How do I tell what piston (std/A/B) I need? KX125

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#1
I see listings on aftermarket pistons (03' KX125) referring to std, "A", "B" etc. Appears these are .01mm larger for each incrementally, and I'm told you really need to know what the bike came with before ordering. Anyway to tell prior to tear down? Once torn dow, how will I know? Is it stamped in the piston somewhere, or needs to be measured to know for sure?
 
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#3
I asked this same question about my '05 KX and never got a reply. FWIW, I'm pretty sure the B size is "standard". Both my KX's and my KDX used B pistons. I also once ran an A (they sent the wrong part) in my KDX and it was fine. Went back to a B next change.

I've used Pro-X pistons in all the bikes.
 

holeshot

Crazy Russian
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#4
I used a "B" on my '03 KX125 ..not sure how I arrived at that though.
 
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#5
Should be stamped on the cylinder.

The correct way to determine the size needed is to tear it down and measure the bore, add the recommended clearance, and order the piston that matches that size. Most poeple just throw in a B, C, or D.
 
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#7
Many times the the over-bore is stamped into the crown of the piston. You may need to buff the surface a bit before you can read it. You can't do that until you have torn the engine apart, however. On the other hand, I wouldn't recommend ordering the parts until you do have the engine apart

It it isn't stamped in the piston then you can establish what size it is by measuring its diameter. Ideally you would measure the cylinder bore, but that can be quite difficult if you don't have the special tools. A standard set of calipers will measure the piston.

If you don't have a set of calipers and have no idea how to use one then take the piston and cylinder down to the machine shop and have them tell you what you need.

Do you know the history of the bike? If you have owned the bike since it was brand new and you know for a fact that the cylinder has never been bored then you can be fairly sure that you need the standard size piston/rings.

Rod
 

holeshot

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#8
It says on page 72 of the '03 KX125 manual that the "letter" mark is on top of the piston towards the exhaust side. In the illustration, a "B" is shown.