01 srx 780 Pistons Breaking. Why!!

viperking

viperking

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Boscobel, wi
I have an 01 780 srx, the bigbore has about 1000 miles on it and i went to load it tonight for a trip up north in the morning and it didn't sound right when i started it, i rode it about 80 miles a few days ago and it ran great and has been sitting since, the motor didn't sound right so i pulled the top end off and found that two of my pistons are starting to break apart on the edge of the skirt! , what is causing this!! the two cylinders have have some very light scuffing but they are still useable and the center head shich was the worst cylinder of the three has a few light marks in the top from piston chunks, is this head still usebale? it only has a few very light marks from piston fragments, well here's some pics any help with why this happened would be greatly appreciated thanks! also does anyone know where i can get new 780 pistons at?
 

Are the pistons scored the same all the way around them, or just on the intake side? Are the skirts damaged only on the intake side, or exhaust side as well? You'd be crazy to run pistons that are scored and chipped on the bottom like that. Pictures of the inside of the piston (top) would be helpful.

I also ave a few questions about your setup.

Who's 780 cylinders are they?
Compresion ratio?
Jetting?
Airbox, and if so, any modifications to it?
Do you still have the thermostat installed? If so, is it opening?
What type of oil are you running?
What fuel are you running?
Is your oil pump setup properly?


What kind of piston clearance are you running in that motor? Excessive clearance is a major cause of skirt failures, however, that does not really look to me like the skirts are failing. Ususally, when they fail due to excessive clearance, they do so much closer to the piston pin.

Also, I'm sure you already know this, but you are definitely going to have to pull the base apart and clean it out before you reassemble this motor.
 
ya wow them look bad ...but need all the above answered .....and need to stt the damage to the jugs
 
Mr vipers 780 kit, not sure on the compression, i don't remember the jetting specs off he top of my head but it was getting plenty of fuel, i had run out the last of a tank of 91 and had gone about 35 miles on a tank of 93 when i finished riding and parked it. the jugs don't have much damage at all only a couple very light scuffs on them, and some slight scarring to the center head, oil pump was properly set and no airbox modifications sled had amsoil in it
 
what does the head look like ? oil burned to the top of it ? or is it clean ?if i remember correct wisco olny made a run of so many and that was it ...so i think you need to find someone who has a set ...but need to contact don and ask him what size pistons he used ...thought i read some were artic cat pistons ?? for a 780 ...but i also remember reading about 2 different bore sizes ....did the third piston have damage at all to it ?
 
the skirt was fine but the third piston had some light scuffing on it, the sled has no thermostat on it either, the heads have some oil on the top of them but they aren't overly covered in oil and not overly clean as well
 
the skirts are broke only on the intake side and and the pistons are scored mostly on the intake sides as well,
 
viperking said:
the skirt was fine but the third piston had some light scuffing on it, the sled has no thermostat on it either, the heads have some oil on the top of them but they aren't overly covered in oil and not overly clean as well
Click to expand...
the sled has no thermostat, thats your problem! been there done that . the starter and center cyl's always cold seize like that because they are the first to get the cold water from the pump, no thermostat makes the problem worse because the water temp takes along time (if ever )to rise enuf for the cyl's to expand fully and even when they do every time you go from low snow to good snow the water temp drops enuf for the cyl's to contract on to hot pistons and your clearence is gone then they start scuffing.780's need to be fully warmed up and the water temp needs to remain farily consistant,thats where the thermostat comes in. just my .02cents on this problem from experence.
 
just to add a bit more, this can even happen with the thermostat in if the outside temp is really cold. here where i live if you don't ride in -25 to -35c you don't ride very muchat all and i do. last year i went to go for a ride christmas night -32c ,let the sled warm up for my usual 10-15 min . water temp was up on my guage so i left went for about a km taking it easy, water temp still up then i nailed it sled stood up as usual then stuck solid and rolled to a stop. since it was so cold out the water temp didn't get hot enuf for the thermostat to open and warm the water in the coolers when i nailed it the thermostat opened and dumped a load of -32 water in to the cyl's the mag cyl being closest to the pump stuck the piston and cracked the cyl between the main and sub exhaust ports because of the thermal shock,the center cyl got a little tight but had only and little scuff and the pto cyl and piston suffered no damage because the mag stoped the engine before the cold water had time to contract the other two cyl's enuf to do damage.moral of the story is run a thermostat and make sure the coolers are good and warm(water temp stabilized) before you leave. works for me;)!
 
9801srx said:
just to add a bit more, this can even happen with the thermostat in if the outside temp is really cold. here where i live if you don't ride in -25 to -35c you don't ride very muchat all and i do. last year i went to go for a ride christmas night -32c ,let the sled warm up for my usual 10-15 min . water temp was up on my guage so i left went for about a km taking it easy, water temp still up then i nailed it sled stood up as usual then stuck solid and rolled to a stop. since it was so cold out the water temp didn't get hot enuf for the thermostat to open and warm the water in the coolers when i nailed it the thermostat opened and dumped a load of -32 water in to the cyl's the mag cyl being closest to the pump stuck the piston and cracked the cyl between the main and sub exhaust ports because of the thermal shock,the center cyl got a little tight but had only and little scuff and the pto cyl and piston suffered no damage because the mag stoped the engine before the cold water had time to contract the other two cyl's enuf to do damage.moral of the story is run a thermostat and make sure the coolers are good and warm(water temp stabilized) before you leave. works for me;)!
Click to expand...

I think you are right on here. That was one of things that Don made a point of making sure I understood. It has to be warm all the way through the system. Be it on the first start of the day, a 10 minute stop to dig a buddy out, or 2 hours at the local pit stop for a burger. Takes a while to get used to waiting, and my friends all look at me funny, but I have had no issues. Good fuel, jet it correctly, and warm it up good. This is my third season on mine and I had it apart prior to the season starting and it all looked like new.

Have you asked the guy that built the motor what he thinks the issue is? That would be where I'd have started.
 
9801srx said:
the sled has no thermostat, thats your problem! been there done that . the starter and center cyl's always cold seize like that because they are the first to get the cold water from the pump, no thermostat makes the problem worse because the water temp takes along time (if ever )to rise enuf for the cyl's to expand fully and even when they do every time you go from low snow to good snow the water temp drops enuf for the cyl's to contract on to hot pistons and your clearence is gone then they start scuffing.780's need to be fully warmed up and the water temp needs to remain farily consistant,thats where the thermostat comes in. just my .02cents on this problem from experence.
Click to expand...


Removing the thermostat is not an issue. Yes, it does affect the amount of time it takes to warm up the engine because the coolant flows through the entire cooling system instead of basically just through the engine before the thermostat opens. This is the only time not running a thermostat would affect the engine. The result, however, will be a slightly cooler and more consistent running engine. This only works properly if you install a drilled out plug in the line that travels from your coolant rail to the bottle. You need to restrict the flow there so you don't have a large portion of coolant just feeding from the engine, back into the bottle, and back into the engine again, which will make it run hot. Your motor also runs at a constant temperature with no thermostat, unlike what happens when your thermostat opens after the engine coolant is warm. I do not run a thermostat in any of my equipment.


If the piston is only damaged on the intake side, I would have to agree with 9801srx by thinking it was a possible cold seizure. The forged Wiseco pistons do tend to heat up and expand alot quicker than the cylinders, and that, compounded with a slower warm up procedure, could be the result of your issues. The best monitoring tool you can purchase for any snowmobile is a water temperature sensor.
 
Valin , what happen to mine ....lol..... are your skis still for sale ? ill take them
 

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Vinnie, i'm going to say it's because you have an Xtra 10 in that sled. :)


Skis are going to be picked up tonight.



wierdcat.gif
 
My thoughts

Looks to me like you are rattling the hell out of those pistons due to lack of fuel quality combined with the fact that you are lean.....there is NO piston wash at all there. Bad gas AND lean = DETONATION CITY. What jet's are you running? It would also be interesting to know what gas you REALLY got at the pump....I'll bet it's not 93 or even 91. Sad to see.....been there sone that.

Madmatt
 
Piston wash

Wait, let me revise my statement a little. There is NO piston wash on the left & center piston in the photo. The one on the right has a LITTLE wash but not enough for me.

Madmatt
 
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