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15
Joined
11/3/2025
Location
Arlington, WA, USA
I have a 2022 yz250 with 120 hours I did a top end kit at 70 hours and saw no real wear. I scotch brighted cross hates into the cylinder in an x pattern to clean it and after I washed and cleaned the cylinder tell I would not get any dirt material from it.
When I originally bought the bike it had 30 hours on it. I compression tested it and got 218 psi. I have had the same compression sense the rebuild.
Now I’m rebuilding it again and the cross hatches around the exhaust port and intake are a mirror finish and there is two small score marks from the piston on the right of the existing port.
I am still in spec all the way through the cylinder with my ring gap what should I do. Should I rebuild it or get a new cylinder
If you can't feel the score marks with your finger nail, and the bore measures in spec, it's good to go.
Proper hone and a B piston
Need to accurately measure the cylinder with a bore gauge per the manual. It's not as easy as throw a B piston in since the YZ250 uses the A-B-C-D sizing. Many of them come stock already with a B or C piston. Do you still have the size mark on the back of the cylinder? It is an ink stamp so, unfortunately, many of them get washed off after a while. What brand and size piston did you use at the last rebuild?
It will run fine on a rebuild, but it's not perfect. I would replate the cylinder now or the next piston.
I've seen this on KTM 2-strokes (TPI) that are built lean from the factory. They run hot and are hard on the nickasil coating (more so than carb bikes or TBI). A lot of them have this type of wear on the cylinder after only 80h.
As you correctly pointed out, the hone marks get polished out of the coating particularly around the ports where the piston and ring have less support by the cylinder. The polished coating won't hold oil as well and will wear your ring faster. But they will hold compression the piston and ring should be well supported as long as your clearances are in spec.
Definitely check piston to bore clearance and ring end gap on the way back together.
Most people don't think about a rattle but I think about piston movement as it crosses the port. Every crossing is a piston rattle that hammers the ports. It's like having a worn out knee in the middle of a good leg.
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It’s a c cylinder and I’m using an oem c piston
I did not have the time to replate the cylinder so I ended up ordering a new cylinder from Yamaha
I thought honing niksil will damage the coating by impregnating the material. And possibly damaging the ports with taking the cylinder out of spec
I can just barely catch the score makes with my finger nail , sucks cause the cylinder is still in spec
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