kx85 big bore kit

It's a little hard to tell. Below find a quote from Eric Gorr about 144cc overbores for 125s.

"Regarding piston life, thats a tough one. On my site I recommend ring changes every 5 hrs for experts and 10 for intermediates with piston changes double the time of the rings. But I have customers who have gone 2 seasons on a piston. Rich and I logged 100 hours on the same set of piston and rings on a 1992 RM144 that we rode at the OT National track. Rich called it the "Survivor Bike". We used good oil (MX2T) and changed the filter when it got dirty. However I know expert riders who can grenade a 144 is under an hour because they constantly rev past the power peak."

In my experence they will not change at all. The KX was offered as a 100cc from the factory and is totally reliable in that configuration. I maintain a couple of them and have not noticed any differnece in wear rate when comparing the 80/85/100 cc versions of the little kx engines.