blew top end. Rebuild advise

[chrisme]

So the other day I was whizzing along, and I start hearing bad sounds, loosing compression, etc. I figured that something probably broke inside the engine and since it was about 7 miles home I just kept riding. At this point it would only go about 15mph and was making a bad metal on metal noise. Then finally after about 1.5 more miles it died and that was it. Low on compression and wouldn't fire. I walked home... picked up the bike with my car, and started to assess the damage.

One of the ring pins is missing off the side of the piston. The piston is effed and the cylinder wall is badly scored. I found that the bad noise was the piston hitting the head! (How would that happen? I can tell because in the same place the piston and head are all shined up and scored).
And finally, probably what ultimately caused it to stop running, something hit the spark plug and bent the electrode all up.

So my question is, why did the piston hit the head? And, since the bottom end seems to be fine (has never had an issue at all!), would it make sense to rebuild? (New piston, new cylinder).
The piston hitting the head isn't a bottom end problem, is it?

thanks for the advise.

 

[Mac]

Chrisme,
Yea, if there is no physical damage to the bottom end, make sure there is no foreign objects float'in around down by the crank, get a jug(cyl assy), piston, rings lower cyl gasket too, I just changed one on my son's yesterday. Same thing happened to his, a piston ring indexing pin came out, ate the top end. I don't know what type of motor you have, I run PK80's, Pirate has the part's for those, I think they may have others, got the parts in 2 days, (the're in Mass.). As far as the piston hitting the head, probably debris from the part's breakage just got in the way. hope that helped.

Mac

 

[chrisme]

I didn't realize pirate was in Mass. cool.
I've had it all apart before because my gaskets were leaking, I just made all new gaskets out of gasket paper. But I never though the indexing pin would come loose. Grrr. I'm gonna get some high temp epoxy or something when I rebuild it to prevent this happening again.

And I've had the piston hit the head before... but that was because I was trying to get the thinnest possible head gasket for the highest compression tune on race gas...... I have no idea why it happened this time :/

 

[Mac]

chrisme,
I would not put ANYTHING on those pins. Any adhesive or retention compound will fail at those temps/conditions. Not to mention it will no-doubt inhibit the ring motion.(there is not much, but there is some) all you can do with the new piston is make sure there not loose prior to installation. let me know how it works out for ya.

Mac

 

[chrisme]

Alright. Thanks for the insight.
The brakes on my car just broke, so depending on the carnage, this might take the back burner

 

[chrisme]

So I fixed the brake calipers out of bits from the parts bin Bike is back on the work bench

 

[Drewd]

Gluing a loose ring alignment pin (assuming we are talking about the same thing) with the normal set JB weld is a tip I've advocated for years due to similar engine failure 4 years ago. The JB weld had held up to the engine temperatures with no problems. I'm sure some fairy godmother will say this is a dumb tip but all it takes is a blown engine miles from home to convince you otherwise.

 

[Bikeguy Joe]

O.K., I'll fall for it....JB Weld inside an internal combustion engine is not a good idea, and you have been lucky.

Now insult me and call me a fairy god mother or what ever.

 

[Allen_Wrench]

I know this offering probably contains no help in it but, from past experience, I don't think your piston hit your head. (Sounds painful if you put it that way.) I think a chunk of your ring got between it and the head, then between it and the plug, then it did laps in your jug until it made a real mess. Even if it wasn't a piece of ring, it was a piece of something in there. That's what I'd diagnose as "terminal indigestion".

 

[chrisme]

"terminal indigestion".

Hahahaha, that's funny.

And I still need to order parts. Last week was just so busy. Working about 10 hours a day at 4 different jobs. No fun.