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1 - In 1936, the OHV twin Knucklehead was born with a 61' displacement. A stepped crankpin was used in these 61" Knuckle engines, from 1936 to 1947. It is, also, the same crankpin
as is used in the 1936 to 1947 Big Twin 74" and 80" Flatheads. The 74" Knuckle uses a tapered crankpin, which is much stronger.
2 - The 61" heads will fit the 74" cylinders. They have smaller combustions chambers, and smaller ports.
3 - If you use the 74" flywheels with the 61" cylinders, you will have to make a spacer plate for the bottom of the cylinders (approx. 1/4" thick). Panhead flywheels (1948 to 1954) will
fit, but the 61" wheels are 1/8" smaller in diameter than the 74" wheels. If you use 61" wheels in a 74" case, you will have to build up the oil scraper, tho prevent the rear cylinder from
getting too much oil.
4 - The Flatheads and Knuckleheads, of around 1940, used no oil control rings. They have a cast web in the crankcase openings that have approximately 1/4" on either side of the rods.
Pistons with or without oil control rings can be used on these cases.
5 - The Big Twin Flatheads, to 1939, and the OHV's, to 1940, had the forked rod as the front rod.
6 - Knucklehead and Panhead main bearings interchange for the years 1936 to 1953.
7 - 1948 to 1953 Panhead timing covers will fit a Knucklehead, if you unplug the upper oil gallery. 1954 to 1969 covers will work if you change the pinion shaft bushing to the pre-54
bushing, and unplug the upper oil gallery. If you don't want to unplug the oil gallery, tee into the oil sending unit, to feed the heads.