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

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I have a 402 I have been able to decode some of it block # 3969845 P date code 04-70 front block code T0908TKB I have found it to be built in the Tonawanda plant Sept 8th and was used in a 1971 truck Head code is 3933148 1969 open chamber 122.1cc oval port my question is everything I have found says in 1970 the compression was 10.25:1 and in 1971 it dropped to 8:1 compression so do I have a 10.25:1 being the engine was built Sept 70 or is it a 8:1 compression being it was put in a 1971 truck how do i find out what I have thanks for the help

well if your piston are stock? them its very low 8:1 or so with a open chamber head.if that head is a true 122cc i do not keep up on all the head numbers i know them by the head chamber and the better casting numbers .every thing i build i cc the heads so i know what they are and can move the ccs abit if needed

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

Time to get out the graduated burrets and plexi glass.... Welcome to the site neighbor...

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

well put this in it 224/224 500/500 on 112 all it call it a day this will make good power as some 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

just remember that cr could be base off a steel shim head gaskets small bore??? and if re built not to long ago and they used the gaskets could be a 454 502 head gaskets . most all head gaskets went to the bigger fire ring gaskets . so then they are 4.250 to4.500 bore gaskets so it could be 10.5cc head gasket @.038 or some are 050+ as much as 13+cc. but them the block could of been O deck and heads cut so ??? or not the same pistons ? if the cast 180 dome then 10.3 with 96.5 cc and 8.5 with 119 heads

thanks for the info but that confused me more then the information I have gotten off the Internet

every thing is volume CC you need to know your cc s if you heads are open and they re built it with a new gaskets ??then it not the way it was from GM . thats why when i build them i cc heads and head gaskets .you want to guess ? who knows what you have BUT if you have a 122 cc head with a stock cast pistons there no way in hell you have 10to1