I changed my timing belt and belt tensioner a week ago and all went well,untill lastnight.When I heard a thumping sound and found my crank pully bolt was loose again.Itook everything back apart in the dark during a ice storm and now I think I have a major problem.The pin that locates the pully is sheared off flush with the outer plate of the timing belt sprocket.is there any cheap way to fix this?,do I have to replace the belt sprocket,if so how hard is it to get off ? how expensive will this be ? Can I line the pully and gear up on a drill press drill a hole through both pcs. And drive a pin in ?

That combo (protrusion and notch) serve to align the crank pulley and cogged gear. There really isn't any holding power there if the bolt is loose (as you experienced). IMHO there isn't any reason to worry about perfect alignment. I would simply install the crank pulley in the correct position as best I could and tighten the bolt to the correct torque. Just make sure the pulley mates up tight against the gear. You can replace the broken piece (gear) when the weather gets better.

How did you hold the crank from turning when you replaced the bolt after changing the belt?? Sounds like it wasn't torqued properly. There is a tool made for holding the crank/pully from turning so that it can be properly torqued. There is also the "rope trick" which you can find by doing a search (or it is probably part of the FAQ's under "Features" at the top of this page. Proceed carefully with the rope trick and pay close attention to the detail of starting from TDC or bent valves can result. Good luck.

Randy