Agree, if missing this will cause this. When I first got my engine together my belt would come off n the burnout box at the track, then be warm at the other end, put it back on but it did this a few timeS. Found out the spacer in the alternator where the bolt goes was too wide and stuck out a little, ground it down some and it never happened again. But I agree with others, get the correct pulley on there too.