two stroke piston failure

dirtbikersa
Edited Date/Time 7/27/2012 4:25pm
Good day all, was out riding the weekend when I ran into some troubles. I suspected something was wrong when the bike was not starting so I just left it. Anyway I got around to tearing it down today and was surprised at what I saw when I removed the piston. See pics -









The piston has just under 50hrs on it but any ideas as to what could have caused this? The cylinder is still in perfect condition and no signs of damage. I would think the broken piece from the piston has blown out the exhaust port and would not of gone down into crankcase.
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project racer 7/15/2012 5:36am
7/15/2012 5:36am Edited Date/Time 7/15/2012 5:51am
Looks like detonation / pre-ignition damage has caught up with that one. Ignition timing, poor fuel quality, lean jetting could be some of the areas to check.

Best be sure where the tramp metal went before buttoning a new top end on it. If even a small particle is still floating around in the cases , it will come up almost instantly upon start up once re-assembled. Also check the big end bearing on the rod . Sometimes when a piston fails /breaks, the rod bearing will get contaminated with the piston material. I see the ring locating pin missing in one picture.

IMO ,too many stray pieces to possibly come up and do more damage. Split the cases,clean every thing up, check the rod / crank, and freshen every thing up while it's down.

Or, pull the bottom end out of the bike, turn it upside down, jam a high pressure water hose in the lower end ,douche it out, immediately fill the cases with a water dispersing lubricant several times, put it back together,and pray.
dirtbikersa 7/15/2012 6:25am
7/15/2012 6:25am Edited Date/Time 7/15/2012 6:31am
I am a bit baffled as to how the piston deteriorated like that. I always run 98 pump and nothing lower, I did run a tank or three of AVGAS at one stage. I also doubt it is due to running lean because whenever I checked the spark plug it was dark brown to almost black. The bike does smoke when starting and idling at first, but once ridden it runs pretty normal.
The cylinder head has been mildly ported and cleaned up and I run a Phathead Racing cylinder head with slightly higher compression.

I hear you on their possibly been small particles in bottom end, but the more I think about it the more impossible it is. The top of the piston never gets low enough for it to get sucked into bottom end. The transfer port from bottom end to top of piston would not suck it in, so my logic would say that it got blown out the exhaust port.
captmoto
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captmoto 7/15/2012 2:25pm
7/15/2012 2:25pm
It definitely got hot right at he ring pin. It looks like the ring then spun and hooked a port. What does the cylinder look like.?
dirtbikersa 7/15/2012 2:42pm
7/15/2012 2:42pm
cylinder still looks perfect. Not a scratch or mark anywhere to be seen...

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mx5471
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mx5471 7/15/2012 3:29pm
7/15/2012 3:29pm
You need higher octane gas. It's classic. I just went thru this problem after the bike sat 4 years. Fixed the top end , ran c12. It rips and no problem since.
project racer 7/15/2012 3:59pm
7/15/2012 3:59pm
dirtbikersa wrote:
I am a bit baffled as to how the piston deteriorated like that. I always run 98 pump and nothing lower, I did run a tank...
I am a bit baffled as to how the piston deteriorated like that. I always run 98 pump and nothing lower, I did run a tank or three of AVGAS at one stage. I also doubt it is due to running lean because whenever I checked the spark plug it was dark brown to almost black. The bike does smoke when starting and idling at first, but once ridden it runs pretty normal.
The cylinder head has been mildly ported and cleaned up and I run a Phathead Racing cylinder head with slightly higher compression.

I hear you on their possibly been small particles in bottom end, but the more I think about it the more impossible it is. The top of the piston never gets low enough for it to get sucked into bottom end. The transfer port from bottom end to top of piston would not suck it in, so my logic would say that it got blown out the exhaust port.
I understand what you are saying. Check the pipe and see if you hear a rattle when you shake it. You never know. With ports the size that they are, on the scavenge stroke there is still a possibility of metal entering through the ports, especially if it fragmented.

It's a chance you take when you go back together with it. Throw the lower end in the parts washer,and flush,flush,flush.

Get an air nozzle and blow it out throughly. All you can do if you are not going to disassemble the lower end.
pete24
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pete24 7/15/2012 5:19pm
7/15/2012 5:19pm
that thing has been detonating for quite some time look at the outside circumfrance of the piston, you mite not want to do the bottom end, but someone is going to do it within the next ten hours, just do it now
Spinnin t's
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Spinnin t's 7/27/2012 12:38pm
7/27/2012 12:38pm
What kind of piston is that?
MR. X
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MR. X 7/27/2012 4:25pm
7/27/2012 4:25pm
How does the head look? it looks as if that piece of the piston was bouncing in there a few times.

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