What caused this piston failure? 340 fanner

[Gunit111]

Understood, but what it does describe is that the pistons were installed in the correct orientation based on the arrow markings on the crown...

Are you saying the factory pistons are defective? Arrows on the wrong side and therefore ring locator pins are 180* out?

The pistons were installed with the correct orientation; locating pins were in the correct place. If the pins were in the wrong spot you would have likely snagged the end of a ring in a port and had a much larger problem than you had and if the pistons were backwards you would have a knive-edged skirt. Pistons have an orientation mark because they are not made symmetrical from the intake to exhaust sides; Locating pins, skirt length and taper, and the wrist pin location are often not symmetrical.

 

[whoisthatguy]

Understood, but what it does describe is that the pistons were installed in the correct orientation based on the arrow markings on the crown...

Are you saying the factory pistons are defective? Arrows on the wrong side and therefore ring locator pins are 180* out?

From your pictures, they clearly show the piston pin and the piston ring ends occur on the same side as your failure. Since you stated that the failure occurred on the exhaust side, then the pistons were installed backward if those were the original pistons. But I doubt that those were the original pistons because Polaris pistons do not have an arrow that looks like that and they do not have the embossed numbers on the top of the piston, as your pictures reveal. Ask the person that sold you the sled, whether he did a new top end just before he sold you that destroyed sled. The exhaust port is one large port, whereas the intake ports are multiple smaller ports. Free ends of piston rings will have difficulty not grinding away at any port when that free end occurs where the port is. None of the ports are supposed to occur where the piston ring ends occur.

 

[BeartoothBaron]

Just a recommendation after fighting the 550 engines. Remove the oil pump and mix your fuel at 40:1. They love oil.
If you are running ethanol fuel you will need to jet up as well.
The oil boils/air entrains at the pump from engine heat. The 550 Polaris fix doesn’t work well. Pre-mixing your fuel is the best answer.
Check those pilot jet, make sure they aren’t plugged.

Hopefully not too much of a threadjack, but I've got a couple Fuji fans I plan to get going (one a 440, the other a 488). I'll keep this in mind with the 488; the 440 has no oil pump to begin with. Would running better oil help with the boiling and aerating problem? Any recommendations for oils for fanners? I'm thinking of Castrol 2T snow for pre-mixing, but I did quite a bit of research a while back and couldn't come up with a great answer. The big issue seems to be flash resistance: fan motors see more piston/ring heat than liquid cooled if they're run WOT for long periods. I suspect that could cause issues for some, but most fanners are in more utility-oriented sleds, so it doesn't crop up.

Starting at the "bad" end, it seems to me running cheap TC-W3 oil in a fan sled that's run wide open a lot is a bad recipe. A snowmobile-formulated oil is better, but if there's an ideal oil for fan sleds, I've never heard of it. Hard to say if any recent oil formulations for two-strokes are even designed with with fan-cooled engines in mind; could be a "basic" oil (like Polaris blue) would be as good or better for a fanner than much more pricey synthetics. One thing I've thought about doing for a while is running oil intended for chain saws, maybe Stihl's or Husqvarna's oil. Should check all the boxes: made for fan-cooled engines that run hard and hot and designed for pre-mix. Maybe I'm missing something, but the only downside I can see is it could get spendy quick...

 

[retiredpop]

Without getting into an oil debate I always found Shell Advance ( dino oil) to be pretty good. I think the synthetic might just be a waste of money for premix.

 

[TRS]

Polaris blue works great.
I have stayed away from the full synthetics in the fan “cooked”sleds.
As an example. I had an issue with Klotz, at elevation, it wouldn’t burn off. It gummed up the piston and cylinder.

 

[TurboSportTSi]

So...I'm trying to do a pressure decay test before tearing into the motor again, but can't seem to find pipes or caps that fit the carb boots, as they are smaller than my newer sleds with bigger motors. The y-pipe outlet is the same size, so that's taken care of.

I was able to get parts that fit the intake side of the carbs so I did the test using the carbs, but the carbs are leaking under pressure. Can this test be done with the carbs in place? Can't tell if the test is invalid or if the carbs have an issue.

 

[TurboSportTSi]

Update: I was able to coerce the test kit directly into the factory carb boots...the motor holds pressure perfectly! Looks like I won't have to go into the bottom end. Still not sure if carbs should be able to hold pressure.

Pulled the jugs off and the mag side (which was a used replacement) scuffed a vertical line on the piston, dead center on the intake side from the bridge in the port. Must've been out of round. If it wasn't for that, I'd be tempted to throw a new set of rings on it, ensure the oil system is bled, and see what happens. Not sure what's up with the oily buildup on the mag side crown.

I think I'm going to send both jugs off to be bored 0.020" over and, once it has been confirmed that they CAN be saved, I'll order an oversized top end kit. Any other suggestions while things are apart....again? I want to replace the carb boots, but the replacements all have terrible reviews and the factory ones are holding pressure for the time being.

Anything obvious in the attached pictures?

 

[Gunit111]

Carbs would be a pita to seal up (the biggest problem is the throttle cable). Build a flat plate and drill it to match the boot adapter bolt holes; I use lexan.

Can you get oversized pistons w/chrome rings? Usual practice for a plated cylinder is to re-plate it (or exchange program) and use standard pistons.

First picture.... Does that scuff catch your finger nail or feel it? If so, then you need to recondition the cylinder. If not then re-ringing it is an option as long as the piston to cylinder clearance and piston diameter is in spec.

Second pic.... wipe the cylinder down with muriatic acid and leave it for 15 mins and rinse. Worn/scuffed through or flaking plating will easier to see.

If you do go with the 20 over option or the replating, send the new pistons with the jugs so the machine shop can match the final bore size with the piston clearance. If you use an exchange program have that same supplier provide the new matching piston kits.

 

[whoisthatguy]

Update: I was able to coerce the test kit directly into the factory carb boots...the motor holds pressure perfectly! Looks like I won't have to go into the bottom end. Still not sure if carbs should be able to hold pressure.

Pulled the jugs off and the mag side (which was a used replacement) scuffed a vertical line on the piston, dead center on the intake side from the bridge in the port. Must've been out of round. If it wasn't for that, I'd be tempted to throw a new set of rings on it, ensure the oil system is bled, and see what happens. Not sure what's up with the oily buildup on the mag side crown.

I think I'm going to send both jugs off to be bored 0.020" over and, once it has been confirmed that they CAN be saved, I'll order an oversized top end kit. Any other suggestions while things are apart....again? I want to replace the carb boots, but the replacements all have terrible reviews and the factory ones are holding pressure for the time being.

Anything obvious in the attached pictures?

It looks like both pistons are shot due to the gouge on the intake side. Since 2 sets of std pistons and rings can probably be bought for $100 on ebay, it is probably cheaper to get standard nicasiled cylinder heads, or get yours nicasiled, instead of jumping up to a larger size setup. Or you might get away with just sanding down any cylinder gouging with 2000 grit, and then go with new pistons. Consider the value of the sled after repair, compared to what you are intending to pay for repairs. You don't need perfect compression on a fan engine, because it is usually for beginner sledders.

 

[TurboSportTSi]

It looks like both pistons are shot due to the gouge on the intake side. Since 2 sets of std pistons and rings can probably be bought for $100 on ebay, it is probably cheaper to get standard nicasiled cylinder heads, or get yours nicasiled, instead of jumping up to a larger size setup. Or you might get away with just sanding down any cylinder gouging with 2000 grit, and then go with new pistons. Consider the value of the sled after repair, compared to what you are intending to pay for repairs. You don't need perfect compression on a fan engine, because it is usually for beginner sledders.

As it turns out, neither piston has any gouges at all. Can't catch anything with a fingernail, so it looks worse than it is. The one original jug has some scoring right where one of the head studs is, so it's probably warped from a previous overheat. I'm going to have a machine shop freshen up the bores and will get new pistons if I need to, based on what they say. Might just be a ring replacement...

Like you alluded to, I'm definitely not looking for perfection here, as long as the motor lasts for a few years. Performance is dead last on the priority list.

 

[TurboSportTSi]

Carbs would be a pita to seal up (the biggest problem is the throttle cable). Build a flat plate and drill it to match the boot adapter bolt holes; I use lexan.

Can you get oversized pistons w/chrome rings? Usual practice for a plated cylinder is to re-plate it (or exchange program) and use standard pistons.

First picture.... Does that scuff catch your finger nail or feel it? If so, then you need to recondition the cylinder. If not then re-ringing it is an option as long as the piston to cylinder clearance and piston diameter is in spec.

Second pic.... wipe the cylinder down with muriatic acid and leave it for 15 mins and rinse. Worn/scuffed through or flaking plating will easier to see.

If you do go with the 20 over option or the replating, send the new pistons with the jugs so the machine shop can match the final bore size with the piston clearance. If you use an exchange program have that same supplier provide the new matching piston kits.

As usual, you're right on the money (have I mentioned how incredibly helpful you have been??? Thank you so very much.). It was leaking from the throttle cable. I was able to get the test done properly, so all good now...just wasn't sure if the carbs needed some attention. Sounds like it might be pretty normal.

10 and 20 over pistons are available, so that shouldn't be an issue if it comes to that.

The scuff you are asking about doesn't catch a nail horizontally, but does vertically, which is odd. The other jug has some scoring, likely due to overheating, so I'm just going to get them both to a suitable level. I will evaluate plating after the machine work since they need touched up anyway, at a minimum.

From what I've seen so far, exchanges are actually pretty pricey ($350-400/jug + core), so I'm going to have a machine shop hone the bores just enough to get a decent surface on both and we'll go from there.


And...as embarrassing as it is, I may have found the cause of the big variance in compression... After scraping and cleaning all gasket surfaces, I somehow missed the head gasket on the original jug (it was a dull gray and not too far off the color of the cast aluminum jug) and put the new one on top of it. Wow...

 

[Gunit111]

Two head gaskets would be a problem! Oops!

One other suggestion for you; Weather you buy new pistons or not, buy new rings one oversize bigger than the pistons you are using and file fit the rings at 0.004" per inch of bore. I have been finding very large end gaps on new matched rings.

 

[TurboSportTSi]

You know, it's funny you say that, because I've had the opposite experience, at least lately.

The last two sets of pistons I've installed, the ring gaps were on the tight end of spec! I even had to swap rings around on my wife's Rev because one was at min spec or slightly tighter in one bore, but another fit better.

I was sweating it because I've never had to file rings and was hoping to not break that streak.

 

[TurboSportTSi]

I'm tellin' ya, the original gasket matched the jug almost perfectly. It wasn't until I looked at them side by side was it obvious that they looked different. Craziness.

 

[Gunit111]

You know, it's funny you say that, because I've had the opposite experience, at least lately.

The last two sets of pistons I've installed, the ring gaps were on the tight end of spec! I even had to swap rings around on my wife's Rev because one was at min spec or slightly tighter in one bore, but another fit better.

I was sweating it because I've never had to file rings and was hoping to not break that streak.

I put a 900 top end on my 95 ZRT800; .018 end gap on supplied rings. Too much imho but it is just a ditch banger. A friends early 90s Indy 650 .028 - .030 on supplied rings. My sons '96 680 Ultra .032 on supplied rings. The Polaris' were both SLP piston kits I believe and the Kitty was a Wiesco kit. On the 680 the ring to ring land was a bit snug as well. Now, this could be (or likely is?) an argument of OEM vs aftermarket or to some extent forged vs cast.

With your ring gap running on that thin port bridge you do not want .030 gap. That gap will grow 8-10% by the time the rings seat in.

Extra gasket: Prepping a sled for vintage (leafer) STOCK class snow drags, I placed a set of base gaskets on the crankcase. Twice my son handed me a perfect prepared jug for installation and twice I fought with the base gasket on the jug while clearing the pistons ring. Never noticed the problem for a week... until the tech inspector wanted to have a word with me. Sometimes you know right away that it doesn't matter what you say, the guy wagging his finger in your nose will not believe anything you do say. The person who hasn't missed something, has never done anything!

 

[TurboSportTSi]

Haha, agreed! Glad (?) I'm not the only one that fights with base gaskets. I thought it was cool that 2 strokes have convenient cutouts on the bottom of the cylinders for compressing rings, but so far, it hasn't made it very easy! Maybe more gap would make for an easier install...and less torn base gaskets!

Good point on the width of the port bridge...it isn't very wide at all. I want to say the pistons I used spec'd a range of like 7-13 thou. I could see that bridge being ~50 wide, so a 30+ thou gap would sure make a guy nervous!

 

[TurboSportTSi]

Small update for those still interested. After getting some other projects completed, I wanted to revisit this sled. Looking at two things: engine/clutch performance and the stiff-turning track.


Stiff track:

I've got the skid out and the only two things I've found that would make the track hard to turn by hand are a gritty feeling driveshaft bearing (PTO side) and a warped brake rotor. I have pulled the pads out and the track was still hard to turn in spots, so I am ignoring the rotor for the time being. The jackshaft bearings check out and the chaincase side driveshaft bearing feels great. There is no way that the driveshaft bearing being a little rough would make the track hard to turn...though I am replacing it, of course. Would like some closure on this but not sure I'll get it... Damn thing feels like the parking brake is on sometimes. It wouldn't even roll down my old tilt trailer, when my other sleds would. Anyway...


On the engine/performance side of things:

I pulled the secondary clutch apart and cleaned it. All looks great inside! Has the correct spring (Silver) and everything slides smoothly. The buttons are a bit worn, so I am replacing them while I'm in there.

Ordered a puller for the primary, but disassembled it as much as I could and found that the clutch cover bushing has all kind of gouges in it. The shaft doesn't appear to have any big marks on it, but the cover does not slide smoothly at all. The weights all move freely and everything else appears serviceable. Has the correct spring (Dark Blue/White), but considering replacing or changing. Will probably order buttons...would be silly not to.

The clutch cover bushing wasn't coming out easily, so I came across this while looking for a removal tool or a new or rebuilt clutch:
https://gilomeninnovations.com/stock-oem-polaris-clutch-cover-for-p90-standard-primary-clutch/

For $40, I will just order a new cover w/ bushing...I am assuming they didn't change much over the years...the bushing/cover design is the same, visually. Also, I had no idea how old the drive belt is, so that has been ordered as well, so we are starting as fresh as reasonable.

The other little project is that I have decided to install a tach in the factory blank spot. I think I've found a compatible one, so I'm excited to get it installed. Should help troubleshooting immensely! For all I know, the 'loud exhaust' was due to the engine at max RPM just to start moving. Based on the clutch cover issues, I could see that being the case. There will be no question now.

 

[whoisthatguy]

You appear to have overlooked all of the bogie wheel bearings. I guarantee that the bearings all need replacing. Do not buy Polaris bearings. The three rear bogies are probably already nearly frozen up. They are the only ones that you need to pay more than $5 each for. The side bogies can be $2.50 bearings that you buy by the dozen on Ebay. The driveshaft bearings require replacement at both ends because the left side blows first so you do the right side at the same time, along with the right side seal. You buy both together in a kit off of ebay, with the seal and snap ring. The secondary clutch bearing rarely goes out because there is no stress on it and everyone keeps it greased. While you have your chaincase off, you should change the top bearing as well.

For the primary weights, you check for sideplay because that can cause a clutch to explode. You reduce sideplay by buying new weights and new bolts that hold them in place. For the shaft, you sand down any corrosion with 2000 grit sandpaper and apply graphite. Forget about replacing the clutch cover as that is impossible to do correctly. So don't screw up any inserts inside of the clutch cover. The copper cylinders also require spacers at each end. You can buy them all in a kit and replace them, along with the guides.

Don't waste your money on a tach because it will cost you more than the sled is worth.