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hi i need a new piston for my KDX 125 i bought a standard 56mm and its oo small how do i meausre the size of the cylinder so i know what size pison and rings to get.. Any help on this would be great thanks

Most would look at the old piston and get the number off of it, then you know what to put back in.

No juliend, most would measure the cylinder to check for wear to be sure they get the correct piston and/or to be sure they aren't in for even more repairs .

I use calipers since I don't have bore gages. It's a little tricky but I measure 2-3 spots on top and bottom and get an average. This technique hasn't failed me in 30+ years .

they must be real nice calipers...

Coop wrote:No juliend, most would measure the cylinder to check for wear to be sure they get the correct piston and/or to be sure they aren't in for even more repairs .

I use calipers since I don't have bore gages. It's a little tricky but I measure 2-3 spots on top and bottom and get an average. This technique hasn't failed me in 30+ years .

PFFT. If the cylinder looks like it's worn, yes. With a plated cylinder it's pretty easy to see. If you're just looking to replace the piston with what was in here, the best way is to order the same size piston you removed. Unless you happen to have a bore gauge handy, which most people do not. Your caliper method is next to worthless. Your 30+ years of good experience with this method is hardly anything more than luck.

everyone loves a smartass, btw ;)

Well if it's luck, it's the only thing in my life I have ever been lucky at. Measuring at more than on location helps you I.D. if the cylinder is out of round.

I was actually taught the caliper method from an old guy who owned a Kawasaki shop I wrenched at around 1998-1999. So really I haven't been doing it for 30 years ( I have been riding that long though), but I have only been using this method for 12 years or so.

I was honestly just trying to joke around and have fun with my response. I do always measure my cylinder when doing a top end though. It's a good safe practice, even if you don't agree with my measurement method .

That's how Mister Wizard told me to do it

I concur. I'm not opposed to checking one with a caliper, but I wouldn't count on it as any sort of accurate measurement of the bore condition. It's impossible to get a good reading on the length of the ring wear. A bore gauge would be best. If the plating looks good, I'd not be scared to put the same size piston back in as what came out. Measuring the top and bottom of the jug can't hurt, but it won't tell you anything about the area around the ports where wear is likely to be more extensive. IMO whether or not you do this is up to you. Not a bad idea.

If you no longer have the other piston, than a caliper will tell you close enough to what the bore size is to know what piston you'll be shopping for. There's no other way to tell.....


I meant no offense with the smartass comment either, just poking back

J.

Nah no offense, it was an accurate comment .

You sure it doesn't have a 200 motor in it?