Change piston

i depends greatly on how you ride (and how clean you keep your air filter!)

Well,it really depends how hard the bike is used,and how much you need top performance.Pro guys have a new piston every time they race,they need 100 % power available from the engine.A 9 year old riding a kx 60 can sometimes go 50-60 hours on a top-end,before the bike gets harder to start,and performance drops off.If the piston breaks,youre in trouble off course.If the rider keeps the engine in the powerband all the time,its 6 hours on a ring,12 hours on a piston,typically.These guys can really feel the new piston on a 65/85/125.Bigger bikes are typically run at lower revs,and the top end will last longer.If you want to read more about it,try doing a search on these forums,a lot of info can be found...

IMHO, the best way to find out if the piston needs replacing is to measure the piston and the bore and see if the difference between the two is within the specs listed in the shop manual...

Sand, high rpm, and not wanting to take the time to measure your parts adds up to getting real good at splitting your cases to completely rebuild the top AND bottom end. Now that's alot of unnecessary work! The only way to tell if new parts are needed is by careful measurement. Disaster will happen if you try to guess when to rebuild.