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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
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.
I might add depending on the year These cars used a SGI block and its stamped on the back left of the block (see image) Lots of info here. Try to save your block. A "GM 5.7 SGI" engine refers to a Chevrolet 5.7-liter (350 cubic inch) small-block engine, likely produced from the late 1980s to the mid-1990s, characterized by its "SGI" designation indicating a Spherical Grafted Iron block with a one-piece rear main seal. The "LG" part of some codes, such as 5.7LG SGI, identifies it as part of the "Large Gas" engine family. These engines were known for their robustness and were used in various vehicles and industrial applications. )A "GM 5.7 SGI" engine refers to a Chevrolet 5.7-liter (350 cubic inch) small-block engine, likely produced from the late 1980s to the mid-1990s, characterized by its "SGI" designation indicating a Spherical Grafted Iron block with a one-piece rear main seal. The "LG" part of some codes, such as 5.7LG SGI, identifies it as part of the "Large Gas" engine family. These engines were known for their robustness and were used in various vehicles and industrial applications
Last edited by Mikeylikesit70; Sep 29, 2025 at 08:00 PM.
Reason: more info
LT1 Block Casting Number
