You might try looking HERE, but I'll bet yours is not a GMC engine. The number on the pad to the rear of the distributer on a real GMC six cylinder MAY have a one letter prefix describing slight variance in bore size, main bearing size and rod bearing size when manufactured, then 3 digits describing the size of the engine (yours should be 228) followed by a 5 or 6 digit engine serial number.
The letter prefix decoder is shown below. Although both of the GMC maintenance manuals I have describe the letter code as a SUFFIX, I have only seen them as a PREFIX in the real world. Maybe the guy who wrote the book didn't know the difference.
Last edited by Bill Hanlon; 09/03/20227:54 PM. Reason: new info
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
