" overbore to use the 425 piston and get that 362ci; a bit more reasonable, but still a big overbore.

There might be a difference in the piston skirt length between pistons, say between 350 and 400 pistons. This might contribute to cylinder taper, might not. Probably not a problem unless your piston-cylinder wall clearances are loose. Less of a skirt might result in less overall piston weight, which might result in an ability to rev slightly faster. Don't know for sure.

Pistons with a higher compression height, say a 455 piston in a 350, will cause the piston to stick out of the top of the block.

[ Thanks to Dave Brode, Bob Barry for this information. ]

Rods

Code   Year(s)     CID/Block Code      Notes
GM     1973        455/Fa
XC     1972        455/F               442

The 1965-1967 400 and 425 used the same crank and rods. Greg Rollin

Connecting Rod Center to Center Lengths

Engine              C-C Length
65-67 (early) 400     7.0"
425                   7.0"
68-69 (late) 400      6.735"
455                   6.735"
260                   6.0"
307                   6.0"
330                   6.0"
350                   6.0"
403                   6.0"
Diesel 350
[ Thanks to Roger Hitson for this information ]

Piston Dish Volumes

                dish    dish    calc'd   msr'd
CID      CR     dia.    depth   vol(cc)  vol(ci)  Notes
260
307
330 HC   10.25  2.42    .063      4.7      5.0
330 LC    9.0   2.5     .240     19.3     19.0
350 HC   10.5                                     W-31 piston (til '70??)
350 HC   10.25                            14.0
350 LC    9.0                                     At least til '70
350 LC    8.5   2.85    .230     24.0     24.0
403       8.5
E400 FT         0       0         0
L400 LC
L400 HC
425 HC-T 10.5   2.75    .047      4.5      4.5    Tornado/Starfire piston
425      10.25  2.75    .065      6.3      7.0    Std. HC 425 piston
455      10.25  2.85    .150     15.7     15.0
455      10.0
455       9.0   3.010   .350     40.8     37.0
455       8.5                             40.0

The flat top piston was used in the 350 Ram-Rod, aka W-31. Early 400 motors used dishless pistons. Some 425s, but not all high compression 425s.

From what I've seen, it appears that in 1971 (when they needed to drop all of the compressions in GM cars - 10.5 to 8.5), that their first method of doing so was to dish the pistons more. In 1973 they increased the chamber sizes on the heads and went back to piston comparable to 1970 and prior.

[ Thanks to Chris Witt, Brent Fields, Ken Snyder, Bob Barry for this information ]


Strengthening / Machining

If you have an engine apart, and have a 4" grinder and a die grinder, it's recommended to polish the rods' side beams. Most importantly, it removes the stress risers (which weaken the rods by providing a start for a crack which leads to failure). So this in effect strengthens the rods. Less important: it allows the rod to shed oil a bit faster to reduce reciprocating weight during engine operation.

Just grind fairly smooth with the 4" grinder, then smooth over with a flappy-sander in the 4" grinder, then a little 80 grit flap wheel in the die grinder will make them a joy to touch and behold. Then run over them with a 240 grit flappy wheel will polish them to shiny. Easy, though time-consuming - about 2 hours to do all 8.

Beef up the block by installing either a main stud kit or a main stud and strap kit. For longevity on the connecting rods, have them stress relieved and install ARP rod bolts.

I installed a main stud kit in my 455, and the only problem I had was with the windage tray. The trick with the connecting rods is to install ARP rod bolts, and have the rods resized. Then take them to a machine shop and have a .500 WIDE slot in the top and bottom of the main bearing journal machined .010-.012" DEEP. Then I would have your rotating assembly balanced. A good machinist will balance the harmonic balancer, connecting rods, pistons, crankshaft, flywheel/flexplate and pressure plate.

[ Thanks to J2RKT@aol.com, Steve for this information ]


Gotchas

425 pistons are not fungible with 455 pistons due to a different distance from pin center to top of piston ("compression height"). The compression height for the 425 is 1.595" and the 455 is 1.720". Compression height is the distance from the center of the piston pin bore to the top of the piston.

Compression height refers to the placement of the wrist pin. With any stroke and rod length combination, the pin has be in the right place to complete the picture for a block of any given height. If the stroke is increased the engineer has to either shorten the rod, move the wrist pin location closer to the piston crown by ½ of the increase, or make the block taller.

If there is room they usually leave the rod alone and change the piston. This is probably cheaper and makes a better rod angle which cuts down on piston and cylinder wall wear. Sometimes there isn't room below the rings and other places, so it's easier to shorten the rod. Either of the other two options are cheaper than making the block taller because this changes lots of other things, such as intake width, pushrod length, location of the brackets, ect.

The 1965-67 400 block will take a 350 piston with a .057" overbore. The 350 pistons are readily available in a variety of compression ratios.

The diesel rod is slightly shorter than the gasoline rod, for the increased ring-land necessary for a diesel, I'm assuming.

[ Thanks to Chris Walter, Steve, Bob Barry for this information ]



Pistons, Connecting Rods 1995 - Present

Identification


Piston Dish Volumes


Strengthening/Machining



Table of ContentsIndexAcknowledgements

HistoryEnginesBlocksHeadsCranksIntakesExhaust • Pistons
TransmissionsDiffsBrakesSuspensionSteeringCamsCarbsInterchange
Best BBBest SB260303307324330350371394400403425455Diesel
RebuildingBuildupSwapRestoreOption CodesWheelsIgnitionComp Ratio
The W'sThe H/O'sThe 442'sToronado88 / 98 / StarfireCutlassJetfireWagons
Basic Tech How ToMiscellAll VehiclesAdditional Information

 Rocketeers have navigated these pages (since April 10, 2000).

© 1996 - 2000 by the members of the Oldsmobile Mail List Server Community. All rights reserved.

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