GMC - The Numbers Game

by Ron Dettling

        For those of you who rummage around wrecking yards, swap meets or other likely places to find basic GMC engine parts, blocks, cranks, heads, etc., it is highly frustrating to find a treasure that you know is GMC, but which you can't easily identify.  Unfortunately, even knowing the GM part number is not helpful, since the major pieces are not marked that way.

        The following list of casting and forging numbers may help in determining your degree of interest in a particular part.  The information came from several old books, and show several crankshaft forging numbers for most later engines.  I don't know what the differences are, but it appears that they are interchangeable.  If any Inliner has any further information I'm sure it would be of interest to others.

*** Important note on GMC head and block interchange****

There were several combinations of pistons - flat-top and domed - to match a variety of head configurations. All GMC heads have the same bolt pattern and port spacing, so any head will fit any block - BUT unless you have the correct pistons to match the head chamber, you will have problems such as inadequate compression ratio or worse, valves hitting top of pistons. Be sure you use the correct combination! 

        In addition to the above, the following connecting rod forging numbers were found:

       Hope this helps!

    * unable to confirm, if anybody can help please send.