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503 six cylinder engine was used in some 55.2-59 GMC 6xx and 8xx series trucks. These are BIG trucks. I don't know about the physical size of the engine, but I'd bet it wouldn't fit into the space offered by smaller trucks.
As hard as small GMC 6 cylinder engines are to find parts for, spares for this engine are probably made of Unobtainium.
Maybe Kenneth would have an opinion.
Last edited by Bill Hanlon; 09/28/2026 12:00 PM.
'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.
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