Piston Failure Diagnoses (With pics)

[studboy]

OK. I'll start out this thread with a disclosure:

This is actually a snowmobile piston failure. While I know that this may be politically wrong, this is costing me much frustration and $$$ (this is a special order piston and it has wrecked the plating...for the third time) and I can't think of a better place to get help. Besides, this will translate over to failures that we experience in our bikes as well.

That said, this is a piston from a 2003 Arctic Cat Mod 800 motor. This piston has about 10 hours of run time.

Judging from what I could gather, the culprits are either: (1) Lean jetting. (2) Improper warm up. (3) Too hot of spark plug heat range.

2 and 3 are out of the question because I am running the same plugs as always (I can also provides provide pictures if needed, but they looked fine to me) and the sled had been running for more than an hour before it melted down.

This is the third time now that this exact same thing has happened. It runs great for weeks and even months before this happens. I can't figure it out. There were no air leaks that I could tell. The temperature and altitude and oil were all the same as usual. There are no signs of detonation. The other piston and cylinder are fine. The thing is that I have been running the sled for ten hours on this piston.

When I rebuilt it this time I was extra careful to check for air leaks. I jetted up the pilot and the main jets two sizes. After one ride I made sure that everything (intake, carbs, base nuts, etc.) were still torqued correctly. The carbs are clean.

This thing shows no signs of running lean and has performed great every time until it squeaks the piston.

What is going on here?

It has the four corner seizure pattern. These photos were taken after I cleaned the smeared aluminum off of the cylinder using acid.

[Vibeguy]

What piston to cylinder clearance are you running? Based on the 4 corner wear pattern you might be a little tight.

[no1clyde]

Afew years ago I got involved with a jetski that did something simular. It turned out to be a mod to the cooling system that sent too cold of water into that cylinder and caused a cold seizure. We changed the cooling system back to stock and it never did it again. The jetski came with the cooling system mod before they got it so it took us some time to figure out but the pistons looked very much like yours. Good luck Ed

[floridaflash]

it appeers to be a forged piston, if so you need greater clearence piston to wall, also forged pistons need to warmed up well

[studboy]

I'm not sure on the piston/wall clearance. My local shop sent it to be re-plated last time. I'll need to check that.

The motor was plenty warm. It had been being run for at least 30 minutes before this seizure occurred.

I'm hoping that it was just too tight piston/bore.

[SkiDaddy]

The sled has a thermostat right? Does the water flow thru this cylinder first?

Maybe it's partially malfunctioning; sticking partly closed when it should be fully opened, then occasionally slamming wide open, "quenching" the cylinder.

[o2bmxin]

Looks to be a four coner seizure, caused by Piston expanding faster than cylinder, causing the cleance to be reduced

Culprit: too quick of warm up, too lean of jetting (main), or too-hot spark plug range,

[mks yz]

Looks to be a four coner seizure, caused by Piston expanding faster than cylinder, causing the cleance to be reduced

Culprit: too quick of warm up, too lean of jetting (main), or too-hot spark plug range,

^^ my thoughts exactly i had an 02 zr 800 mod it was about bullit proof, but you have let it warm up really good right at first then just take it easy, by warm up im not talking about the normal jack stand rev it up until it clear out stuff, run it 1500-3000 for a couple minutes and feel the coolant return hose when its good and warm your good, by the way i "think" i ran a br10eya its been a while but the plug color is a give away one of the easiest motor i ever jetted sea level to 3000 460 mains 2 clip on needle, so on your next motor do compression checks and get a bore light that damage looks like it was done over time until it just let go. so check the cyl with a bore light to try to catch it fast if on the next one. my 2 yen

[studboy]

I have been cautious letting the motor warm up.

If that were the problem, wouldn't the other cylinder/piston show scarring as well?

[mks yz]

I have been cautious letting the motor warm up.

If that were the problem, wouldn't the other cylinder/piston show scarring as well?

i just wanted to recap on the warmup thing cause to some people rapping it out on the jack stand is a cautious warm up. however your right, normally it would show on both cyl. but it does take the mag side longer to warm also you have two piece heads so less heat is transfered between cylinders allowing for different warmup cycles depending on coolant route , i forgot to ask you which cyl is burning down mag or pto. i would check the clearances, also i would check your cooling system out, thermostat ect. could you post a pic of your spark plugs and give jetting specs? also a four corner sieze could be bad fuel, water in the tank, do you run iso heat? since its a mod did you scrap the oil injection or is that still stock? also with some warmup caused burndowns a small amount of damage is done in the initial warmup and it will run fine until you get it on the hill or making a lot of power/heat then the damage compounds until it lets go. keep us posted and good luck

[mks yz]

my best guess is that since the crown on the piston looks good that it is a clearance issue, what kind of piston do you run?

[DWinMI]

Looks to be a four coner seizure, caused by Piston expanding faster than cylinder, causing the cleance to be reduced

Culprit: too quick of warm up, too lean of jetting (main), or too-hot spark plug range,

BINGO!

[studboy]

i just wanted to recap on the warmup thing cause to some people rapping it out on the jack stand is a cautious warm up. however your right, normally it would show on both cyl. but it does take the mag side longer to warm also you have two piece heads so less heat is transfered between cylinders allowing for different warmup cycles depending on coolant route , i forgot to ask you which cyl is burning down mag or pto. i would check the clearances, also i would check your cooling system out, thermostat ect. could you post a pic of your spark plugs and give jetting specs? also a four corner sieze could be bad fuel, water in the tank, do you run iso heat? since its a mod did you scrap the oil injection or is that still stock? also with some warmup caused burndowns a small amount of damage is done in the initial warmup and it will run fine until you get it on the hill or making a lot of power/heat then the damage compounds until it lets go. keep us posted and good luck

The sled has a one piece head, so the coolant is shared between both cylinders. I have been letting this one warm up more than my other sleds (Mod. Thundercat, Etc.)

The mag side burned down. I will try and get some spark plug pics tomorrow.

They look pretty spot on. I am running a 420 main, and two sizes up on the pilot from where I was at. I ran a 400 main and two sizes smaller pilot for almost two years with no problems before it seized the first time (ran out of gas). After that I haven't been able to keep it together for more than ten hours despite being careful with it.

I need to check the thermostat. The sled had been running for at least four hours that day before it burned down. No signs of detonation.

The sled is running premix now. 32:1 amsoil.

I really need to take it and have the piston/bore clearance checked out.

I will also check out the thermostat.

[SkiDaddy]

Maybe the crank is twisted. Did you ever check it?

[floridaflash]

I personely do not like Amsoil, I have seen a lot of motors after using it including outboards, it does not lubricate very well

[studboy]

While I'm not sure that Amsoil is the best lubricant, I do get it at a good price and I'm pretty sure that it wasn't an oil related failure.

[no1clyde]

studboy the jetski I was working on was always warmed up and had ran awhile when it would do it too. I forget what they had done to it but it would get a big dose of cold water into that cylinder and do size it. We put it back like stock on the cooling system and it fixed it. I know your sled is a different thing but I would look real close at the thermostat and anything else in the cooling system that possibly cause tomething like this. good luck Ed

[mks yz]

studboy what did you find out?

[Wiseone]

it appeers to be a forged piston, if so you need greater clearence piston to wall, also forged pistons need to warmed up well

Since when?

Modern Forgings run as tight or tighter piston to cylinder clearance than casting's

you are refering to 70's technology.

Check the recomended Forged clearance to your OE casting.

Most Forgings are set up at .0015 or .038mm and no more than .002 or .051mm. For all 250's. Again compare this to your OE manual for the OE cast piston.

Classic four corner seizure. there is much information on this type of failure online.

Ed

[studboy]

Just for an update:

I had the piston/bore measured and the clearances are correct.

What most of the guys at the machine shot thought is that the thermostat is to blame, they said that they've seen this before.

Snowmobiles (especially this one) have a large amount of cooling available. The thermostat was sticking partially shut (warming the cylinders/pistons) then it opened wide open "quenching" the cylinders with cold coolant.

It makes sense to me. The jetting may be a little lean as well but it was a warmer day than usual and the sled ran good on two sizes leaner main and pilot for two years before seizing the first time.