kmx 125?

Well worth replacing the little end bearing whilst its apart,i would go with a genuine piston and rings though,The nikasil coating will be fine if there are no scratches,some light ones are ok as long as you cant feel them with your finger nail.if theres any wear its usually by the exhaust port.Nicasil is very hard wearing and great at dispersing heat so does allow for a tighter fitting piston,usually its the rings that wear down but as you have some damage on the piston,i would replace that aswell.Also rotate the crankshaft and make sure it feels really smooth with no up or down movment on the rod,a little side to side is ok.you can check clearances with a manual but i recommend taking the barrel into a local bike shop and have them measure it as most manufacturers make 2 or 3 sized pistons where nikasil is used and will usually have a number on the piston.it may well be that its due a slightly larger piston.The shop can tell you this,you can also have the barrel honed to help retain oil(well worth doing)most play of the piston will be due to centrafugel throw of the crank but dont worry about this as if the lining is ok its not worn.as adam said pistons are not perfectly round but cam shaven to accomadate the thicker part of the piston walls,where the wrist pin fits.but now we going to far lol.good luck and be sure to fit the piston with the arrow pointing to the exhaust and make sure the circlips are fitted properly with the opening facing up or down so there not stretched as the engine rotates

*
May 2010

gary the piston may well be slapping as this is very common
a few facts are worth telling you and not that many people know this apart from the technicians on here i expect but. your piston is not circular from the top or botom view although it looks like it is ,it is in fact an oval so there will be play from side ot side but shouldnt be much from back to front as its wider there it does need some play though to stop heat seizing as it will be too tight in the bore if its too big once its expanded with the heat of running.
also the piston is not the same size at the top as it is at the bottom ! pistons are tapered and the top is smaller then the bottom as the top expands more as it getrs hotter and when expanded it will be about the same width as the bottom.so it will have more play at the top than the bottom.i would say put a new piston in it if it has marks on it ,these barrels are nickasil plated and cost a few quid to have replated so if its just the piston thats worn lucky you! a piston will be about £30 and a top gasket set is about £18 these are pattern part prices as genuine stuff will be better quality but more money.hope this has helped you out gary

adam bike doctor
May 2010