yz piston problem

[ukracer]

Just came to change piston on my 03 yz 125 and it is a right mess on the exhaust side. It has stock jetting run on good fuel with 150ml of oil to a gallon and i keep the filter and airbox as clean as can be. It has had about 20 hours use since the last piston change.
Last time there was a flat spot on the piston where the exhaust bridge is but i had it relieved. This time it is about 20mm across. I think i have caught it in time as the cylinder is not too bad but obviously want to try and stop it from happening again.
Any advice would be great.
Thanks Ian

 

[ukracer]

What i really want to know is why it has happened?

 

[jmossbarger]

It sounds like you have wear from the exhaust port. sometimes if you releive the bridge too much the piston will actually catch on the top edge of the port on the up stroke. It causes the kind of wear that you described. A good chamfer on the top edge of the port will help, but it is mostly caused over time and it difficult to prevent. It really doesn't hurt anything as long as the piston ring is still free in the groove.

Good luck,
Joe