Is there any need to rebuild my bottom end? 07 YZ450

My bike has lost a little hp, a little harder to start and used a small amount of oil between changes. I decided it was time to freshen up the top end. I tore down my motor last night, and found the piston and cylinder to look a little on the rough side. The cylinder has ring grooves worn in at the top and bottom of the stroke (which honing will not remove) and has 2 scores on the intake side. Also matching scores on the piston below the rings. The scores are approximately .010 deep and maybe .030 wide and run from just below the ring to the bottom of the skirt.

My question is, what caused that score? Crank and rod bearings seem to be ok, the bottom end was quiet before the tear down and everything worked good. Only issue I've had is sometimes 3rd gear can miss shift, but not all the time. I'm not sure if really there is any reason to tear the bottom end apart, but my engine guy is a little concerned about where the metal came from. I have not been getting any metal on my oil filter either. Any advise? Suggestions? Much appreciated!

ah mate just a thought on my behalf on the scoring maybe it has ran a little hot which could have caused it just an oppinion im most probably wrong but you never know....

Thanks. I probably should have been more specific about that "score" Actually both marks are more like a gouge that something foreign has caused.

Until you find out what caused the gouge, plan on tearing it down further.

ouch 02! Well, for checking your bottom end - you should have NO play up down on the rod, some side to side is ok - grey will chime in with numbers I'm sure.

The scoring on the cylinder/piston has to be from something foreign as you said - are there any (no matter how small) chips off the piston? If not, it likely came from down below.

How's your clutch/plates - all intact?

If everything on the outside checks out, it had to have come from inside the case.

Have you pulled the stator cover and checked the (I think it's called pickup coil) at the stator - (the part just outside your flywheel to the front of the bike)

Check for metal magnetized to it - it works similar to a magnetic drain plug, sometimes catching stuff before it makes it to the drain.

Check your oil pump for scoring too. At least those points should give you an indication of how severe it may be or how much stuff moved through..

Check the manual in section 2. It shows the allowable clearances.

I'm thinking it came from my transmission. The crank appears to be really good, everything was top notch before I took the motor apart. I'm going to cross my fingers.. I'm already in large coin for the top end, not really wanting to spend anymore!

I know how you feel!

I had a score in my cylinder too, pretty much gone once I had it honed, likely came from my grenaded bearing.

See you at the track this weekend if you're staying local!

Back together and running GREAT! Nice hp gain in the bottom end!

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now