07 YZ450 Burning oil. Time for a rebuild?

Hey guys looking for some input on rebuilding my 450. I noticed with the last few rides i'd come back to low oil, barely even on the dipstick low (we ride 5+ hours at a time every weekend). I make sure I top it off before each ride & change it once every 2 rides & filter every 3-4. It starts & runs well but i can see some blue in my exhaust when i give it a rev in neutral. Im not tacking it off redline but its noticeably blue. Also comparing it kicking over with a couple other friends 450's mine kicks over pretty easily. Im suspecting I have low-ish compression but obviously id check that first before i start the tear down. I was wondering if it would be worth it to replace the piston, rings & cylinder or If I should just hone the cyl & throw new rings in there. Im not trying to spend a fortune on this thing & i see no need to bore it out any larger for the type of riding I do (trail riding), I do however want to do it right so that this isnt a problem for a while. I appreciate any input, thanks. 


Note: I just replaced the timing chain, guides, and tensioner (about 6 rides ago) my valves are in spec & i dont have any odd noises tapping or anything crazy like that. 

On my 07 WR, which is a little different, I had to add about 50ml of oil every few rides to bring the oil level back up to the dipstick.

Got the shop to check out the compression and sure enough it was in need of new piston and rings.


I see no reason you'd need a new cylinder unless it's had to be re-bored a few times already, in which case it may be out of spec. You may not even have to re-bore the current cylinder,

because generally if you're replacing the piston and rings before they fail the cylinder shouldn't be too damaged.

I'd just check if the width of the cylinder is in-spec and if so just replace the piston and rings.



I just had it all opened up with the head off when I did the guides, chain & tensioner the cyl walls looked great but I have no idea if it was ever bored before, im assuming no but i wont be able to tell until i pull it apart. Compression test is my next step, I appreciate the feedback. 

Just FYI, you'll probably need to do a leak down test vs. a compression test due to the auto-decompressor...... easy to rig up..... done it a few times testing crank seals on 2-strokes. Some of the TT experts can confirm most likely.

Two-stroke and four-stroke leak downs are fundamentally different, unless the 2T test is conducted on the rings rather than the crank seals.


Also, one does not bore  YZF cylinder oversize for wear.  They are Nikasil plated aluminum.  If the bore is worn out or damaged, it's bored about .006 - .008" over and then replated.  Any significant damage is welded up, then bored again, then plated.  At the end of either thing, the bore is precision honed back to standard size again. 


The only overbores are big bore kits, but they too come in one size only.

Appreciate it guys!1

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