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Posted By: mechanic58 Differences between GMC and Chevrolet L6's - Wed May 10 2017 04:10 PM
Fixing to acquire a '53 GMC 400 with a 270 in it. I already a few stovebolt chevrolets and I am wondering what parts - if any will interchange between these two engines. Looking at picture on google images it looks like there may at lease be some parts that do interchange. The cylinder head looks entirely different though - but do they interchange? At first I assumed the GMC engines (248, 270 & 302) were completely different, but now looking at the pictures it doesn't look like they're very much different than the chevrolets. Just curious.
Posted By: EdPruss Re: Differences between GMC and Chevrolet L6's - Wed May 10 2017 05:05 PM
Nothing much interchanges. GMC's had two(at least) heads, large and small intake ports, not all heads fit all engines because of piston domes being different, for example. Patrick has a good catalogue that explains this. GMC always had full pressure oiling. GMC engines about 3" longer, radiator within core support to accommodate this, rather than behind like Chevy.

Ed

Posted By: mechanic58 Re: Differences between GMC and Chevrolet L6's - Wed May 10 2017 05:48 PM
I did wonder about length. It's hard to tell that looking at pictures. For example, the timing covers look to be identical. It's misleading. I did notice the radiator in the Jimmy I am getting is forward of the location of the radiator in my 4100.
Posted By: mechanic58 Re: Differences between GMC and Chevrolet L6's - Wed May 10 2017 05:50 PM
I did notice that the cylinder heads look to have far superior combustion chamber design compared to the Chevrolets. That's why I wondered if the heads would interchange. Guess not.
Posted By: Hotrod Lincoln Re: Differences between GMC and Chevrolet L6's - Wed May 10 2017 06:10 PM
Some accessories like distributors, fuel pumps, and carburetors will bolt on, but no guarantee that they will work as well as the correct items. I believe starters and generators will also swap.

One trick dirt track racers used to use was to adapt a GMC oil pump to a splash-oiled Chevy engine to get better oil pressure. I'm not sure how much tinkering was involved in that swap, however.
Jerry
Posted By: panic Re: Differences between GMC and Chevrolet L6's - Wed May 10 2017 06:27 PM
Every dimension is wrong (block, deck, bolt pattern, head, length, cam, valves) except crank journal diameters, but the cranks can't be exchanged.
The oil pump looks like it can be swapped, but since the 1955 Gen-2 pump is even bigger no one has tried in 60 years. The distributors may swap?

The basic stovebolt head design was good enough for Mercedes to use until 1971 in the 280 SL ("pagoda" roof), and the post-WW2 Toyota Type “F” L6 OHV engines are based on this head.
I disagree, vertical OHV valves were made obsolete in 1949 by Cadillac and Oldsmobile (and eventually Ford SB, FE, FF, Chrysler LA, B, RB, Pontiac, Buick nailhead, Chevy SB, etc.) flat chamber floors are a terrible idea. The highly rated "H" head's chamber is almost completely open with no squish. The "D" chamber has quench but badly shrouded and placed valves.
Posted By: Hotrod Lincoln Re: Differences between GMC and Chevrolet L6's - Wed May 10 2017 10:21 PM
Oliver farm tractor inline sixes are almost clones of the stovebolt engine, also. On one tractor engine I rebuilt, I used 235 valves and guides to replace the antique Oliver parts that were no longer available. The only noticeable difference was the diameter of the valve stems, and the Chevy guides fixed that problem.
Jerry
Posted By: ol' 55 Re: Differences between GMC and Chevrolet L6's - Tue May 16 2017 04:45 AM
That sure is crazy talking early chevy 6 cylinders and you bring up an Oliver tractor which I so happen to have there Hotrod Lincoln. This one is a 4 cylinder called a Super 55 which is pretty dam strong. Yeah has a 6 speed with 2 reverse speeds, this was Pops ol tractor when he was farming almonds in the southern california area. Little town called Quartz Hill.
Posted By: Hotrod Lincoln Re: Differences between GMC and Chevrolet L6's - Tue May 16 2017 04:58 AM
I owned a diesel shop and 24-hour road service in the late 1970's on Highway 99 in Atwatwer, about 70 miles north of Fresno. There were lots of almond orchards up that way, too.
Jerry
Posted By: Norcal Dave Re: Differences between GMC and Chevrolet L6's - Tue May 16 2017 06:50 PM
Were 50-53 GMC's 12 volt or 6 volt?
Posted By: Johnny N Re: Differences between GMC and Chevrolet L6's - Tue May 16 2017 07:03 PM
Will clutch and pressure plates interchange between Chevy and gmc

228?
Posted By: showkey Re: Differences between GMC and Chevrolet L6's - Tue May 16 2017 07:33 PM
Originally Posted by Norcal Dave
Were 50-53 GMC's 12 volt or 6 volt?


My '50 GMC 100. 1/2T 228 was 6 volt positive ground
Posted By: Cletis Re: Differences between GMC and Chevrolet L6's - Tue May 16 2017 07:57 PM
GMCs were 6V pos. through 1955 first series. Six cylinder trucks continued with 6 V pos. for at least one more year after that while the V8s were 12V neg.

Clutch and pressure plate will interchange as long as they are the same diameter. Bigger trucks take bigger clutches.
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