CT70 Camshaft Sprocket Failure

Well, during the summer I had a catastrophic failure. I can't imagine the engine had more than 5 hours on it since I rebuilt it when the camshaft sprocket broke to pieces. As far as I can tell, I got lucky and it failed with the valves closed though. Now I'm trying to figure out what caused the failure so I can make sure it doesn't happen again once I rebuild the engine again. I have three theories.

First theory is the 50 year old sprocket just finally gave out. I replaced the cam chain and rollers during my first rebuild, but reused the sprocket since it didn't appear too worn and didn't look cracked or damaged in any way.

Theory two is the sprocket bolts worked loose allowing the sprocket to wobble, catch on something, and bust. You can see where the sprocket rammed into the cylinder head behind where it's mounted in one of my pictures. I don't think this was the case though, I think the sprocket broke and then gouged a mark into the cylinder head.

Theory three is my cam chain wasn't being tensioned properly, allowing the loose chain to bind up somewhere and break the sprocket. In my pictures you can see the chain was rubbing along both sides of the cylinder head along its travel. Inside the bore appears a bit shined up, so I'd say it was rubbing in there too. There's also a mark inside the case.

If I were to guess, I'd say it was a loose cam chain binding up and busting the gear. All the rub marks on the cylinder head, barrel, and case lead me to this conclusion. My engine has the spring loaded auto tensioner, so perhaps it wasn't doing its job? I didn't replace the spring in the tensioner since I couldn't find a new one, so maybe it's worn out.

Has anyone seen anything like this before? Any other ideas?? I appreciate your time!

sprocket looked junk before breaking. those gear teeth just dont vanish. option 3.5 worn teeth made chain slip an few teeth. The interference head cause a valve or both to smack the piston stopping the cam in its tracks and with crank inertia chain kept chain right on zinging snapping the cam sprocket. just my thoughts Happen at high rpm?

I would agree with lukelaw1, that sprocket was toast long ago. Dratv usually carries that tensioner spring and rubber tip thingy.

I'm not so sure that's the case. If the piston connected with a valve, wouldn't you expect to see a mark on the valve or piston?

I might be wrong, but I think the piston is upside down, the arrow should be pointing down, and the dimple is clearance for the spark plug tip, no?

Yes

Good eye Dirtbikr188, that piston is backwards.

Hmm good catch! I could've sworn I triple checked orientation when installing the piston. I didn't notice there was a dimple for the spark plug in the head. I'm surprised that wasn't an issue for me.