I did a search here for a block casting 12621766 with no results. Anyone can verify what block this is, a search online shows Its the current updated 6.2 block (LS9).

 

I am going to find out what that block is because i am trying to figure out what block i want to use for my build. I am at work today but its sunday and not everyone who is an expert on the block line is in today.I happen to work at GM powertrain where they build those engines and am covering the line where they machine them right now.

 

From a search I came up with a block casting 12621766 is a LS3/LS9 (6.2L/376ci) 2009-Present. The LS3 uses the same block as the Corvette ZR1's LS9, except for the "SC" engine's larger head bolt threads, piston oilers, forged steel main caps and its deck plate honing process.
http://www.southernperformancesystem...k-casting.html
http://www.camarohomepage.com/ls3/index.htm

 

Some reading I did years ago indicated that those are the order batch counts, i.e. that block was in the 2nd order of '870 block castings from the foundry that supplied GM. I've seen many different counts over the years, Big & Small blocks, GM10, GM6, GM4, etc.

 

I know it is an old thread.

But I was searching and this gave me the wrong information, and I hate getting bad data... So I thought I would correct it.
12621766 is an LS3 engine block.

The correct GM part number for the LS9 engine is:
12621983 for 2009-2011.

Have a nice day!

 

I'm afraid you are giving out more wrong information.

Different part numbers share the same casting. "12621766" is the casting number. That number is literally cast into the back of the block.

After 12621766 castings leave the foundry, they are final machined into different blocks with different part numbers.

Here's an LS9:


LS3:


L99:


LSA:


According to my copy of the 2015 GM Performance Parts catalog, the LS9 block is p/n 12621983 as you mentioned. The LS3 and L99 block is p/n 12623967, and the LSA block is p/n 12623968.

All of them have 12621766 cast on the back, but none of them have that as a part number.

 

Thank you torquetube for the correction!! I sure hate to spread misinformation and you are correct. I was following along with the OP's 'suggestion' and incorrectly intermingling the terms 'casting number' and 'part number'. I normally try to be clear and specific. I could try to assert some lame excuse here, but I really should just pay attention before I post.

Anyhow, I appreciate the correction and for setting me upright!

And confirming your provided Catalog data My 2009 ZR1's LS9 carries the cast-in casting number 12621766 (located just below the deck and at the rear of the LHS). The same LS9 block from 2009 also carries the application-specific, sort-of stamped part number GM12621983 (which only applies after machining and is located on the front RHS, under the AC compressor bracket location). Exactly as you stated. And also, the LS9 block and any other aluminum 6.2L LS block are not interchangeable or compatible due primarily to the LS9-unique machining for oil squirters and larger head bolt size

It then follows that in order to categorically determine if a block is suitable for a specific LS model engine, the part number must be identified, because this casting number block is specifically machined to produce more than one unique (and non-interchangeable) part number block. On the other hand, if you find this casting number on a block in a junkyard... it is super unlikely to be an LS9 block because so few were produced and folks tend not to scrap them.