Re: Should I have to pay for this?

In a hydo-lock situation...it's far more likely that the connecting rod will simply bend. Seen plenty of those instances too.

Ian

Yes, you are right. You may recall that's exactly what happened to my daughter's Quad Four about four years ago. The connecting rod was the weak link and bent like it had been made of putty. You and others on this NG helped me get that machine back on the road for which I am eternally grateful.

My thinking on the event now being discussed was that if the spark plug threads are indeed the weak link as proposed by some of the posters earlier (compared to the connecting rod), and if the threads gave way before the connecting rod did then the pressure would have been relieved as soon as the spark plug blew out. Maybe that saved the connecting rod from bending.

I'm not arguing with anyone on this. Just would love to know what really happened to that engine. Probably not enough is known to make a reasonable determination.

You are correct - we do not have enough information to determine what happened, but from experience, I can say we can be relatively sure the plugs have been removed on this engine previously, and one of 2 things happened. Either that plug was REALLY tight, and some of the thread came out with the plug (and the thread was possibly even "chased" before installing the new plug OR The person who removed the plugs cross-threaded the new plug putting it in, and again, possibly even "chased" the thread before installing the new plug.

In any event, the thread in the head was compromised. I HAVER seen plugs blow out of aluminum heads - and in all cases one or the other of the aforementioned had occured previously.

With 100,000Km plug change intervals we WILL see more of this happening in the future, mostly due to the first scenario. Seized plugs are becoming almost commonplace today.

Long before a Pontiac Sunfire would blow out plugs from a hydrolock situation you would either lock up the starter, split the starter drive, or bend the rod - most likely in that order.

Your reasoning makes excellent sense. What continues to trouble me is the time frame of the failure. Was it strictly coincidence that the plug failed when it was in the shop for seemingly unrelated routine maintenance? Certainly could be. If the spark plug threads were damaged in the past as you suspect then the eventual time of failure could not be predicted. This may have been that time. Thanks for sharing your experience.

I would agree with your last sentence. But there is enough known to rule out the shop that worked on it having to be responsible for this repair. Unless they were messing about with other things that we know nothing about.

Ian