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Help - burned piston and cylinder again!

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Last fall my sons 04 cr85 lost compression (totally) on a ride. This spring, I pulled the cylinder expecting to replace the piston, and ring, only to discover that the piston had become extremly hot around the exhaust port. This led to the ring seizing to the piston, and the plating on the jug getting torched. I replaced the jug with a new Honda jug, piston kit from Wisco, and gaskets from Cometic. After only a few hours of riding (5-7) I checked the compression, only to find it down to 70#.👍 I again pulled the cylinder, and this piston, jug is doing the same thing, only it has not gotten as bad as before yet. I checked the direction of the piston, placement of the ring gap, the ring gap measurement, cylinder to piston clearance, everything. What I did notice is that the head gasket has a raised "stamping" on it. The gasket was put on the right way, tab to the reed side, but would putting the gasket on "upside down" create a flow issue where that side of the jug is not getting proper flow? I can see where it could make a difference, and read another post that stated the raised part goes up (I had mine down), but would that make that happen? I need to know before I burn another new jug, and top end kit. ANY help is greatly appreciated. Thanks

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was the gasket on right the first time it happened? it maybe a lube issue not coolant.

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I'm 40 and lucky to remember last week.👍 Honestly, I don't know. That is the only possible common denominator here. Hard to believe that I could get it wrong twice by chance, but it is a 50/50 chance.

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I'm 40 and lucky to remember last week.👍 Honestly, I don't know. That is the only possible common denominator here. Hard to believe that I could get it wrong twice by chance, but it is a 50/50 chance.
maybe , how about water pump function, and rad condition, basically if coolant holes all line up the gasket is on correct, markings usually read right side up, if you can read writing on gasket its on right side up, what imean is lettering is not backwards, check coolant hole alignment w/ gasket that none are blocked off, then move on to other things, w/p, rad, proper lube, pipe condition, blocked silencer or air leaks in pipe,

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Was the Wiseco piston one of the ones that you are supposed to drill the skirt on before installation?

Some wiseco pistons are meant to have holes drilled in the skirt for lubrication purposes, but don't come like that from the factory because they cannot guarantee that they will line up with the bridges between the ports due to the tolerances of honda's castings.

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Yes, it was supposed to have holes drilled in it. I did not drill the holes on the first one, but did on the second one. I have also checked the radiators and water pump and everything is fine there. I don't know how to check the crank seals for leakage. Any ideas? If that were the case, wouldn't the piston be scored all the way around? I have considered crank seals but don't know much about that issue.

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What premix oil and mixture are you using?

The exhaust port bridge is the thinnest part of the 85 cylinder in contact with the piston and ring, so this is the part that gets hottest. If there is a lack of lubrication, this is where it will show first 9 times out of 10.

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Have you tried adding a little more pre-mix to your fuel and re-jetting for it? Or maybe jetting just a little richer on the main.

I know this sounds baisic and maybe you have already tried this, but it sounds like a lack of lubrication and therby too much heat in the area of the exhaust port to me.

You say the piston got very hot around the exhaust port by chance was there any sighns of detonation. Something like pitting on the head or the piston eaten/coroded away in the same spot.

I have not owned an 85 but I know those little motors turn very high RPM's compared to say a 250.

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Other than break in, I have left the mix the same. This son does not race the bike, mostly trail and "local" track riding, so if anything, I leave the mix, and jetting a little on the rich side.

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ok here's an angle, was alot of mud caking on the engine/ this would prevent it from disapateing the heat, especially on a small eng. just a shot in the dark?

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If you flip the head gasket over, does holes line-up the same? I've also heard that the raised edges go up.

Any corrosion in water ports?

When it's running, have you looked into radiator with cap off to see if it's flowing.

Is there any dirt in radiator fins? That will boil a motor over in miniutes. had a truck do it to me while offroading. had to tow it home.

I believe an 85 should run 32:1.

Check sticky "pressure testing a 2-stroke", it tells you how. that will tell you if your sucking air and running lean.

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I've been away for a few days, so sorry for not replying! Last weekend I replaced the left (magneto) side crankseal. After pulling the mag. I noticed it seemed to be leaking. Took five minutes to replace, and $9.00 for the seal. Wish I had noticed before smoking two pistons, and a cylinder:foul: Lessons learned though. I found an older two stroke book (1996) that went through every possible scenario with a two stroke, one chapter showed a piston that looked exactly like mine, and it said that exhaust side burning is caused by a leaking mag side seal. Thanks for all the help and suggestions!👍

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40:1 is a bit lean on an 85. Even is it isn't raced. I'd run more in the range of 32:1 to 28:1.

Where in Michigan are you??

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Is this one reason that honda changed the cylinder and piston in 05??? My son has a 05 and 06 and have never had this problem. We run hp2 at 32:1 with jetting per the chart. Jerry

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I believe that you have to drill some holes in the side of the Wiseco piston so that the exhaust bridge gets proper lubrication. Let me just do a quick google here...

Rob129

03-13-2007, 07:35 AM

What does it mean to "relieve" an exhaust bridge?

This is the jug out of my CR500. See how the bridge is relieved?

http://img.photobucket.com/albums/v235/engineguy129/DSC01051.jpg

Basically, you take about a couple of thousandths off the bridge.

You only drill Wiseco's! Wiseco's are forged and expand funky versus cast pistons. Cast pistons do not need to be drilled.

http://img.photobucket.com/albums/v235/engineguy129/cr500wiseco.jpg

Also,

and,

justarider

03-13-2007, 09:06 PM

Yes, follow the instructions, carefully mark the piston, center punch the piston (very gently) where the holes need to be. Drill it, chamfer the holes ever so slightly, and clean clean clean the piston. Your exhaust bridge with thank you. I've done about a dozen of these. Carefully do the holes and you'll be in good shape.

The instructions to drill the piston can be found here at this link, also in the first quote above.http://img.photobucket.com/albums/v235/engineguy129/cr500wiseco.jpg

Good luck! 👍

Terrafire

*oop, guess I was a little late there, I guess heres how to drill a piston anywho.*

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