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I'm needing to replace the topend on my son's KX65 and stopped by a local shop to see about them inspecting it. Specifically, I was hoping they could measure the bore and provide that info. so I could order the correct piston and put it all back together. The owner stated that was more or less a thing of the past with modern plated cylinders. He stated the plating is so hard that unless it's scratched or gouged, just clean it up and put a standard size piston back in and make sure the ring end gap is correct. He stated they really don't wear these days. Does this sound correct? I was under the impression you needed to measure the bore, pick up the correct size piston (A, B, C, etc.) that would give the correct piston/cylinder clearance and then set the ring end gap. What is the consensus? How is everyone doing their topends these days?

I am no expert, but I vote: your way

I would agree with shop owner.
If no obvious signs of wear bore should be good.

If the plating gets compromised and there is actually wear and bore is over size, then the only solution for repair is bore and replate or replace cylinder.

Light scuffs in plating can be cleaned up with scotch brite. If plating has been worn off completely in any area it needs professional repair.

It sounds like you disassembled and brought cylinder to shop for inspection?
Or did you bring them the bike fully assembled?

I would think if it was just the cylinder and the owner was actually looking at it he would advise condition and course of action, or actually measure it for you just to appease the customer.

If you really want to know the numbers, take the cylinder to a machine shop and have them measure the bore for you. Have them measure in several places so you know the whole story. With these numbers, check against published standard bore dimensions, and advice of piston supplier.

Typically, forged pistons want a bit more clearance, but you should check with your piston supplier for their recommendations.

It's more or less correct. Plated cylinders that are still in good shape do generally wear slowly and little. But as pumpguy said, most machine shops can tell you pretty quickly what the actual measurement is, and it shouldn't cost too much. There's only one size OEM piston available (44.46mm on a standard bore of 44.5mm) which is a B. An A is .01mm smaller and C and D are .01 and .02 mm larger respectively.

IMO the OEM piston is about 65$ and the Wiseco is like $100. It's a chainsaw engine that needs to be redone every season so I'd clean up the bore if needed and throw an OEM piston and rings in and ride. YMMV

Checking for out of round is one thing to measure for, as mentioned with the multiple measurements.

I had stopped by the shop to inquire about them just being able to measure the cylinder bore as I don't have those instruments. That's when he talked about them not needing it these day's, etc. He had stated he would be happy to look at it and give his opinion.

I pulled it all apart this evening and the cross hatch is still present but you can see a few streaks. Nothing at all that catches your finger nail and no plating loss around any ports. The piston (Pro-X "A" piston) has some blow by but no gouges or scratches. I can't detect any vertical movement in the connecting rod so I'm hoping a new piston, rings and wrist pin bearing will be good for a while longer. Hoping I get a chance tomorrow and I'll see what they say about the removed parts.

Thanks for the help guys.

I was in a hurry last night putting it back together and older (not old) age is leading to eyes that don't see as well. I didn't have the best light in the shop and lost most dexterity in the fingers from the cold so after I put it back together, I began to wonder if the rings were correctly positioned around the locating pins. As best I could see, the rings didn't shift. It went together, the engine turns over by hand smoothly, no weird noises, etc. Assuming everything is within spec (bore, piston, ring end gap, etc.) can the cylinder be installed over the piston without the rings being correctly positioned by the locating pins? Is there enough clearance for the rings to overlap the locating pins and still install the cylinder?

That should be a negative. If the ring is sitting on top of the locating pin, you shouldn't be able to get it inside the bore.

Good deal. Just paranoid I suppose.