lowdown on 2stroke topend

Im pretty sure i know the asnwer to these but it cant help to check..

1 what (if any) are the signs when a top end is in need or close to being rebuilt? and if the bike is running fine and is overdue, what could happen

2 am i right in saying you can wear out 2 rings in the time a piston wears out?

3 what are the signs of a worn ring

4 what are the signs of a worn piston

5 if a cylinder has been bored out, the piston is made bigger to accomodate, not the ring made bigger? so for eg my yz125, if it was still stock the piston diamter should be 54mm. if it was lets say 56mm it would be bored out?

6. does the cylinder walls wear out from normal use and would need to be re-bored/plated? or only when dirt has got in?

Hard starting and low compression is the sign that it's time for a new top end.

Metal fatigue is hard to nail, so replacing an old but good looking piston is just good insurance.

If the bore doesn't look like new, bring it to a mechanic, if they wouldn't run it in their own bike then it's time to replate.

1 - Lack of compression. Also, if a cast piston is left in too long it can break apart.

2 - Rings usually wear out first, but it's best to replace the piston and rings together. A tell tale sign are black marks from blow-by on the piston skirt.

3 - Lack of compression

4 - Skirt slap. Where the piston skirt knocks against the cylinder. You can also wear out the ring landings (ring grooves) causing the ring to operate something like a windsheild wiper.

5 - The ring needs to be larger too. The piston is always a hair smaller than the cylinder bore otherwise it would be press fit. lol

6 - Yes, they wear out from normal use, but alot faster if dirt is involved. Best bet when doing a top end is to measure the cylinder with a dial bore guage, or bring it to t machine shop and have them measure it. If the measurements are within spec (from your service manual) and there are no deep scratches or damage then a new piston and rings would be fine. Most of the time, under normal wear, the cylinder will taper out towards the bottom, and start to become slightly oval from piston slap and you will develop a small ridge near the top from the ring. Your service manual will outline where these points are.

A good rule of thumb for a weekend warrior is to replace the piston/rings with a forged piston annually. More if you race/ride alot. The fresher the piston, the less things will wear out.


the reason i ask i bought it 2nd hand approx a year ago, havent road it that much im guessing maybe 25 hours max ride time. the fella i bought it told me it had had a rebuild by a mechanic only a few rides before i got it but he could have been talking shit. i havent noticed any drop in compression, i think about 3 or 4 kicks cold start and first kick on hot start.

since in australia parts are expensive as **** and the exchange rate is good i was thinking about buying a kit from usa to either use now or if what you have aid is right, i should be ok leaving it until i see a drop in perfomance????

am i right in saying take the cylinder off and get it checked for correct bore before i buy the kit? or is reboring that rare i shoudnt bother?

wiesco seems the way to go with kits? does anyone know the difference between standard, pro-lite and racer gp series kits? these are the ones i can find on sites i woudl buy from? i am not after top performance, more after reliability and longevity

Stick with the Wiseco Pro-Lite piston. It has 2 rings compared to the Racers Choice which only has one ring and needs to be changed more frequently.

Do the top end as soon as possible, people lie through their teeth about bike maintenance.

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