Problem with 2009 Yam 450r

[machinehead]

Oil is pumping out breather into airbox. ( alot , filled box with a half liter of oil in a short period )
Compression test shows 90 psi, after a teaspoon of oil dropped in chamber, compression reads 120 psi.
Correct amount of oil in motor.
I suspect worn rings, with blow by pressureing up the crank case.
What do you guys think.
Time to tear into top-end ?
Would you just re-ring if the piston looks good, or is it worth the extra for new piston and wrist pin ?
I beleive this was the first year of the R model, and haven't had a great deal of experience with them.

 

[Med1ogre]

My motor did the same thing in my ltr, I put a big bore kit on and had some head work done. I sent my head south to Williams Atv, he does a lot of pro/national quads. Seems to know whats up, and is really cheap. Enough of me, i would definetly spring for a new piston and pin and double check your valve clearances while your in there.

 

[GYMBRAT]

I had to replace my Valves, both exhaust and intake sides on my KTM...