ZZ4 or Circle Track? Casting Numbers Don’t Make Sense.


New Member
Feb 25, 2018
1972 Chevy Nova
Several years ago I bought what the seller claimed was a zz4 8 year old but "never been run" crate engine. It has all of the carictoristics of a zz4. Aluminum angle plug heads, plastic timing chain cover, hencho de Mexico on the block, ect. But I have been told that “ZZ4” should be stamped in the serial number on the milled pad near the water pump. Mine does not. I have been told that it may be a circle track engine that uses the same block as a zz4? I have read a ton of articles on the number decoding and yet mine just doesn’t seem to make sense. Correct me if I am wrong but my assumption from the serial number is (1) GM, (2) Mexico, (12) December, (21) 21st. However the 2P04 has me stumped. Here is what I can gather for numbers off the engine:
Block casting number-10243880
Head casting number-10088113
Serial number-1M1221-2P04
Im looking to get the date of manufacture, motor type (zz4/circle track), HP, etc. Any help would be greatly appreciated. Thanks!

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