Maintenance & Technical > KX500 Original

Chronic KX500 Piston Seizing

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Foxx4Beaver:
You're saying you lost 1psi in 16mins?....if that's the case, you have an air leak somewhere...which will cause it to run lean/hot.
You should be able to hold 7psi overnight.

PBEB:
Well, it looks like we may have our answer! While the cylinder is away, I'll start replacing cranks seals and pressure test it when it's assembled and look for additional leaks! Thanks!

PBEB:
After months of thinking about this, think I have the answer. I’d like to run this by the general KX 500 populous. I don’t think it was a lean situation caused by an air leak or the carb. I don’t think it was a coolant leak into the combustion chamber or an over aggressive exhaust pipe overheating the piston. Here is my verdict. Both seizures happened the same way. The bike travelling at a high rate of speed in 37-degree weather. I think the rush of overly cooled coolant hit the cylinder causing it to contract as the piston was expanding with higher RPM exhaust temperature. I believe this resulted in a classic 3/4-point seizure. The coolant inlet is nearest to the right sub exhaust port which has the most piston scuffing.  Any ideas on this, opinions or fixes?

sandblaster:
I've rode all my k5's in the pacific northwest in just about every cold weather scenario so I'd say nope..
It looks to me more like a out of spec bore with too tight of tolerance for the piston being used.
When you send your cylinder to Millennium, buy the piston from them so you can insure that everything is done right.
Have them deck the cylinder to clean up as well as the head.
After assembly do a leak down.
Pay close attention to the governor rod,,, they are prone to leak because the shafts get worn..

dave916:

--- Quote from: PBEB on May 11, 2017, 12:16:17 PM ---After months of thinking about this, think I have the answer. I’d like to run this by the general KX 500 populous. I don’t think it was a lean situation caused by an air leak or the carb. I don’t think it was a coolant leak into the combustion chamber or an over aggressive exhaust pipe overheating the piston. Here is my verdict. Both seizures happened the same way. The bike travelling at a high rate of speed in 37-degree weather. I think the rush of overly cooled coolant hit the cylinder causing it to contract as the piston was expanding with higher RPM exhaust temperature. I believe this resulted in a classic 3/4-point seizure. The coolant inlet is nearest to the right sub exhaust port which has the most piston scuffing.  Any ideas on this, opinions or fixes?

--- End quote ---

You are fitting the head gasket correctly? http://www.kxriders.com/forums/index.php/topic,6675.0.html

A badly  done repair area around exhaust subport bridges{ common to  hairline crack in that area}
Check you piston ring gap in a few areas around exhaust subport to see if it tight

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