Hey! Was on a little ride earlier and all of a sudden... BABANG and total loss of power.
Upon a 2 second inspection I noticed the spark plug was blown right out. The spark plug must have been harder metal than the cylinder head as it fully stripped out the cylinder yet the plug is fine.

My question is what is my best route to take here, get a used replacement off ebay? There's a machine shop fairly close and I was thinking of taking it to them but I just wanted to know first if this was possibly repairable. I've heard of the helicoil thing but I feel like that wouldn't be solid enough to hold the plug under pressure, and it might blow again? Would it be possible for them to weld in a sleeve or something and rethread it? just curious as to how workable the cylinder head is, what alloy/metal is it made from?

Thanks in advance for any help, much appreciated!

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Damage like this is not possible under normal operating conditions, which makes me ask . . . how could this have happened??

I suspect that someone has over torqued the spark plug one too many times, and has pretty much pulled the threads into a severely weakened state . . . needing only a little combustion pressure and heat to finish the job.
Detonation could also be a factor in such a scenario.

Either a replacement head, or a Helicoil insert would be adequate to fix the problem.
It's not the material of the aluminum head, but rather improper tightening of the spark plug that caused this to happen.

To avoid a repeat of this situation, see your owner's or service manual on proper spark plug tightening torque.

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Likely that at some point someone overtightened the spark plug and damaged the threads. Then days, months, or even years later, they failed.

Helicoils and threadserts work fine if installed properly. Very strong repair... maybe stronger than original. Machine shop could do this. Head should be aluminum and "could" be welded up and but doubt it would be worth the hassle and complications. (warping and aluminum likely has become "oil impregnated" and can be tricky to weld).

You didn't say what bike you have but the easiest and maybe cheapest would be to buy a good used head on ebay... I think I even bought a brand new CT1 head for $25 not that long ago. If that isn't practical for your bike, then I'd pull the head and have the threads repaired with an insert.

And once again... I type too slow!

Short version "What Enduronut just said" .

An alternate option as to the failure mode: Many years ago, my cousin had a DT3 that had the spark plug fall out while riding. It seems that he just screwed it in hand tight. I saw this happen to him TWO times. I suspect that over time, a loose plug could rattle around and damage the threads in the head. Seems as though power drop would be noticable before damage could occur. Except to maybe my cousin.

It's a GT80, with shipping to Canada involved the cheapest I can find a new one is 60-70, taking it to the machine shop tomorrow to see what kinda money he wants to fix it.

It must have been working itself loose, as it was running fine for several hours before until I revved the engine very high.

Yes all of the threads have definitely been abused on this poor bike. Have already gotten a helicoil into the drain plug threads and new cylinder studs. Even the bolts that secure the kicker and shifter are stripped lol.

I shall ask about getting a helicoil in there tomorrow the machinist is usually very reasonable so If I'm lucky it might be fairly cheap!

Got the threads in the head tapped out and ready for the insertion, the type I got is similar to a helicoil although it is a solid sleeve with knurls which get expanded to lock it into place once installed.

The directions call for hi temp rtv silicone. Reading around I see some arguments about the rtv silicone vs using a thread locker like loctite.

Any opinions on this? I will probably go with the rtv silicone like the directions recommend but was just wondering if loctite or something would be better? Thanks in advance