GM part ID for 454 engine

[Michael C.]

I need help with a GM piston identification. It is a NIB 30 over for 454 I think. The part number is 12456437. Can anyone tell me what the piston is suitable for and its correct application: i.e. forged, hypereutectic, or regular cast and the engine it fits? Thank you.

[Clem Z.]

Re: GM part ID for 454 engine

i believe those are flat top pistons with 2 valve notches for a 454. that all i can tell you.

[Joe L.]

Re: GM part ID for 454 engine

Mike-----

GM #12456437 "started life" known as GM #3993877. This is a conventional cast, +.030 SERVICE piston used for 1972 Corvettes with LS-5 as well as many other lower compression 454 applications in passenger cars and light trucks of the period. It is also the SERVICE piston for 1973 Corvettes with LS-4. It, and its successor, GM #353061, were used for many passenger car and light truck applications with low compression engines in the 1973-78 period. However, it is not applicable to 1974 Corvette LS-4; a different piston was used for that application.

GM #12456437 "came into being" in March, 1998 to replace the above-referenced pistons for the above-referenced applications. As clem described, it is a flat top configuration with 2 "opposing and offset eyebrow" valve reliefs.

The original GM #3993877 and GM #353061 pistons were conventional, permanent mold, aluminum alloy cast pistons. It is possible that the 12456437 is a hypereutectic alloy cast aluminum piston. I doubt it, but it's possible. A "clue" might be this: look on the underside of the piston. If there is a "CFB" and/or a GM 8 digit casting number, then it's unlikely that it's a hypereutectic alloy piston. If there is an "F-M" with or without a GM 8 digit casting number, then it's very likely a hypereutectic alloy piston. Please report what you find.

[Michael C.]

Re: GM part ID for 454 engine

Joe, Clem,
Thanks for your prompt response. I am not in posession of the piston but am considering their purchase for inclusion in an LS-5 motor I am building. I would prefer hypereutectic but was enticed by the GM logo on these and the chance that they might be correct. I will take a look and report back my findings.

[Clem Z.]

i think F/M has this same piston under H625P *NM*

[Clem Z.]

the "H" stands for sterling hypereutectic *NM*

[Michael C.]

Re: the "H" stands for sterling hypereutectic

Clem,
If I am understanding you correctly you believe that F-M makes these for GM and also sells them under their number as you quoted above? And that they are hypereutectic.

Thanks
Mike

[Clem Z.]

that would be my guess but your CR

would be less tha 9:1 with 100.9 cc heads

[Michael C.]

Re: that would be my guess but your CR

That would be lower than I had wanted esp. with the med perf oval port heads. I had 9.0 as a goal. Maybe aftermarket hypereutectics are a better choice since i can control the comp ratio with piston selection. BTW what is your experience with big block valve guides? usually require sleeving, etc., or not prone to excessive wear? I haven't miked them yet.

[Clem Z.]

Re: that would be my guess but your CR

what are your head casting # i can check the chamber CCs and see what dome volume you will need. since BBC heads have the guides bored after the guides are installed in the heads NEVER change out a guide as the valve could be way off center of the head valve seat. you need to sleeve the orignal guides to stay on center.

[Joe L.]

Re: the "H" stands for sterling hypereutectic

Mike-----

Getting the information that I described from the underside of the pistons will go a long way to answering the remaining questions about them.

In any event, these pistons will produce a very low compression ratio under almost any circumstances. If you are using uncut, oval port, 100 cc chamber heads (about the smallest big block combustion chamber size commonly found), minimum deck clearance, standard bore, and stock-type shim gasket, then your compression ratio would be ABOUT 9.1:1. If using larger size combustion chamber heads, which are more common, and/or a thicker-than-stock head gasket (which is common), the compression will be LESS THAN 9.1:1. An overbore from standard will increase the ratio, but not by very much---about 0.1 points.

If using the much more common 113 or 118 cc heads, then the compression ratio will be ABOUT 8.3:1 for 113 cc heads and 8.0:1 for 118 cc heads.

[Michael C.]

Re: the "H" stands for sterling hypereutectic

Joe,
The head casting number is 336781: the open chambered oval port head at 113.0cc. I have not cc'd them but they don't appear to have been cut and the block has not as it still displays its date stamp. I have not miked the bore wear but it appears that it will clean up at .020 or .030 over at most. What do you think the 8.3 would rise to if the heads or block required a squaring cut taken?

My piston guy has not returned my calls regarding the markings on the bottom of the pistons. Perhaps tomorrow...

[Joe L.]

Re: the "H" stands for sterling hypereutectic

Mike----

Assuming that you use a composition type gasket at around .040" thick, the amount that's removed from the head and the block will just about "offset". In any event, I think that the most that you're going to end up with is about 8.5:1, even if you were to use a stock-type gasket with the cut heads and block. You could go with a heavier cut on the heads to increase compression somewhat. I don't like this approach, but it can be done.

On the other hand, 8.5:1, if you can get to that, is really not all that bad for a big block. Big blocks have more detonation problems than small blocks. GM builds most of its crate big blocks with cast iron heads at about an 8.5:1 compression ratio and still recommends that only 91+ octane gasoline be used.

[Michael C.]

Re: the "H" stands for sterling hypereutectic

The only marks on the piston were B3576. I did not personally inspect them but the report was no other marks on the sides or underside. Why was Clem Z. sure they were hypereutectic?

[Clem Z.]

i posted that the ones sold by F/M are

hypereutectic not the ones sold by GM.the GM ones could be if they were made by F/M