I was riding a few days ago, stopped for a 5 min break, and when I was ready to go out again, my YZ125 wouldn't start, like it was flooded. I bump started it, it sounded rich for a while, then seemed to open up. I made half a lap before it stopped for good. No compression :( I tore it down, and a large chip was taken out of the top of my piston and ring on the exhaust side along with a hole in the side (not the top) of the piston just below this chip. Needless to say, I also had a good size gouge on the cylinder :silly: My question is why. Nothing had changed. The bike had been riding well, I hadn't touched the carb and the fuel was fresh. It was cool outside, but not cold. The piston was a little old (rings 10 hours ago, piston maybe 30 hours), but this looks more like detonation to me. Any ideas?
help, my piston has a hole in it
- Thread starter dkortje
- Start date
Is it detonation for sure? It sounds like maybe the powervalve and the piston kissed...I found out the hard way on that one. Take a look at your power valve and see if it was hitting the piston. On my bike it was wedged in the end of the pipe after it blew out of the cylinder!
marcusgunby
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- Jan 9, 2000
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Any pics?
nikki
Moto Junkie
- Apr 21, 2000
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Is the pin where the ring ends meet together still intact on the piston?
I had that pin once work its way through the top of the piston head and do some damage on my '98 RM 125. Doesn't sound exactly like what you are describing... but just something else to check.
Did you find any significant metal/steel pieces stuck in the head/piston/cylinder that look like they could have caused the damage?
I had that pin once work its way through the top of the piston head and do some damage on my '98 RM 125. Doesn't sound exactly like what you are describing... but just something else to check.
Did you find any significant metal/steel pieces stuck in the head/piston/cylinder that look like they could have caused the damage?
The bike that did that was my '94 KX250. It took a little chunk out of the exhaust side of the piston before it blew out of the exhaust with no damage to the head. This was due to the overboring of the cylinder without grinding down the exhaust valve to match. I thought that the mechanic would know that, because even I know that, so I didn't check it out. It lasted about 5 hours however, which is pretty amazing.
- Thread starter
- #7
Sorry, I don't have a pic. There were no 'welded' Parts on the head or cylinder though. The power valve sounds like a little of a possibility, as it was on that side and last time I changed rings, I cleaned the valve like Eric recomends with oven cleaner, but the cylinder still has it original bore. Could I have somehow affected the power valve at that time. Right after I did that it did make a clanking noise when I first started it the first time but then that went away and it ran great for at least 10-14 hours before this happened.
marcusgunby
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Alot of 125s make a terrible clanking after doing a top end-i believe its the oil used to lube the piston cause a hydraulic lock for a few seconds.Ive stripped bikes down after doing a top end to find nothing-i just let the engine clear out nowdays and the noise soon goes.
DEANSFASTWAY
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- May 16, 2002
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Hopefully its not peices of the crank coming up . Try and post some pics as marcus said and we can all take a look andplay detective . Sometimes bits of the crank bearing come up ar parts of the lower rod bearing .Sure ruins the day EH ?
Did it look anything like this? http://www.ericgorr.com/images/chipped.jpg
(Without all the scoring etc.)Read about it here > http://www.ericgorr.com/techarticles/pistondiaguide_ta.html
I was having a lot of head gasket failures(The head was not perfectly flat for some reason) on my CR a while back and the piston failed just like in this picture, fortunately for me the piece that chipped off headed straight out the pipe, so all I needed was a new top end!
(Without all the scoring etc.)Read about it here > http://www.ericgorr.com/techarticles/pistondiaguide_ta.html
I was having a lot of head gasket failures(The head was not perfectly flat for some reason) on my CR a while back and the piston failed just like in this picture, fortunately for me the piece that chipped off headed straight out the pipe, so all I needed was a new top end!
- Thread starter
- #11
Zenith,
Yea, I had seen that on Erics site, and thought maybe. It does look a bit like that except for the hole on the side of the piston. My O-rings look ok, but I guess I need to find something flat to check for head warpage, how much is too much and what is the easiest way to do this?
Yea, I had seen that on Erics site, and thought maybe. It does look a bit like that except for the hole on the side of the piston. My O-rings look ok, but I guess I need to find something flat to check for head warpage, how much is too much and what is the easiest way to do this?
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