So I'm replacing the entire piston and cylinder again after just doing this (Last was Namura and this one is Niche - if it makes a difference). On the last one I could remove the exhaust and stick my finger in the exhaust port and push on the piston and I could hear it strike the cylinder walls. The rings also feel loose when at the exhaust port. The bike sounds good when it starts up but after warming up and putting a little load on it I could hear it clacking and chugging. Disappointing since this is all brand new. I haven't removed it yet to confirm but just decided to re-do it all. I'm kind of guessing it has too much gap between the piston and cylinder walls and I'm getting piston slap?? I checked the bottom end when I 1st replaced the cylinder and piston and it had no movement, hopefully that remains the same when I pull it again.

Anyway I'm trying to do this right by measuring everything prior to assembly. The ring gap on both rings, measured from the top and the bottom of the cylinder is 0.15 mm. The cylinder to the piston gap with the rings installed measured from the top of the cylinder is 0.10 mm all around the top.

My question is, the shop manual says in one place, Piston to Cylinder should not exceed .12 mm but then in the graphic below it says 0.03 to 0.05 mm....Are these the clearances at the top 0.12 and bottom 0.03-0.05 mm?

When I physically measure the cylinder at the bottom and subtract the diameter of the piston (5/8" up from the bottom - as directed), I get the 0.03 mm difference.

I just want to make sure this time there are no excessive gaps before I start.

Tomorrow I'll measure the top of the cylinder to make sure it is flat. I need to dig out the flat steel straight edge.

Practice makes perfect.

 

You always measure the clearance from the base of the piston skirt to the bottom of the cylinder bore and subtract the difference. This is what the manual shows."Inspect the piston for scoring or cracks in piston crown
or pin area. Excessive carbon buildup below the ring
land is an indication of piston, ring, or cylinder wear. If
damage is excessive, replace piston. Piston-to-cylinder
clearance should not exceed .0047″ (0.12 mm).
Measure the piston 5/8″ (15 mm) from bottom. Then
measure inside diameter of cylinder. The difference
between these measurements should not exceed
.0047″ (0.12 mm)."

This is the MAX clearance that the piston to cylinder should be and the manuals shows the maximum clearance for the ring end gap also. It doesn't show what a NEW piston to cylinder clearance should be.. Imo a new piston and new cylinder should be close to .002 clearance or even a tad tighter. .0015 is optimum. That's what I've usually found when working on these small engines. Here's a good video on piston clearances and how to measure them. Towards the end of the video it even shows how you can use a plain feeler gauge without having to resort to buying calipers and micrometers.

 

I finally got this bike running. Rod bearing was fine when I double checked it. I swapped out the piston and cylinder again and this time there is joy.ran it around the block twice and there was no issues, except one issue on the last lap and it was the throttle again was stuck. I changed the cable and the oil pump so the only thing left is the factory mikuni. I have messed with it before and it all seemed fine, seems like it operates smoothly. If I blip the throttle or jam down on the brakes I can usually get it down, but I can't give it to a kid this way. I didn't have this issue until I switched to this carb.

Any thing to look straight away at? I'll see how hot it is tomorrow out here, maybe I'll pull that carb apart again, I may have to adjust the float up a little any way. Sometimes when you spike the throttle there is no response and then it takes off. If all else fails I'm putting the old Chinese one back in or I'm just going to spend 25 and get a new Chinese one. Just refusing to give up on this Mikuni at the moment. Unless you think it has to do with the pre-mix in there. Currently it is 50:1 until I break it in.

Other than that, the bike runs great now, not making noises and doesn't feel like it is dragging, idles nice and sounds good.

Thanks OPT for all the help.

 

Resolve the sticking issue. Also check the throttle housing itself as a problem area on sticking.They do tend to run boggy with gas premixed and running the oil pump also.Oil will accumulate in the carb bowls.We did that at the shop also straight out of the crates per factory instructions. This was just to insure that oil pumps and lines had time to get rid of any possible air pockets.Give it a little time,usually the first tank of premix,drain the carb bowls then go to straight fuel. Hopefully you'll notice a difference after that on performance.