402 piston question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

beautimus · 01-15-2005, 02:31 PM

hey,
i just bought a 402 for my 71 truck from board member cadiac.
(excellent transaction by the way) and i have gotten it torn apart
and the piston tops have a dome to them. i am not a big block guy.
so i don't know, shouldn't they be flat? the motor was bored .030
so i know that they aren't the factory pistons. the tops of them are
raised up about 1/4" . if they were small block pistons i would assume that they were about 11 to 1 compression but with a big block, i just don't know.
any body know ?
Thanks, Tony

6772king · 01-15-2005, 09:25 PM

well flat tops are about as low as you can go in a big block 250 domes would go from 9-11 to 1 depending on the heads post the casting # of them if you can ,,,do you know what the motor was originally out of???

da-burb · 01-15-2005, 11:15 PM

I pulled apart a 1970 402 and it had domed pistons. It wasn't rebuilt from what I could tell so the pistons are factory. I think they had .030 on them too. My guess is that yours are stock originals and the .030 has to do with the fact that a 396 bored 30 is a 402 engine.

orangeuavol · 01-16-2005, 02:13 AM

My 402 still had th factory pistons but were not marked as 30 over (.030). We bored it 30 over to a 408 to clean up the walls and the new speed pro pistons were marke .030. The block was a car block and not a truck block if that makes a difference.

beautimus · 01-16-2005, 06:22 PM

found out they are indeed the factory pistons, the dome is due to the combustio chamber shape, actual compression is not that high even with the domes. i have a buddy who showed me the stock pistons from his motor. it had never been apart, prior to the tear down, and the pistons look exactly like mine. so i learned something . thanks to all that replied.

tony

01-15-2005, 02:31 PM	#1
beautimus Registered User Join Date: Mar 2002 Location: woodstock, alabama, usa Posts: 938	402 piston question hey, i just bought a 402 for my 71 truck from board member cadiac. (excellent transaction by the way) and i have gotten it torn apart and the piston tops have a dome to them. i am not a big block guy. so i don't know, shouldn't they be flat? the motor was bored .030 so i know that they aren't the factory pistons. the tops of them are raised up about 1/4" . if they were small block pistons i would assume that they were about 11 to 1 compression but with a big block, i just don't know. any body know ? Thanks, Tony __________________ i survived cancer and kidney stones, but my kids are killing me.

01-15-2005, 09:25 PM	#2
6772king Registered User Join Date: Feb 2002 Location: grove city pa usa Posts: 628	well flat tops are about as low as you can go in a big block 250 domes would go from 9-11 to 1 depending on the heads post the casting # of them if you can ,,,do you know what the motor was originally out of??? __________________ 67 chevy short step 2wd 427 71 chevy short fleet 4x4 396 72 gmc jimmy 2wd 377 71 chevy blazer 4x4 454 69 chevy longhorn 5.9l cummins 72 chevy 400 2wd custom camper 67 chevy lwb 4x4 454-in pieces 67 chevy panel soon 4x4-400 67 gmc dump 4x4 1 ton-305 WTF!! 72 chevy 3/4 ton 4x4 burb 350 70 chevy panel 2wd 302 5 speed tko "THERE IS NO REPLACMENT FOR DISPLACMENT"!!!!!!!

01-15-2005, 11:15 PM	#3
da-burb "Ochre Ogre" Join Date: Apr 2000 Location: Springfield, MN Posts: 3,558	I pulled apart a 1970 402 and it had domed pistons. It wasn't rebuilt from what I could tell so the pistons are factory. I think they had .030 on them too. My guess is that yours are stock originals and the .030 has to do with the fact that a 396 bored 30 is a 402 engine. __________________ Bowtie Truck Stop Inc. Mid-West GM Truck Restoration Parts Supplier Your Key Parts, Auto Metal Direct, Dynacorn, and Goodmark dealer. like us @ www.facebook.com/BowtieTruckStop 1971 C-10 Suburban (Ochre) 1971 K-10 Suburban (Ochre) 1972 C-10 Suburban (Ochre) 1972 K-20 Suburban (Yellow- that just aint right!) Springfield, Minnesota 56087

01-16-2005, 02:13 AM	#4
orangeuavol GO VOLS! Join Date: May 2001 Location: Knoxville, TN Posts: 724	My 402 still had th factory pistons but were not marked as 30 over (.030). We bored it 30 over to a 408 to clean up the walls and the new speed pro pistons were marke .030. The block was a car block and not a truck block if that makes a difference. __________________ http://1972blazer.shutterfly.com/<= blazer build pics It's called golf because all the other four letter words were taken. It's easier to ask for forgiveness than for permission.

01-16-2005, 06:22 PM	#5
beautimus Registered User Join Date: Mar 2002 Location: woodstock, alabama, usa Posts: 938	found out they are indeed the factory pistons, the dome is due to the combustio chamber shape, actual compression is not that high even with the domes. i have a buddy who showed me the stock pistons from his motor. it had never been apart, prior to the tear down, and the pistons look exactly like mine. so i learned something . thanks to all that replied. tony __________________ i survived cancer and kidney stones, but my kids are killing me.



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