Chronic KX500 Piston Seizing

[PBEB]

Hello everyone!

I’m having problems keeping my K5 running. It seems to consistently seize pistons under certain

conditions. If I’m generally play riding, everything is fine, but, when riding hard and fast in open terrain at ¾+

throttle in 4th or 5th gear it will seize in very short order. It's right over the first sub exhaust port. Mechanically everything is fine. Quality gas & oil. All machine work

done by reputable shops, etc. Any ideas on improving reliability? Increase piston skirt clearance? How about lubricating holes drilled over exhaust bridges?

[Foxx4Beaver]

Welcome to the site.

what's your current jetting/altitude?
did you verify the cylinder and cylinder head mating surfaces were flat?...did you lap them to be sure?

[PBEB]

I ride close to sea level, usually less than 2,000 feet. When I ran the stock Keihin 39mm carb it was set with a 175 to 180 main, needle, if memory serves, 3rd or 4th position. Now I run a 40mm Lectron set by Lectron. I seized pistons under both carbs. The head has been milled and reshaped so I feel relatively confident it's flat but I cannot say definitively that the cylinder has been lapped. I'll be sending it to Millennium for plating so I'll include that too. Good idea!

[Foxx4Beaver]

Both pistons have a slightly steam washed look to them(but I don't think that's the case)...like the head gasket(s) may not have been sealing up completely, and allowing coolant into the cylinder.
It's always a good idea to do the lapping yourself, just so that you have peace of mind, that everything is perfectly flat, and you can see it with your own eyes.

What type/brand of head gaskets are you using?...the majority of us are runnin Cometic, and OEM.

[PBEB]

My last rebuild I used a full top end kit from Cometic. I just found in my notes on my last rebuild the results from my leak down test - 16 minutes, starting at 6 psi to 5 psi. I have a 1/2 piece of glass I've saved for lapping. I'll dig that thing out and lap the cylinder when it returns. I checked the squish prior with the Cometic head gasket and it came out to 1.6mm. Does that sound about right?

[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]

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?

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