400 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.

Early 400
Built 1965 - 1967.

Late 400
Built 1968 - 1969.

The short stroke 1965-67 400s are awesome motors, but in 68-69 Olds attempted to reduce emissions by using a reduced bore to minimize combustion chamber quench area and thus lower unburned hydrocarbons (emissions controls actually started in 1968 on 49-state cars). Note that this decision to go with a smaller bore was also due in no small part to the desire to use a common crankshaft with the new 455 in 1968 as well. The result is a terribly undersquare motor (3.870 bore x 4.125 stroke) which won't rev very well. This is the worst bore-to-stroke ratio of any American V8 from the modern era.

One limiting factor to your buildup will be the small bore size of your later 400, which can shroud larger valves. One great way to gain yourself some horsepower is to move up to a 455 shortblock with your stock "C" heads; more cubic inches, and the large valves will breathe even better.

The story on the change from the early 400 to the 68-69 400 was due to design changes to start limiting emissions and with the longer stroke they picked up more torque which lead to more power at lower rpm. Thus their early attempt to reduce gas consumption and increase mileage with lower emission. Plus the redesign of the internals which now shared the same crank and rods as the 455's.

I recall reading that the undersquare design of the 400 and 455 in 1968 was intended to reduce the quench area in the combustion chamber to reduce emissions. Apparently, the walls of the combustion chamber are cooler than the rest (makes sense) and fuel can condense out there and lead to incomplete burning and thus higher HC and CO emissions. By making the bore smaller, the surface area of the combustion chamber is, by definition, reduced.

Of course, while that was the published rationale, I suspect that this decision had more to do with commonality of the expensive crank forging between the 400 and 455 than anything else. Since Olds likely expected to sell many more 455s than 400s, redesigning the 400 to use the 455 crank and rods probably made a lot of financial sense.

[ Thanks to Rob Thomas, Bob Barry, Joe Padavano for this information ]
Rev Potential, Limit
The early 400 engines rev higher than any other big block. They will rev higher than a 425. Got a little less piston mass. These things will turn almost 7000 RPM.
[ Thanks to Jim Chermack for this information ]


Cylinder Boring
The E block (early 400) can only be safely bored .100. That's all. The cheapest way to go is get standard 350 pistons and bore the engine .057". They work great. You can even get the flat top W-31 pistons and have it the same as the originals. I've used these before with excellent results.

Although the 1965-1967 400 and 425 used the same crank and rods, the blocks are different castings. The maximum overbore recommended by Oldsmobile is .060. I've heard of guys using the 425 pistons before, but if it were mine, I'd sure want to have the cylinders sonic checked for thickness. Excessively thin cylinder walls will lead to overheating due to less material to dissipate the heat. Thin cylinder walls will also distort much easier, resulting in poor ring seal.

You would want to at least use torque plates when boring and honing the cylinders (This should always be done on an Olds engine anyway). If this is a (drag) race only engine, I would recommend you use a water jacket filler available from Hard Blok, Moroso, etc. However, if this is a driver, cruiser, etc., I would rather sacrifice the few cubic inches gained than compromise longevity.

Ran my question about the 425 piston into the 400 'E' block by 'the rocket scientist', Chris Witt, and he related seeing a cutaway 400 engine at MSU. He recalled being amazed at the thickness of the cylinder--he measured 3/8". Sounds like going .063 won't pose any problem there. Apparently JM also says it's an OK deal.

[ Thanks to Jim Chermack, Greg Rollin, Peter Sliskovich for this information ]


Piston to Valve Interference
The Olds engine your thinking of is the 65-67 400's. They all used flat top pistons and had the same A, B, C, heads that were used on all engines during those years. The 66-67 W-30's even used that .474 lift / 308 duration cam. So there shouldn't be any piston to valve clearance problems. My Olds engine guru out here says that piston to valve clearance comes into effect after about .560 lift with alot of duration.
[ Thanks to Jim Chermack for this information ]

Buildup Examples

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