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firebird7980 02-03-2013 05:54 AM

Gm piston number what does it mean
I have a gm piston. just try to figure out what the numbers on the bottom of the piston means 3991476 60 thanks.

vinniekq2 02-03-2013 09:13 AM

contact fbird88 I think he has used these pistons

BigEd36 02-03-2013 12:26 PM

From searching online I can't find much about that casting number, but Silvolite shows a piston replacing that part number, it's either 11 or 13 cc dish piston, Silvolite's version is 1.54 compression height instead of the factory 1.56 so they would be considered "rebuilder" pistons that will give you .020" more "in the hole" or piston deck height than standard pistons. The 60 at the end of the number probably means they're .060" overbore sized.

cobalt327 02-03-2013 01:33 PM

I'd tend to agree w/BigEd36- Silvolite Pistons, bottom of the page. Two possibilities are shown, one has a 0.070" deep dish, the other a 0.080" dish. Both are available 0.060" over. The numbers shows up under "O.E. Casting No."

If these are original pistons, they should have a CH of 1.56". But if they're aftermarket replacements, they're prollably going to be the shorter rebuilder type.

firebird7980 02-03-2013 07:11 PM

The 60 is under the the rest of the number . What do you mean ch of 1.56

BigEd36 02-03-2013 08:44 PM


Originally Posted by firebird7980 (Post 1642304)
The 60 is under the the rest of the number . What do you mean ch of 1.56

CH - Compression Height - is the distance from the center of the piston pin to the top of the piston. 1.56" is the compression height of a standard 350 piston. Many aftermarket pistons are what's called "rebuilder" pistons, they are only 1.54" compression height, or .020" shorter. The reasoning is that it's assumed when the engine is rebuilt the block will be decked, so making the piston shorter will keep the piston deck height (the distance the top of the piston is below the top deck of the block) will be the same. If your engine is rebuilt using "rebuilder" pistons and the block is not decked, now your pistons will be .020" further "in the hole" (down in the cylinder). That will lower your compression ratio, and add .020" to the squish/quench height (distance from the top of the piston to the cylinder head) which makes an engine more likely to have detonation problems or require higher octane gasoline to prevent the detonation.

firebird7980 02-04-2013 04:25 AM

Thank you:)

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