Re: 21 stud flathead
The earliest ones had nuts in the valley for the mains, no crankcase vent tube, then studs for mains, no vent tube, 35 started the vent tube, these were all Babbitt mains, then more or less in 36 some started to have insert mains, 37 the water pumps were moved to the block, some of the original parts books have pictures to show the difference
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