Back after a few years. Opinions and discussion welcome on can of worms.

[Dr.Lovewell]

Suggestions and opinions on what could have caused this! I worked on a quad for a guy and he gave me a 1985 250SX but the rear
differential was demolished in it. I put up an ad on CL for one and got a steal on one for $100 but not without consequence. The guy told me that the timing chain fell off the top sprocket and was laying in the bottom end and the motor wouldn't turn. He said it was just sitting there idling and it died and wouldn't start back up. I pulled the motor since I am going to fully restore it and wanted it down to the frame anyway.

I started pulling the top end off and looked in the valve adjusting cover and could see the chain was still there and looked normal. I thought " Uh Oh". Then got the head off and found a disaster with a bent valve, broken valve, and a chewed up piston and combustion chamber including the valve seats. I checked everything out to try to find a reason this happened and everything looked good. The rod is straight and not overextending, normal free play, crank looks good, etc. The only thing I can think of it that the valve lash was never adjusted and the valve slammed shut too hard and that's what sheared the head off of the valve.

The guy said it started sounding like a "chain clatter" before it quit. I run my own small engine repair shop and think I am right but thought I would throw up some pics and see what other opinions you guys have. Just kind of a open discussion on it. I have the first one that was given to me that seems to have an okay motor but am going to do a full rebuild since it's a restore so I'm not too worried about it as it will be fixed anyway but like I said, Any opinions are welcome. Maybe someone has a different logical explanation for it. IDK.

My only uncertainty is the rod has some discoloration on it like heat coloring but I have seen this before and thought it was normal from heat treating during manufacturing. Usually I don't see much of it left because of wear and oil staining but this one is pretty prominent. Let me know what you guys think. I hope I can get the pics up since I'm new again. Thanks!

[RIDE-RED 250r]

It's likely the timing chain got too worn and stretched or the tensioner broke/malfuntioned and the chain skipped on the sprocket throwing the timing off resulting in a battered head.... Loose valve lash doesn't cause that, but a loose cam chain will.

[Dr.Lovewell]

That is exactly what my first thought was but the chain was tight, no damage whatsoever, and the tensioner doesn't have so much as a scratch. Maybe they were just running the Hell out of it. Idk. The only other possibility I can think of is heat causing metal fatigue or just normal metal fatigue from age or wear. Everything looked normal other than the mangled head. Weird none the less. LOL. Thanks for the reply.

[RIDE-RED 250r]

Hmm, interesting...

I guess it would be impossible to tell, but I wonder if you had a valve retainer pop out....

[The Thing]

how do the teeth look on the crank, and it would have to be timing cause what if the guy put the chain back on after it broke.