On a Pontiac 1985 Firebird Trans AM (1 serpentine and 3 v-belts LB9 305 vin F), the alternators need to be slightly modded to be able to bolt up. I removed the outer four small bolt on the new GM alternator casting halves, removed the back cast, reinstalled the internal brushes/springs securely with a paper clip in the rear casting slot, and reattached the casting a 1/4 turn to match the bolt configuration of the previous alternator. Also to add, have to reuse the old serp. pulley (5 dollar pulley from tractor equip. store) removed with an impact.

Wanted to add, most alternators are rebuildable. I rebuilt my previous alternator once, 10 dollars on small part, total overall alternator life was 2 1/2 years in between failures. Originally in 2005 i was persuaded away from the 1985 108amp alternator because didnt bolt up like the original one, very frustrated I settled for a 1983-84 alternator (83amps) because it bolted right up. But instead of rebuilding the 83amp alt. after a recent* failure, instead i bought a 108 amp alt. and split the two halves and rotated 1/4 turn. Lifetime warranty on the alt. should be good.