How many hours till top end needed on 125?

Did someone mention honing? This should get interesting real quick.

Lots of good advice here. As far as the honing goes, I pondered this for quite a while and ended up just using a scotch brite pad to thoroughly clean off the film from my cylinder (go with the direction of the cross-hatching). I was fortunate that I could still see quite a bit of the cross-hatching, so didn't see the point in physically honing the cylinder. If you can't see the cross-hatching it's time for honing. The best advice I can give for cleaning the cylinder is to use some oven cleaner. Don't leave the cleaner on too long though b/c it will corrode aluminum. Oh yeah, when you think the cylinder is clean, clean it again.

2-3 hrs is a good approximation of the time it will take you. Most of the time is spent cleaning the cylinder and removing any old gasket pieces.

Tip of the day : place a clean shop rag over the motor when removing or installing piston circlips!

Originally posted by kawakx125
what happens if you dont change your top end as often as you should.

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Here is an example. This piston is the OE out of a '98 CR250 that was removed last fall. See the crack on the skirt? There is one forming on the other side as well, but it is not as apparent. The chunk missing from the crack was enough to scar the cylinder wall. It was repairable, but it could have been prevented.