hello...what engine? does it have a harmonic balancer bolt? if it does, tighten bolt to install balancer,it will"slide" on as bolt is tightened-you may need a longer bolt to start...if it does not have a bolt, as mentioned use a hammer....mmmm however i would be careful not to hit too hard-slow even taps...sometimes problems can be caused from a hard blow, sometimes no prblem....
