In 1947 the Society of Automotive Engineers drafted a standard to define the shape of flanged propeller shaft ends. Its name is “Aerospace Standard AS-127”. The standard covers sizes 1 through 6. The most commonly used sizes are 1 through 3.
The bushings contain the female threads used to secure the prop bolts and they are a press fit into the flange. The bolt circle defined by the bushings is concentric to the pilot dia within .005. the pilot dia is concentric to the crankshaft centerline within .002.