today i was workin on my car and went to put the water pump pulley on and it wouldnt fit on...the round center peg is to fat to fit through the center of the pulley..whats the difference with the S4 and S5 pulley? why wont it fit

 

The nose of the S5 waterpump is larger than that of the S4.
The fan's thermoclutch won't interchange either (actually, the S5 will- should- work on either but not vice-versa).

 

so can i just grind it a little bit so it fits? do the bolt holes line up?..im running an efan so im not worried about the clutch fan working..

 

i did grinded mine and it worked fine.