Round Back Alternator Rebuilder

Dana67Dart said
Difference... 2 groove was 3", 1 groove was 2.7", parts book calls for 2.5". It would probably charge with the headlights and brakes on at idle with a 2.5".

Pretty sure 2.7" OD might be 2.5" effective diameter.
I found the same OD on the factory type pulleys I have.

The V belt lengths are sometimes sized by effective length, and other times outside length.

Here's one got from OMM with the stamped number still visible.

Just had to check. The stamped number is same as the part number for a 2 1/2" pulley