Scratched Piston

[mxmann]

Hi!

I've replaced the piston in my Rev3. But now (after about 40 hours rode with the new piston) the engine was off again, and my new piston has many small scratches on it. Not deep at all, but there are many of them. Clearly on the intake and the exhaust side. The cylinder is the same. I can't figure out where did the dirt enter. I've checked the air box for leaking, and it's not leaking. I didn't found any dirt or any water in the bottom of the air box. I was regularly cleaning the air filter, and I always used air filter oil. However I did not put too much oil on it, because last time when I put too much oil, the engine didn't idle well. Both manifolds seems ok. I was also thinking that when the engine was off, some dirt entered. But I tried to work as clean as possible. And because it's scratched on the intake and exhaust side, I'm thinking that it sucked in the dirt and did not enter during the engine rebuild. Maybe I'm wrong?

Can you guys help me what else to I check? I'm out of ideas, and I don't want to rebuild the engine until I don't find the source of the problem (because the same will happen to the new engine).

[yoyo]

Hi!

I've replaced the piston in my Rev3. But now (after about 40 hours rode with the new piston) the engine was off again, and my new piston has many small scratches on it. Not deep at all, but there are many of them. Clearly on the intake and the exhaust side. The cylinder is the same. I can't figure out where did the dirt enter. I've checked the air box for leaking, and it's not leaking. I didn't found any dirt or any water in the bottom of the air box. I was regularly cleaning the air filter, and I always used air filter oil. However I did not put too much oil on it, because last time when I put too much oil, the engine didn't idle well. Both manifolds seems ok. I was also thinking that when the engine was off, some dirt entered. But I tried to work as clean as possible. And because it's scratched on the intake and exhaust side, I'm thinking that it sucked in the dirt and did not enter during the engine rebuild. Maybe I'm wrong?

Can you guys help me what else to I check? I'm out of ideas, and I don't want to rebuild the engine until I don't find the source of the problem (because the same will happen to the new engine).

Did you replace the small end bearings and gudgeon pin at the same time as the piston? If not then there may have been some wear in the old ones and it's caused the new piston to pick up. You say small scratches - you will get some if the bore of the cylinder has worn slightly, even a new engine will get some slight scratches as the piston beds in - it all depends on how small the scratches are.

Could you have mixed up a wrong ratio of oil/fuel? This can lead to the engine running hot and cause in extreme cases a seizure?

I'd be inclined to take the barrel and piston off to be inspected. Are there any corresponding marks on the barrel? If there are, then maybe re-honing it will sort it. (TBH I don't know if it has a nickelseal lining) If it has then it will have to be replated to get rid of any marks.

I doubt if it was dirt that caused the problem else you would have had it just on the inlet side. If dirt had got in then it would have swished around a bit and then remained in the bottom of the crankcase, unlikely to have been pushed back out through the exhaust ports.

Shaun.

Edited March 25, 2008 by YoYo

[copemech]

Please note the fact that air filters are not 100 percent efficient.

Use proper "sticky" air filter oil, too much better than too little.

I have come to like the Jitsie filters, dual layer may help.

Hope that helps?