is it about time for me to re-build?

Im kind of new to the mx dirtbike engines but if you could get the compression checked and it was still fine I dont think you need a rebuild. I think all you need to replace on these a lot is the rings on the pistons that probably wear out (losing you compression) since these weed eaters are reving WFO all day long. When you kick it over is it still the same resistance as when you got it or is that getting a lot easier than when it was newer. Id think this would be a basic way to tell.

Im sure some guys are replacing these after every single race but do they need to?

The compression will tell you very little about what is really going on.

How do I know so much???

We found an old compression tester that belonged to my friends father when he was a diesel mechanic years ago. We tested his boys brand new CR 80 and got a certain figure. We decided to check this monthly and replace the piston or rings when the figure dropped. 8 months later (weekly riding) the figure was stiill 95% of where we started. We both thought...sure, Honda just wants to sell parts. Screw them!

We continued to ignore the slight ticking the bike made. Heck, the compression was still good. Why worry???

The next ride the poor old piston let go. I believe the largest piece of it was 1/4 inch. We were very lucky. It only took us three hours to clean the mess up. But, the compression remained excellent until the end.

I put the compression tester away and will probably never bother with it again.

Seriously, no joke.......change the piston and rings on a regular basis. It is cheap insurance.

See dont listen to me :). Rcannon was it a bearing or something that went and took out the piston or the piston itself went or what? There could be a million reasons the piston was fine one second and bad the other. He could have replaced that piston that morning but if it was time for a bearing to go or a lean condition happened it would have done the same thing. Im not attacking you here, Im just wanting to learn.

The bearings were fine. No problems there. The piston broke up from being ran too long (probably close to 50 hours, or more). You can get by with that on a 250, but not an 80. Obviously, we should have changed the rings earlier, but the bike was not mine. Still, compression was great until the very end. The intact pieces of the piston showed very little wear.

If it were mine, I would take it down about every 10-15 hrs and measure and inspect. Add new rings if the piston was in spec, and live happily ever after.

Have you ever delt with one of those people that always knows best??? He now is on a regular piston schedule if that says anything.

The only sign we had was th esparkplug coming loose 10 minutes before it gernaded. This SHOULD have been warning enough!

Still, for 100.00 and a few hours work, a very costly and embarrasing breakdown could have been avoided.

compression

I know what you are talking about I replace my piston about every 20-30 hours of riding.
The old compression testor used to work ,but now they have to make rings that last 100 hours or so.