Dear Mr. Know-It-All,
Hello, sir, I have a 2002 KX85 that I rebuilt top to bottom this winter that is having an issue. A previous owner sleeved it 0.5mm overbore, and the only piston kit I found was a forged Wiseco. I heard that the thermal expansion rates could be an issue, especially during warm-up, but my tolerances were on the loose side of spec. On my last ride, the engine was fully warmed, and I had just gone wide open for maybe 15 seconds and then let off to around 3/4 throttle when it loaded down and acted like it seized. I grabbed the clutch and, after it slowed down a fair bit, let it out slowly and fired it back up, and it ran fine. Could the piston be getting tight in the cylinder, or is this another issue? I was running Liqui-Moly full synthetic at 32-1, I believe. Thanks.
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This photo is not from a KX85, but does show a seized abrasion just above the exhaust port.


Bradly, Bradly, Bradly, I quite frankly detest assuming anything, but in this case, I must make assumptions to attempt to give you a response to your drama. From what you say, I assume this cylinder has a cast-iron sleeve. Cast iron can have some expansion issues. If the engine locked up as you rolled off the throttle, it could be that the needle or needle clip position is too lean. Before you change the needle position, take off the exhaust pipe and look at the front of the piston. If it got tight, it is likely that some aluminum was transferred to the cylinder wall. The rings scrape the aluminum off, and that will compromise the rings’ ability to seal. If there are seize marks on the front of the piston, they will need to be taken care of. Small, delicate patches can be sanded smooth after you remove the piston. Make sure to clean the rings and ring grooves of any metal. If there are large or deep marks, the piston will need to be replaced.


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