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Hey,

I've been fixing a 1992 kx125. When I bought the bike it had little to no compression. My compression tester barely read anything; I could turn the kickstart over with two fingers. I took apart the top end: the cylinder and piston were in good shape, no scoring on either, but the ring was shot. I replaced the ring and gaskets and put it back together, I made sure the ring was the correctly installed: correct side facing up, gap around the nipple. The compression is better, but I can still turn the kickstart over by hand, but I do have a little more resistance. Still, the compression is only 50psi. I didn't see any leaks from the cylinder or head. Any ideas why I still have low compression? 

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William1, the piston felt and fit snug when put the cylinder back on, could it still be the barrel?

3 minutes ago, William1 said:

Worn out barrel.

The piston felt and fit snug when put the cylinder back on, could it still be the barrel?

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For proper compression, the barrel has to be round, proper taper and enough cross hatch to hold oil to help in sealing a properly fitted ring. All three parts must be right (I'll assume the head/gasket is not leaking). Right now, all you know is the rings are new.

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I don't have any pictures right now, but can post tomorrow. I just measured the cylinder bore and piston with my micrometer. Bore is about 54.8-54.9mm and the piston is about 54.5-54.6mm. From reading other posts the OEM specs for a 92 kx125 are 53.9mm-ish for the piston and just over 54mm-ish for the cylinder bore. I'm thinking I ordered too small a piston ring and the person before me may have bought a different sized piston when the cylinder started to wear. But there is no lip or scoring on the piston which tells me it is in decent shape. I'm not sure, at the moment I'm starting with the potential cheaper fixes and going from there. 

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I don't have any pictures right now, but can post tomorrow. I just measured the cylinder bore and piston with my micrometer. Bore is about 54.8-54.9mm and the piston is about 54.5-54.6mm. From reading other posts the OEM specs for a 92 kx125 are 53.9mm-ish for the piston and just over 54mm-ish for the cylinder bore. I'm thinking I ordered too small a piston ring and the person before me may have bought a different sized piston when the cylinder started to wear. But there is no lip or scoring on the piston which tells me it is in decent shape. I'm not sure, at the moment I'm starting with the potential cheaper fixes and going from there. 

And according to another forum on here, the piston spec is 53.945mm to 53.960mm and the cylinder bore is 54mm to 54.015mm. Still, maybe my .2-.3mm difference is too big. I'll submit pictures when I can.

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KDXGarage, I am using an Empire 6in. digital meter. I put a 54.5mm ring on the piston and the compression has come back. I kicked it over for a while and it did start for a second, but cut out as soon as I gave it some gas. The top of my piston was wet with milky brown oil. I also had a coolant leak into the transmission so I took apart the crankcase cover and am replacing my water pump seal and crank seals. Hopefully that does the trick and she'll be running. If that doesn't work my next guess about the oil in the cylinder would be the cylinder's oil seal rings.

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