I bought a 2001 kx85 with a 100 top end according to the seller and when I called the mechanic. The bike has a new crank and runs very good. Is there a way I can order a piston for it before I tear it down. Do I just order a standard 100 piston kit or should I take a look inside first. Is there a marking on the cylinder outside. I am used my other son's KTM 65 so this is new to me. Any suggestions? Also, what piston and where from on line? I searched and heard wiesco are good but with KTM they are not so favored at least with the 65sx owners I know or read from.

Didn't have any luck at all with Wiseco's, Vertex does make a piston for the 100.

Nope, you'll have to pull the head to measure the bore. If it was overbored or is scored, you'll have just purchased a $100 paperweight.
That is assuming the CL seller didn't pull your leg.

I will tear it down and take a look when I have to. I am sure the cylinder is good because it runs like a clock and has all the power. I looked at vertex but didn't see a 100 piston. Will look again. Maybe I need to put in a newer year on the site. Should be the same piston 06 or newer I would guess. How canI tell if it is overbored or a true 100 jug? Thanks

My mistake it was for the older style 100, oh man it's time for a new piston in ours also. It looks like OEM or Wiseco is it unless you order from overseas. And my above reference was for the Wiseco's in our 85 motors, always hanging a ring and scuffing on the exaust bridge.

http://www.shopwiki....996112&s=457021&

I'm pretty sure all the stock bore KX100s use the same piston. Of the research I've done, it's also the same piston as the KE100, but I'd do a cross reference on your year to be sure before making that leap.
There may be a marking on the piston itself as to what size it is, but to get to it, you still have to do some disassembly.
Might I ask, if you're pretty sure the piston and cylinder are good, why are you looking to replace it? The KX100 piston/rings for your average rider should last quite a while.

Best thing to do is:

Do a compression test first, if you're at 120 or higher, you'll be good for a while, if it drops below 110, you'll need rings soon at the very least.

If someone dropped an actual 100cc jug on an 85, you will be able to tell, it should be marked somewhere on the left side of the jug......99cc
If that's the case and you are 100% sure that it's an unbored 100 - go ahead and buy your spare piston right now.

If someone over bored the 85 or a 100 (called a bigbore), it's a standard size over bore and I think there are 2 available, the 100cc which is Eric Gorr's standard - uses a standard wiseco kx100 piston and you could use any kx100 piston, some are also bored to 107cc - which requires an oversized piston usually available from any of the aftermarket manufacturers.

I ned to get a compression guage. My son is 12 and jut getting to where he can get the most out of the bike. I put a timer on the motor but will do top end on compression. The reason I ask is my 8 yr old has a ktm 65 that needs a piston at about 20 hrs or so.

I have a new, unopened, complete gasket kit from K&S (71-2009) that fits 96-'04' KX100 & '02-'04 RM100 PLUS one new Wiseco ring for same bike


I'll sell for $40, shipped. PM me if interested.

also, visit the kawi 2 stroke forum and the FAQ section at the top. It'll help you alot.