need some help replacing the rear sprocket on a 2005 450yfz, can anyone help please

 

Not sure if its the same as most quads. If so you need some big wrenches to brake the axle nut lose. Have someone sit on the quad and hold all the brakes and get the axle nut lose. (use some PB blaster or a little heat from a propane torch if needed). One that is done you can loosen the front sprocket and pop it off then remove the hub/wheel and slide the sprocket off that side.

 

Aren't the big lock nuts on the disc brake side? most quads I have seen the sprocket is bolted on to the flange on the axle with 5 or 6 bolts. It's a simple matter of loosening the chain, taking it off the sprocket, then either sliding it over the hub, or, if the center hole is not big enough, remove the hub, install the new sprocket and put the chain back on, tension it, install hub and wheel (if hub had to be removed) and you are good to go.