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1 - All lower ends are interchangeable for all years.
2 - On engines up to 1939, the forked rod was rotated to be
the front rod, to correct oiling problems. On 1939 and later engines,
the forked rod is the rear.
3 - 1937, 38, and 39 timing covers have no fins. They will
bolt onto all years.
4 - When U and UL 74" cylinders have been bored
to their maximum oversize, have them sleeved. Do not bore them to
the UH and ULH oversize. The walls will be too thin.
5 - There are two types of telescoping valve spring covers.
A screw-based and a spring-loaded. Do not mix covers and bases.
1937-38 is 3 piece similar to 45, fits 1930-38 big twin & all 45
threaded blocks. 1939-48 are spring-loaded, use different blocks.
6 - There are five types of cylinder heads. The
1937-48 U-Series all had 9 bolt holes. Both came in either
iron or aluminum. They came in low, medium, and
high compression ratios, in iron (there are minor differences
in the castings in 1937-39 having to do with the primer cup, but all
are interchangable if same ratio). They came in medium and
high compression ratios, in aluminum. None of them have numbers
stamped on the side.
7 - The 1936 to 1939 models use the VL small crankpin with .250" rollers for standard.
8 - To swap Big Twin Flathead and OHV flywheels, remember that the timing marks are not the same. The Flatheads time at 11/32" BTDC, and the OHV's time at 7/16" BTDC.