Re: 21 Stud 37mod
Ron, we are agreeing except for the year the crankshaft was introduced. The 91A part number is explicitly a 1939 part number. The 2.4 main crank carried through 1938 including the 24 stud motors. I have a 1938 24 stud motor with the earlier shorter crank, it is an oddball engine that is basically a 21 stud engine with three additional fasteners per head.
Also, from the Green Book (please pardon the formatting, I had to use ___ to show the different columns):
68 6303-A___Crankshaft assy-2.40" dia. main bearing-24.47" overall length-used with 78-6335 & 78-6336___Pass, Comm & Truck 90 HP (8 cyl)___36-38
91 6303-A___Crankshaft assy-2.50" diam. main bearing-26" overall length-slinger type rear main bearing seal-used with 78-6335 & 78-6336-replaced by 91A6303-B, 91A-6335, 19B-6336 & 19A-6347___Pass, Comm & Truck 90 HP (8 cyl)___39-42
91 6303-B___Crankshaft assy-2.50" diam. main bearing-26" overall length-bus type rear main bearing oil seal having oil grovves-used with 91A-6335, 19B-6336 & 91A-6347___Pass, Comm & Truck 90 HP (8 cyl)___39-42
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