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! 

YEAR

ENGINE

BLOCK CASTING NUMBER

HEAD CASTING NUMBER

CRANK FORGING NUMBER

CAMSHAFT FORGING NUMBER

39-52

228

6107326

2136477
6107412
2194819

6107342
2191127
2194570

2191529

53

248

6107328

2194819

2191127
6107342

2191529

54

248

6107328

2194819

2194570

2191529

55

248

2324002

2194819

2194570

2191529

46-52

270

2135412

2136477
2193417

2135416

2136000

53

270

2135412

2194819

2136119

2191529

54

270

2135412

2194819

2194012
2342204
2362229
2192941

2191529
2192486

55-63

270
(side motor mounts)

2324003

2194819

2194012
2342204
2362229
2192941

2326722

52-54

302

2193980

2193983

2194012
2342204
2362229
2192941

2192486

55-62

302
(side motor mounts)

2324004

2193983

2194012
2342204
2362229
2192941

2326722

Military

302

2194202

2193983

61700332*

 

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

2135419

Late 270, early 302 - narrow bearings, .990" pin

5150059
6107428

228, 248, early 270 - wade bearings, .990" pin

2341204

Late 302 - narrow bearings, 1.050" pin

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