Are there differences in the size between the v6 and v8 alternators on our cars? I'm trying to use the alternator off my old 2.8 motor as I just had it rebuilt about a year ago and I know it's in good working condition but it seems like the pulley is too short and won't live up correctly with my water pump pulley and crank pulley

 

basicly 3 different physical sizes of alternator came on thirdgens,'85-86 v6 was one of the engines that came with the largest size-17si type,85-86 TPI also came with the 17si.I think v6 belt is different from v8,but pulley position is the same.What brackets are you using on the v8 engine?

 

v belt brackets off an 87 firebird 305 tpi

 

with correct pulley you should be able to use the '86 v6 alternator with the '87 v8 setup-87 used the smaller cs130 alternator,so you will need to modify the upper alternator bracket some or use the upper bracket meant for 85-86 TPI(will also need modified a little)or the upper bracket from 82-83 with 70 or 85 amp 15si(CFI will need modified,carb version likely bolt on)probably will need the pivot bolt spacer from 15 or 17si v8 too,but that part can be replicated or substitute used if needed.