[ace.cafe]

Ace - Could this be the result of too tight of cylinder wall clearance for the piston?

No , I don't think so.
The piston was well run-in, and has had quite a few miles on it, and was tolerating his normal riding for over a year.
Then, this.

I am sure that something happened, which subsequently caused this
 It's not common.

[jedaks]

I noticed the last month that sometimes the bike smelled hot after a short ride but it ran pefectly.

I can't give you any technical explanations. I feel a little sick at heart.

[Arizoni]

The uncolored areas at the top and bottom of the hole thru the piston indicate the gudgeon pin was rotating in the piston rather than (or in addition to) rotating in the small end of the rod.

I was always under the impression that the movement should be in the bushing at the small end of the connecting rod with little to no movement in the holes thru the piston.
Could a rotating gudgeon pin have worked the retaining ring loose?

[ace.cafe]

The uncolored areas at the top and bottom of the hole thru the piston indicate the gudgeon pin was rotating in the piston rather than (or in addition to) rotating in the small end of the rod.
I was always under the impression that the movement should be in the bushing at the small end of the connecting rod with little to no movement in the holes thru the piston.
Could a rotating gudgeon pin have worked the retaining ring loose?

It shouldn't cause a problem. The pin floats in both rod and piston, with the ends fixed by the clips. The ends of the pin are 45° chamfered, so that if the pin pushes sideways, it jams the round clip harder into the groove, so they won't come out. So, that's why you see the whole side area around the clip blown out. It got pushed so hard that it blew out the whole end of the pin boss.
Not normal.

[Arizoni]

The retainer seems to be convoluted with 4 lobes rather than a simple snap ring so the pin may not have compressed it into the groove.

One things for certain.  Something applied a hell of a lot of side (or end) load to the gudgeon pin to shove the retainer out and to push all of that aluminum out of the way.

Very strange.

[Kevin Mahoney]

You don't see that everyday!!
As long as we are talking about piston pins....
http://en.wikipedia.org/wiki/Gudgeon_pin

[jedaks]

I am very foxed I can tell you.
I am leaning towards the cause being overheating rather than geometry of the crank. I could be wrong on that. But this morning I drained the oil (which was full of tiny non-metallic sparkles and the oil filter was loaded with them. They are the "scrapings" from the piston. When I roll the crank it is butter smooth and quiet as anything.

I think it was overheating because the underside of the piston is absolutely black, presumably from burnt oil.

[cafeman]

I am very foxed I can tell you.
I am leaning towards the cause being overheating rather than geometry of the crank. I could be wrong on that. But this morning I drained the oil (which was full of tiny non-metallic sparkles and the oil filter was loaded with them. They are the "scrapings" from the piston. When I roll the crank it is butter smooth and quiet as anything.

I think it was overheating because the underside of the piston is absolutely black, presumably from burnt oil.

I too would agree it would likely be from overheating, question is, what sort of loads were you subjecting it too leading up to it clanking? I know on mine (scored piston, excess blow-by) I had a few last rides in succession that were pretty "spirited" where things seemed fine and the bike was running great, grins and adrenaline. With the increased blow-by smoke and oil from the breather it got to the point I had to pull the top end too. Then I found the reasons for it all. Too much prolonged "on the throttle" and definite lugging. Somehow I forgot about "not" lugging it, I was riding with my son and also got carried away too many times . Care to recall the type of riding you were doing the last few times?

[jedaks]

I don't ride hard but I did go for a 90 minute ride last weekend, 100kph (bike is a 535 with roller bearing, steel conrod and alloy barrel). Bike performed perfectly but maybe got too hot.

Anyway, I was wrong about the barrel not being harmed. Initially I didn't see this score but in better light I found it this afternoon. Also is a photo of the underside of the piston,which looks like a dirty oven.

[jedaks]

I was able to recover the pieces of the circlip from the engine cases.

[cyrusb]

If, as you say, you were riding normally, and your timing is good, the only other cause for overheating would be lack of oil.

[cafeman]

If, as you say, you were riding normally, and your timing is good, the only other cause for overheating would be lack of oil.

. I just looked closer at the picture of the piston top  and it kinda looks spotted. Does the crown or edge show any signs of detonation?

[cyrusb]

Not a good view of the piston top, but it appears weird, and shiny. Could think too lean but the op says the plug was sooty. Maybe the soot was oil and not fuel? I don't think the op mentioned oil smoke, that would answer that.

[cafeman]

Just watched the youtube video, and you mention the rod is offset with 1/4" sideplay. Was this the same as it was after the lower end was first put together? I'd be leaning towards a circlip not being fully seated and with time and loads it finally popped out causing the piston damage. But, as much as it would suck, the possibility of a repeat is too great. I would tear it all apart and methodically go through it all again.

[jedaks]

Thanks for the replies.
I've satisfied myself the crank is good and that the circlip just gave way. (I had re-used it after decarbonising the head and piston 6 months ago).
When I watch the conrod move through the stroke, the small end almost exactly bisects the case halves.

I was able to recover all the parts of the broken clip. I have recently been told the open end of a gudeon pin circlip should always be at 12 o'clock or 6 o'clock. I'm not sure why but next time I'll be sure to put it that way