'01 YZ426 burns oil like a 2 stroke

Just picked up a 2001 YZ426 real cheap. Bike would not start and when kicked over there was very little compression. First thought was valve clearances, so I pulled the head cover to find that the intake cam was timed 4 teeth off? Got it reset, checked the clearances and they were all well within spec. Put it back together and it kicks like a mule just as it should, and seems to run fairly well, but is burning oil bigtime, I mean splooging like a 2 stroke. What would be the first thing to look at? I already pulled off the whole top end to see if there was anything obvious like rings incorrectly oriented, or piston/jug damage, but everything looks good. I will bring the jug into work tomorrow to get good measurements on the bore with a bore gage.

What should I be looking at? Never worked on a YZ before but I have 2 CRF450R's that I've gotten pretty intimate with...

You're certain it was oil, and not coolant you saw "smoking"?

Valve stem seals maybe?

May be worth the time to pull the exhaust and inspect and clean the header, mid pipe and can for accumulated gunk.  If it was timed 4 teeth off, who knows what else may be inside and it may just be remnants of the previous owner.  Start with a clean and fresh exhaust, along with clean and properly working upper end, jug etc.  It may be the only way to determine what is going on.

I suspect it was the piston, after looking more closely and getting measurements, the front and back sides are pretty worn and it's alost .005" out of round. The wrist pin was pretty worn also so I'de say the piston has alot of hours on it. The cylinder looked pretty good with roundness and taper being under .001, which I think is decent?

Have a new hi comp weisco piston on the way. I have decarbed the head and exhaust, so we will see how she runs with the new piston.

The piston is not supposed to be round

It expands when hot

The shape of the piston has nothing to do with thermal expansion, but it's true that the piston is not supposed to be cylindrical in shape.  The only place on the sides of the piston that it should be truly round is the ring lands, the area above the skirt.  But there isn't intended to be any wear or contact in that area, so it doesn't matter, really.  If the piston were full skirted, like a 2T piston, then its proper shape would be that of a barrel with the sides pushed in at the wrist pin bores so it looks oval shaped from the top, and barrel shaped from the side. 


The reason for the shape is to reduce friction and also to manage piston thrust against the bore, especially during the reversals of direction that occur at the top and bottom of the stroke. 

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