Those would be the correct modern belt sizes, according to 10 ER guru Skip Campbell, for a 1/2 hp motor.
What I’m confused about is why Skip specifically called out belt sizes for a 1/2 hp motor. As I understand it, the distance from the mounting feet to the shaft centerline should be the same for
any Frame 56 motor. Not to mention shaft diameter, mounting-hole pattern, etc. In other words, all Frame 56 motors should be mechanically interchangeable in all critical mounting dimensions. This does exclude body length and diameter, which are not, strictly speaking, mounting dimensions (although they could affect clearances to the bench boards, etc.).
Perhaps the 3/4 hp motors available at the time were not Frame 56?
