396/427 block

[Tom H.]

I recently purchased a bbc engine, casting number 3855961. Reading Chevy v8 Engine Casting Numbers" It says this is a 396 or 427 block. The crank is a 3904815, which the book says can be either a 396 or 427. This engine has .030 over pistons in it, with a bore size of approx. 4.14, which I believe makes it a 402.
Did Chevrolet make a block that could be either size, depending on how it was bored? Are all 961 blocks identical, regardless of finish size? If so, can this block be bored to 427 specs?
It would seem odd to me that there are two blocks with the same casting numbers, but different interior specs.

[Joe L.]

Re: 396/427 block

I recently purchased a bbc engine, casting number 3855961. Reading Chevy v8 Engine Casting Numbers" It says this is a 396 or 427 block. The crank is a 3904815, which the book says can be either a 396 or 427. This engine has .030 over pistons in it, with a bore size of approx. 4.14, which I believe makes it a 402.
Did Chevrolet make a block that could be either size, depending on how it was bored? Are all 961 blocks identical, regardless of finish size? If so, can this block be bored to 427 specs?
It would seem odd to me that there are two blocks with the same casting numbers, but different interior specs.

Tom-----


Block castings are usually specific as to basic bore size but may be used for engines with different stroke lengths (since stroke length is, of course, a function of the crankshaft). As far as I know, the GM #3855961 casting was used for 396 cid engines only. Vaguely, it seems to me that I once heard someone else say that some were used for 427's. I doubted it then and I still doubt it now.

I would not attempt to bore a 3855961 block to 4.25" bore size. Even if sonic testing determined that it could be done for a particular block, the resultant cylinder wall thickness would be too thin as far as I'm concerned.

The GM #3904815 crank is a nodular iron CAST IRON crank used for 396 cid engines. It could be used for 427 engines with re-balancing but I don't think it was ever used for a PRODUCTION 427 application.

[Tom H.]

Re: 396/427 block

Thanks Joe. It did seem odd to me that two blocks with the same casting number would have different bore sizes. I'm hoping that this one will clean up with a hone, and I can use the pistons that came in it to stay a 402. The cast crank doesn't bother me, as it will not get extreme use. If I have to go .040 to clean it up, can you recommend a piston for me that will yield around 9.5 to one. I'm using a set of 3917215 heads which I believe are 101 cc. The engine must have been decked at one time, as threr are no stampings on the pad.
Thanks again for the information.

Tom

[Joe L.]

Re: 396/427 block

Thanks Joe. It did seem odd to me that two blocks with the same casting number would have different bore sizes. I'm hoping that this one will clean up with a hone, and I can use the pistons that came in it to stay a 402. The cast crank doesn't bother me, as it will not get extreme use. If I have to go .040 to clean it up, can you recommend a piston for me that will yield around 9.5 to one. I'm using a set of 3917215 heads which I believe are 101 cc. The engine must have been decked at one time, as threr are no stampings on the pad.
Thanks again for the information.

Tom

Tom-----


I think a Keith Black #160 piston would work well for you. It's available in .040" oversize. It will produce about a 10:1 compression ratio. You could lower that a bit by using extra thick head gaskets.