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Discussion Starter #1
Machzkeith here, first time poster long time reader. So this past season my 99 Mach Z started what I would describe as a sound like a piston was moving around in the cylinder (piston chatter?). So its off season now and I have it pulled apart. What I found was the PTO side piston was heavily worn on the skirt on the intake side. Later after I cleaned it up i noticed a crack in the skirt. So my question is what would cause the piston to wear so bad on one side like that? Did the wearing of the skirt cause it to crack or the crack cause it to begin to wear? Question the third...are the wrist pin bearings supposed to fall apart when you remove the piston from the con rod? Any input would be greatly appreciated! Thanks.
 

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I would suggest that you developed piston slap due to normal wear and that would explain the scuffing and after a while the crack devoloped.
 

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Discussion Starter #4
Its a 1999 Mach Z with 8400kms on original motor. Compression was MAG 118 CTR 118 PTO 108. Rem PTO was the bad cylinder.
 

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cageless bearings, so the answer to the question is yes. the answer to the piston will vary. I doubt you truely ever know.
 

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Welcome !

Check your PTO crank bearings for fretting or possible failure. Without seeing the piston crown my best guess is something other than injector oil has peppered the intake side of that PTO piston. The wear occurs first then the consequent excessive clearance provides room for catastrophic piston slap which usually locates its stress cracks where the casting becomes thin under the wrist pin boss. My intuition tells me that you might have PTO bearing debris blasting that piston. If not then compare the crowns of the other 2 pistons. They provide a picture in time of what was happening before the crack occured. Usually damage from the pin down is a result of contamination or cold seizure. Piston pin up is heat related. The piston is barrel faced and larger at the bottom to control expansion. There's a spread of 300 degrees difference between the combustion (top) side and the bottom rear side of an average 2 cycle piston under normal operation...did I say normal.
 

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I am with ya doc trash from somewhere.
 

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Discussion Starter #8
Hey there, thanks for the welcome and the info. Need more input! I took some more pics so you can learned me! So by what you guessed now that u can see the crown of the piston....it is significantly thicker build up on it than the other pistons, that build up should tell me that I need to replace the PTO (both sides I will) bearing right? What does the build up on the bottom side of the piston tell me? Note the MAG side is minimal, center is medium, and PTO is high why the difference? And what should my action plan be? My intentions were to bore and repiston all 3 cylinders and replace PTO and MAG side bearings. Do the center bearings need to be replaced as well? Are there other things I should do while I have this apart or any advice on boring or anything else? Thanks for the help here, I am impressed with your guy's abilty to diagnose a faulty bearing by looking at a piston, well done!
 

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Wash on the ctr and mag pistons looks pretty good. Even the PTO piston looks like it had a good wash pattern going before the problem started. It looks to me like the PTO leaned out, heated up and then the detonation began. That piston was hammered by deto. It looks like the ctr piston was getting some extra heat as well, but was still a ways behind the PTO.

My guess? Crank seal on the PTO side went bad which leaned it out causing the detonation and superheating of the piston dome, which in turn caused the oil in the air/fuel mix in the crankcase to stick/burn to the bottom of the dome. Now, quite possibly the PTO bearing went first in this chain of events and the extra heat generated by a bad bearing pushed the seal over the edge allowing outside air in which leaned the mixture. Then the center cylinder started heating up since it was next door. All the heat and detonation busted the skirt on an already worn piston. JMO from afar.

I would buy a reman crank and rebuild.

Doc?
 

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Wash on the ctr and mag pistons looks pretty good. Even the PTO piston looks like it had a good wash pattern going before the problem started. It looks to me like the PTO leaned out, heated up and then the detonation began. That piston was hammered by deto. It looks like the ctr piston was getting some extra heat as well, but was still a ways behind the PTO.

My guess? Crank seal on the PTO side went bad which leaned it out causing the detonation and superheating of the piston dome, which in turn caused the oil in the air/fuel mix in the crankcase to stick/burn to the bottom of the dome. Now, quite possibly the PTO bearing went first in this chain of events and the extra heat generated by a bad bearing pushed the seal over the edge allowing outside air in which leaned the mixture. Then the center cylinder started heating up since it was next door. All the heat and detonation busted the skirt on an already worn piston. JMO from afar.

I would buy a reman crank and rebuild.

Doc?[/b]
Doc I am not, but I must say it looks like you could be right.
 

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Just to add again that a re-man crank from Doo would be the best way to go and also, you cant bore your cylinders, they are nickasal. Just ask around the net and you will find a good cyl.
snowmobileworld or amsnow are 2 good places to find triple parts.
 

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Just to add again that a re-man crank from Doo would be the best way to go and also, you cant bore your cylinders, they are nickasal. Just ask around the net and you will find a good cyl.
snowmobileworld or amsnow are 2 good places to find triple parts.[/b]
Good Point !

The crank should be done and one less thing to go wrong.
 
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