Thread: HELP I need to find out what compression my 402 has

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well everything I have found on the internet says if its a 1970 it should have 10.25:1 and if its a 1971 it will have 8:1

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Originally Posted by mistjef

well everything I have found on the internet says if its a 1970 it should have 10.25:1 and if its a 1971 it will have 8:1

well if the heads are 122 cc chamber there no way its a 10.25:1 . older heads are around 90 to 100 cc then it would be 10.25.1or hi 9s chevy never made a bigger cast piston for the 396/325 and the 402/350 engines they all had a small dome pistons. and made some lower cr cast pistons but the 325/350 hp had the bigger domes in low hp engines they move up the ccs on the head s the old engines use what we called bath tubs chambers very small cc s they stop using that chamber in 1967 or so

I guess I am trying to hard to make it have 10.25:1 compression I want to put a thumper cam 522 in it but the Comp website says u need 9:1 compression and the motor that I have only has 20,000 miles on it so I dont want to change pistons yes all I am after is the sound

I believe you’ll find your engine casting number is 3969854. If so, you have a 1971 Truck 402 – Advertised horsepower is 310. It originally came with a TH-400 transmission.

The heads are basic big block oval port, low performance, 366-427 truck, 122 cc combustion chamber with 225cc runners. Advertised C/R was 9:1