Can I bore My 2001 yz 125?

there are only four possibilities left:
1: valves are not fully seating - adjust rocker arm / cam
2: piston rings are bad - replace rings
3: holes in pistons - replace
4: cylinders out of round - most complicated repair, bore / machine cylinders and replace piston rings with oversized rings.

of course you could just have 'gunk' on the valves preventing full closure........

I would be checking compression it may not be visible score on cylinder but it does not take much to loose compression on small bore motor with one piston ring and 11 years old

Contact Kustom-kraft.com they can provide you with the sleeve you need, or bore and hone your existing sleeve.

what did the spark plug look like. did it have and shavings on it

The std bore is 54mm and some available piston sizes are (Vertex Pistons)

53.93
53.94
53.95
53.96
53.97

yea, for most bikes they only come in .020 increments now. if your looking to freshen up the top end, then youll have to go to a 56.00mm piston. there will be almost no change in displacement or jetting from 55 to 56mm. if your replacing the piston, its best to have your cylinder bored anyways. since the worn cylinder walls may have adverse effects on the new piston rings, and they may wear a bit prematurely. a bore only costs about 30-40 bucks most places, and its cheap insurance. plus you will have the peace of mind knowing that your bore is matched specifically for your piston. I would definitely recommend a fresh bore.

excessive piston clearance.really needed bore job with oversized piston,not just honed and rings.measure piston to cylinder clearance,check spcifications and verify

• If a hole in the piston then install new piston and rings. Next, clean the carbon off the head and install a new head gasket.
• If cylinder wall is scared the piston has seized up. Pull the cylinder off and take it to the dealer. Have them bore the cylinder and return it along with a new oversize piston and rings.
• While at the parts counter, get a new stock spark plug and a supply of Yamalube Two Stroke Motor Oil. You need to add 4 ounces per gallon when filling up. This will be a 32/1 gas to oil ratio. NEVER ride the bike without at least the needed 32/1 mix.

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