YZ426 piston, rings and gearing

1 reply to this topic
  • gsev

Posted August 07, 2010 - 01:34 PM


I finally have my 2002 YZ426 running good. It now starts first or second kick when cold and the same after it has warmed up. I have a question. Today I noticed that it puffs a little blue smoke when accelerating. I have about five easy hours on the oil change, and I have noticed the oil level has dropped a small amount. The bike seems really strong, the valve clearance is well within tolerance and the cam chain looks good. The engine does not make any unusual noises.

Should I just ride it and add oil, if needed between changes?
Is it possible that a set of rings, using the original piston would cure the problem?
Dennis Kirk offers a "Pro X" piston assembly for reasonable that is supposedly a fractional oversized piston designed for slighlty worn cylinders. Has anyone used one of these and how can I mic the cylinder to no which one to order?

What gearing do you use on the 426 for woods riding?


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  • grayracer513

Posted August 07, 2010 - 03:46 PM


Fractional oversizes are not generally offered for engines with plated cylinders. Any cylinder worn enough to require a larger piston will not be worn "straight" and evenly along the bore axis, and so would need to be replated and honed to shape and size. Since that would need doing anyway, any such repairs are made with standard size pistons, either replacing or refinishing the cylinder back to it's original size.

To evaluate yours, you need to have the bore measured carefully by a qualified individual. IF it passes this inspection, it can be refinished by honing briefly with a bottle brush type hone and reused.

The piston may be reuseable, but given the age of your bike, it's not likely. If it were my money, I'd buy the OEM replacement part and skip the shiny stuff in the catalog.

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