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1969 Camaro
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Discussion Starter #1
I am looking for info on stock 402 BBC pistons. I have a 402, and the pistons
are not dish. they have a short dome that is flat on top. The part number
is GM 3993850. I can't find a place to check the part number. I want to
know what compresion ratio it will have with 781 or 292 heads. Any help
would be great.
 

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1971 was the first year for open chamber heads as a compression killer (they had been offered over the counter since 1968 as a high performance part).
1972 was the last year for the 402 cid BBC.

336781 was the last 454 head made introduced in 1974 and went out in early '75, an open chamber (113.06 cc) pass head had oval ports, 2.06/1.72 valves, application was Chevelle, Corvette, and Impala/Caprice/Estate Wagon.

My parts books only go up to 1975 which was the last year for the BBC in a car. There isn't a 292 head listed there. If you have a BBC 292 head then it is a 119.2cc open chamber peanut port truck heads with 2.06/1.77 valves.

3964291 was the last closed chamber BBC head made (date code 1969), and comes in both peanut plug (5/8th hex) or "N" style plugs (13/16th with washer) It had a109.037cc combustion chamber with 2.19/1.88 valves.

3904392 was a high performance (rectangular port) aluminum BBC head made in 1967 for the 435 hose Corvette. Closed chamber (106.8 cc) with 2.19/1.72 valves.

3904992 was another high performance (rectangular port) aluminum BBC head made in 1969-'70 for the L-88 427 Corvette. It differs in that it is an open chamber head. You wouldn't confuse either of these with a steel pass head.

The dome on your 402 pistons will either be for the bath tub shaped closed chamber heads or the dome goes to the edge of the piston (quench area above the ring lands anyway) for an open chamber head. Both were designed for an 11.0:1 compression ratio with their respective heads. If you run a closed chamber piston with an open chamber head you will loose 6cc for about 0.4:1 in compression

Larger Dave
 

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Discussion Starter #3
The 6272292 heads are 1971 402 or 454 heads, and the 336781 heads are 1973-1985 454 heads. Neither are penut port, both are oval port, but
I dont know what cc. Both sets of heads look alike, so they would probably
be the same cc.
 

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PDW HOTRODS said:
I am looking for info on stock 402 BBC pistons. I have a 402, and the pistons
are not dish. they have a short dome that is flat on top. The part number
is GM 3993850. I can't find a place to check the part number. I want to
know what compresion ratio it will have with 781 or 292 heads. Any help
would be great.
There were 4 of those pistons in the 402 and 4 #3993849's. These were rights and lefts. If you use the 292 heads you should come in at about 8.5, I think those heads are about 116 cc's. I don't have a chamber volume for the 781's handy so I can't calculate the C.R. Thanks, Gary in N.Y.
P.S. I believe the 781's were used in the early to mid 70's, which would most likely make them "smog" heads also, and probably the same as the 292's, a large "open-chamber".
 

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1969 Camaro
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Discussion Starter #5
If I want higher compresion than 8.5-1 should I look for different
heads, or pistons. I have built small blocks in the past, and
this will be my first bib block. I would like it at least to be like
a 350 hp 396-402. A litle more would be GREAT.
 
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