After sitting in my garage for two years I finally have had time to look at my Dad's "bike" (please forgive the bush Alaska terminology). I got a compression tester and verified that it had no compression and then pulled the snifter valve core and did a leak-down test with the air compressor running 80psi right into the combustion chamber while at TDC. The air flooding out the exhaust gave me a basic area to look at. The cam shaft was not turning so I pulled off the, recoil starter, flywheel, starter coil, and ultimately the broken crankshaft sprocket.

I ordered a new sprocket and woodruff key and installed them after pulling off the chain Tensioner and the camshaft sprocket. I welded my own socket out of 1 1/2 steel pipe to torque the nut on to spec. After timing/reassembly I noted that there still was no compression so I took off the cylinder head and found a bent valve. There was obvious indications of the exhaust valve striking the Piston. So i have a new exhaust valve, valve guide and exhaust valve seal on there way from cyclepartswarehouse.

Is there anything else that I should look at before I reassemble the engine and set the valves at .006? I don't know if the failure by the sprocket on the crankshaft precipitated the Pistion strike and valve bending. Or if there is a trifecta - an unknown third failure?
Is it common that a sprocket would just fail?

Thanks for all your help
I have perused the forums here for tons of info and learned a great deal about my Polaris project.

2000 Polaris Magnum 325

 

Main reason for lower crank sprocket problems was that the slotted nut could back off,even though it's left hand threads and you wouldn't think it could, then the loose sprocket could split the woodruff key,jump timing and could possibly do top end damage. At times even piston damage.If your sprocket just broke,the slotted nut was tight,then it was a fluke deal.Could have just been a bad heat treatment on it. When you get the new valves back in,loctite the threads for the slotted nut when you reinstall it!! This can keep it from ever backing off as Polaris didn't apply any from the factory when new. Your Magnum also falls into the year gap on 425/500 engines with soft heat treatment on some exhaust cam lobes. May or may not be a problem on yours,but many were affected from the 95 Magnum up.I'd also inspect the cam. Plus the biggest culprit on this was the 500 engine from 1996 to the 2000 model year simply because more were sold. 2001 Sportsman HO models had the different cam,carb.Here's what the cam looks like when ground down compared to the new one.Bad Cam Pic - ATVConnection.com ATV Enthusiast Community

 

When I removed the slotted nut it was fully tightened. The woodruff key was rolled over pretty hard but the fracture on the sprocket appeared to be a flaw in the metal.
Seeing one of your posts earlier I added loctite to the nut on the crankshaft.
Do you have the specs for the 2000 magnum 325's cam shaft so that I can measure and make sure that it is within recommended tolerances? It didn't look bad to me but without a new one side by side it's hard to say.

Thanks

 

Misread your post,getting blind I guess. Thought it was the 425 magnum. No cam problems on the 325 engine that I'm aware of. At least I never had any. But yes I did have the slotted nut back off on them along with the 500 engine. Good that you used loctite on the threads.

 

Got my new valves in and installed and put the engine back together. The motor cranked but would not turn over. Just to check that everything was working I sprayed in some started fluid and it started then died moments later. Figuring that I had a air lock in the fuel lines I blew in some air into the gas tank with the air compressor- next attempt she started right up. It did have some issues with the carb. and dying unexpectedly but I am not surprised with it sitting around for two years before I opened it up. It runs fine now so I think that the Seafoam in the gas is doing its job.

Thanks for all the help.

 

Check that the tank vent line isn't kinked up also,can cause vapor locking. Sitting up two years,I'd still install a new carb kit,but that's just me. Seafoam,Stabil,etc are better at protecting fuel and keeping carbs from gumming up rather than cleaning one up after sitting. Glad you have it running though..