Another 340 J & O head thread (what do i have?)

moper said
Those look more like aftermarket pistons to me. You will want to do some measurements rather than guess on the compression ratio. It may be higher or lower than you think depending on the machining done on the block previously. It's not worth it to guess, and there is no such thing as a "10.5:1" or "8.8:1" piston. There are just pistons of differring designs that get put in a block with a head gasket and head combination. Whatever that combo yields is what that piston "is". So you have to do a little measuring, do a little math, and then you know what YOU specifically have in that engine.

Very true. Most of these compression ratios were with thin head gaskets no longer commonly used and honestly most 100% stock heads I've had cc'd at the stock ratings. We actually did a set before/after and IIRC one set grew 2ccs after getting cleaned up by the machine shop. On the other hand, it's about as common to see them shaved down to 3-5ccs below stock. Most simple rebuilds will end up shy of the factory CR for that combination.

eduardostewart said
if i can tie the engine to the vin i'll be happy. i'm pretty sure my dad didn't replace anything internal like that back then.

Should be on the side of the block low down. IIRC you get two of the VIN that tell you what car it came from and then you get the serial of the VIN. The last two I saw were a Duster and a Dart Sport/Demon. The Duster is likely the most common by far.