.670" bore x 2-1/2" OD x 5/8", 21/32", 11/16", 3/4" belts; cold rolled billet steel
A belt/pulley mismatch will cost you horsepower (mileage) and belt life; more than many would imagine.
2-1/2” diameter: small enough for adequate alternator speed on lower RPM engines, large enough for long belt life.
12-volt alternator conversions often require a wide-belt pulley. A v-belt alternator typically comes standard with a 3/8" pulley good up to 0.41" belt width; the 10SI does. 0.47" is the minimum belt width for the 1/2" pulley. 0.625" puts you into my 5/8" pulley. Off-road, industrial and ag engines might also use wide belts. Cogged v-belts are a good way to go on the small diameter alternator pulleys.
