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Recently purchased a used GTO and trying to determine what engine is installed..
stamp code, front of block is: YC 346883
But below the ID is 4659 which has been stamped over with hypens and below it is stamp 4685,,as a corrected #..??
 

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Yes - always begin with the 4 character date code, cast into the top of the block near the distributor. You must start with this because Pontiac reused the same two character stamping (passenger side front) in different years to mean different things. The date code is one alpha character denoting month (A=January), followed by day, with a 1 digit year at the end. The engine foundry would "swap over" and begin casting blocks for the next model year in about mid year, so a date code of say F278 (June 27, 68) would probably be a model year 1969 engine. Next get the block casting number - located in different places in different years. The web sites that BigD posted above can help you know where to look. This tells you if it's a 389, 400, 421 etc... Last thing to look at is the two character stamping on the passenger side front near the top of the deck. (YC in your case). This tells you details of how it was built (which cam, carburetor, etc) and also which type of car it was installed in and what kind of transmission was behind it.

Bear
 
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