I have a 425 BBO 1965 block. Just resealed the motor and put in a new cam and noticed the pistons are aftermarket. They have a stamp on the underside that say "901P OCT 93" I have tried to research online and cannot find out what pistons they are and was wondering if any of the long time Olds guys here might have a clue. Thanks!

 

Alright so removed my heads and no luck with part numbers but does anyone know if these look like low or high compression 425 pistons?

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I'm gonna say that they are 10.25:1 pistons just based on the pointed notch. The lower compression pistons had a more rounded notch I believe.

Scott

 

Oct 93 is obviously the casting or mold date, so these are aftermarket of some type. Since they are over 20 years old, it's likely that they are no longer in production, so an internet search may not turn up any info.

 

To my eyes, those dishes are way too deep to be "high compression".

 

Measure the depth of the dish. It looks to be about 1/4 inch in the pics. If so, they are low compression. High compression will measure about .060 deep.

 

Are you sure it's not converted to a 455 with a cast crank?

 

Confirmed it's a 425 yestersay. Measured the stroke. Not the greatest pic. But when your 100% eye level with it the deck is just a hair under 4 or right on it basically.

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From looking at some pictures of when I was measuring stroke from the center of the piston to the deck measures in at what looks to be 4 3/8" and from the flat edge is normal 425 stoke which is 3.975". Calculating the difference gives me a dish of about .40. I know it's not exact, basically eyeballing it for now. But sounds like it might be high comp

 

I forgot to mention compression check results before I opened the motor when a little below operating temp were at 165. And the head gasket that came off looked like a thicker felpro. Would it be safe to assume my cr ratio is prolly somewhere in the higher 9's?

 

If it were me, while the engine is open I would measure the head combustion chamber volume, the piston dish volume, and the piston-to-deck clearance. Those numbers would allow determining the exact compression ratio, with no guessing.

 

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