LT1 Block Casting Number
 

I'm trying to get a new block for my car but I have a question. Are the 2 bolt main and 4 bolt main blocks the same casting #? Looks like my block has a "327" as the casting number. Is that correct?

I called a place that had some cores but the guy told me all of his 327 castings had 2 bolt mains.

Yes - "There are two blocks, one with two-bolt mains and one with four-bolt mains. They both have the same 10125327 casting number, so there’s no sure way to know which one you have until you get the pan off. However, if it came out of a Corvette, it should be a four-bolt block, and if it came out of anything else, it was supposed to be a two-bolt. GM used the two-bolt block for everything but the Corvette because it had plenty of strength and it weighed a little bit less".

I have the 4 bolt block but when calling around noone else seems to have those - guess there were too many caprices, camaro's and firebirds and not enough corvettes :D

My guess is that in the manufacturing process, the exact same mold is used for the 2 bolt vs. 4 bolt blocks. The block itself is the same core part. The "327" is part of the mold. I think it is a machining process that makes the difference for the 2 bolt cap and 4 bolt cap. That would explain why you can't ID the "block" itself, because they are essentially the same cast part.

There are many Stories on this but the casting number is the truth...now how they came up with the number..... ;)

Get a 2 bolt main, and then have the other bolts added... splayed caps rock. :D