Compression differences between cylinders

[elf]

Is there an acceptable difference between cylinders? My 2010 600 RMK with 6800 miles is moving on to my kids now. In doing some post season repairs/maintenance to it I checked compression last night. The mag side is at 123 PSI, that side was rebuilt at about 3000 miles, so it has 3800 miles on the new jug/piston. The PTO side, which is still original, is at about 112 PSI. I figure next summer might time to freshen up that side with at least new rings, maybe a whole top end. So, will I have any issues with running it this way for another yr? Its running great right now.

Overall, the sled is in amazingly good shape. I noted earlier that I had to weld up a crack in a rail and repair a couple egged out holes but when I looked over the track there's not a torn lug on it, I changed out the original slides but really didn't need to. Only on my second set of carbides. While I'm not rough on sleds I also don't baby them. But we do usually ride in excellent snow conditions, not a lot of road riding (outside of forest roads) or road crossings.

 

[ezra]

IMHO at the point u are at it is kind of crazy not to at least just toss a top end in it .what are we talking $250 bucks and a few nights in the garage teaching the kid how to work on his toys ?

 

[ac600sp]

A 2 stroke engine should be above 100 psi. Multiple cyclinders should be within 10% of each other. With that kind of miles and one side being lower I'd do a rebuild before something catastrophic happens and its cost much more. A few hundred bucks and it will be good for many more miles.

 

[srt20]

That's enough of a difference for me to open up an engine.

 

[elf]

So it looks like the consensus is this fall do a top end on the PTO side. I can handle that.

 

[groomerdriver]

So it looks like the consensus is this fall do a top end on the PTO side. I can handle that.

Just trying to learn this stuff.....what happened to the mag side that you did it at 3000 miles? Is it "better" for the engine if you keep both cylinders fresh/the same?

 

[srt20]

Pistons are cheap. Do them both.


IMO it is not critical to have fresh on both. But it makes good sense to maintain properly. I have been forced to run an engine with massive difference in compression,175psi on one and 100psi on the other. It certainly didn't run properly, but it did run. Lost a wristpin circlip and made a little mess on the piston and cylinder. We were in mtns and didn't have much choice.
True story, made a base gasket out of a Leininkugels beer case, covered in RTV. Worked great, no leaks. Was last trip of last year. I didn't take it apart til last fall. Never leaked a drop. Put a 150 or so miles on it.

 

[ezra]

So it looks like the consensus is this fall do a top end on the PTO side. I can handle that.

why do just 1? what are u saving 80 bucks? good money after bad . do it right then u are good for yrs
the work involved is less than 20 min more to do both.
U don't have to but why would u not

 

[elf]

why do just 1? what are u saving 80 bucks? good money after bad . do it right then u are good for yrs
the work involved is less than 20 min more to do both.
U don't have to but why would u not

I guess I hadn't given much thought to doing both sides and I certainly can but it's more like $220 for a piston thru polaris than $80. I guess a question would be if the jug looks fine on the PTO side should I replace that? or just do the piston on that side? I replaced the jug on the mag side when I did that but it was all scored up.
And I'm not nearly good enough to do a second side in 20 minutes. Actually the biggest PITA was getting the fuel rail off the back of the jugs, not enough room to work in there. Wish there was an easy way to remove the airbox.

I guess I should be sure we are talking the same thing. When I say top end I'm referring to piston and cylinder. Is that what you're referring to by a "top end" also. Or are you just talking piston?

 

[srt20]

I guess I hadn't given much thought to doing both sides and I certainly can but it's more like $220 for a piston thru polaris than $80. I guess a question would be if the jug looks fine on the PTO side should I replace that? or just do the piston on that side? I replaced the jug on the mag side when I did that but it was all scored up.
And I'm not nearly good enough to do a second side in 20 minutes. Actually the biggest PITA was getting the fuel rail off the back of the jugs, not enough room to work in there. Wish there was an easy way to remove the airbox.

I guess I should be sure we are talking the same thing. When I say top end I'm referring to piston and cylinder. Is that what you're referring to by a "top end" also. Or are you just talking piston?

Doesn't the airbox come out easy? My IQ has a rubber strap, IAT sensor, and fuel filter. Then pull up and out. Yours is different?
If jug is fine, reuse it. They don't wear out with Nikasil cylinders.
You can use polaris pistons, but its clear they are inferior to almost any aftermarket piston that costs half or less.
Fuel rail should be 2 bolts and gently pry back on the whole rail to remove both injectors at once. Keep track of green orings, probably stay in cylinder.

 

[elf]

Doesn't the airbox come out easy? My IQ has a rubber strap, IAT sensor, and fuel filter. Then pull up and out. Yours is different?
If jug is fine, reuse it. They don't wear out with Nikasil cylinders.
You can use polaris pistons, but its clear they are inferior to almost any aftermarket piston that costs half or less.
Fuel rail should be 2 bolts and gently pry back on the whole rail to remove both injectors at once. Keep track of green orings, probably stay in cylinder.

I would guess my airbox is the same but it sure doesn't look that easy, maybe it is. If that comes out easy then the fuel rail will be easy. What brands of pistons would you recommend if not using OEM? I've had good luck with them so far.

 

[mezz]

I don't want to sound like a tard, but, unless something is wrong with the piston(s) why can't you just do the rings? Really, if you nip it in the bud early enough, you should be good to go with a simple & cost effective solution, again, as long as the piston(s) are good to go.-Mezz

 

[srt20]

I would guess my airbox is the same but it sure doesn't look that easy, maybe it is. If that comes out easy then the fuel rail will be easy. What brands of pistons would you recommend if not using OEM? I've had good luck with them so far.

I don't have a favorite piston brand. Whatever cheap.
Probably SPI if you want cast pistons. Stock pistons are cast.
Probably Wiseco if you want forged pistons. Forged pistons are stronger and more durable. But make sure they are 4032 alloy. 4032 alloy expands much less than 2618 alloy. 2618 alloy is the pistons that people would seize.

I have used both and had a ring locator pin come out on a SPI piston. It wrecked cylinder and piston. BUT, the engine is not stock and detonation is a very likely cause of that happening as well. So that very well could have been my fault.
Had no problems with Wiseco. Wiseco will be more expensive than SPI. I have used SPI on stock 600s and had no problems as well.
Had excellent durability out of Wossner pistons too. They are also a forged piston.

 

[srt20]

I don't want to sound like a tard, but, unless something is wrong with the piston(s) why can't you just do the rings? Really, if you nip it in the bud early enough, you should be good to go with a simple & cost effective solution, again, as long as the piston(s) are good to go.-Mezz

Thats also an option, but if you take it all apart you would be foolish not to measure piston to cylinder clearance. I would put money that the high mile low compression piston is well worn. I would also bet that the 3000 mile piston is worn down from new, significantly as well.

I know its highly controversial, but my opinion is that the 600 poo engine suffers from the same design flaw as the 800. And I've seen it personally. And thats all I will say on that subject to avoid polluting this thread.

 

[elf]

I don't want to sound like a tard, but, unless something is wrong with the piston(s) why can't you just do the rings? Really, if you nip it in the bud early enough, you should be good to go with a simple & cost effective solution, again, as long as the piston(s) are good to go.-Mezz

I'm not a good enough mechanic to answer that. I'm perfectly fine just doing rings also as long as the piston is good. I've certainly gotten a lot of good feedback about doing this, enough that I'm wondering what I should do.

Rebuilding the mag side the decision was easy, had a melted piston and scored cylinder. Somebody had asked what happened and the theory is I leaned it way out. The reason being is that we stopped for gas and I put 11.5 gallons in a 11 gallon tank so I was running the crap on the bottom of the tank. Filled sled up at the Holiday in Tofte and 4 miles later down it went. Was running a long forest road at a good clip, the 600 road up by the Trestle Inn for those familiar with the area. Luckily I was only 100 yds from the junction with Hwy 7.

 

[ac600sp]

I'd do both Pistons. Why not, your already in there doing one side, a 2 stroke runs better with even compression on multiple cyclinders. If the other cyclinders is not scored I'd home it out and install a new piston and rings. I'd recommend the factory parts for replacement. They can be found on eBay if you have the factory part #'s.

 

[ezra]

I guess I hadn't given much thought to doing both sides and I certainly can but it's more like $220 for a piston thru polaris than $80. I guess I should be sure we are talking the same thing. When I say top end I'm referring to piston and cylinder. Is that what you're referring to by a "top end" also. Or are you just talking piston?

why use OEM ? I would toss in some spi kit if u are more of a let it warm up some what and go. or if u are a guy who is sure sled is warm before hitting the gas get the wisco kit . U can get a set of wossners prob 1 of best out for less than oem poo I think last 1 I helped do wossners were like 130 MSRP each think we got both for 190 .IMHO all 3 are better than what poo has been putting out .with 3k on the 1 piston why risk it ? and yes just gasket kit pistion kit and a nice diamond hone that any local shop can do for like 20 bucks a hole as u brows the new inventory.
prob no diff but if buying over counter I ask them to bring out a few and I find a close to matched weight as I can find. call me crazy but drop a few on a scale some day

 

[groomerdriver]

I know its highly controversial, but my opinion is that the 600 poo engine suffers from the same design flaw as the 800. And I've seen it personally. And thats all I will say on that subject to avoid polluting this thread.

If Elf doesn't mind his thread getting jumbled up, please tell me your thoughts. This fall my oldest son will be buying a new (leftover 16 is most likely) Poo 155 RMK. He's undecided about the 600 vs. 800. Originally I had him talked out of a 2010-14 used 800 in favor of a used 600. He is now leaning 800 in a new sled as the Poo 800's seem to be as reliable as any up in his neck of the woods (Alaska).

 

[snoluver1]

I don't want to sound like a tard, but, unless something is wrong with the piston(s) why can't you just do the rings? Really, if you nip it in the bud early enough, you should be good to go with a simple & cost effective solution, again, as long as the piston(s) are good to go.-Mezz

Replacing the piston is more about unseen fatigue. Even if it looks good and measures good the piston skirt is prone to failure in a 2 stroke. It's fairly cheap insurance if you have the piston out already, to just replace it.

 

[srt20]

If Elf doesn't mind his thread getting jumbled up, please tell me your thoughts. This fall my oldest son will be buying a new (leftover 16 is most likely) Poo 155 RMK. He's undecided about the 600 vs. 800. Originally I had him talked out of a 2010-14 used 800 in favor of a used 600. He is now leaning 800 in a new sled as the Poo 800's seem to be as reliable as any up in his neck of the woods (Alaska).

IMO, the poo engines have life expectancy similar to the other 2 stroke 800s WITH PROPER MAINTENANCE. Piston replacement IS maintenance. Especially on the poo engines. In fact, IMO the poo bottom ends, crank, bearings, etc, is very stout.

Don't be afraid to buy a poo. But realize if you want a long engine life, you might have to replace pistons before damage occurs. I own 3 Poo CFI sleds. None have stock pistons.
My 13 Switchback 800 with 2800 miles on it ran as good as the day the engine was first started. Compression was 123 psi in both holes. Looking at the pistons through the exhaust, they looked great, slight wear on the rings but certainly useable. I took it apart anyway. Piston skirts were well collapsed. IIRC, .08 piston to cylinder clearance. That will run fine, but for how long?? For $206 for pistons and gaskets, and a couple hours of my time, I had a new top end. You need the right tools to properly measure the piston to cylinder clearance. And its not measured with the piston in the cylinder and a feeler gauge. And rocking the piston in the bore with the head of tells you nothing either. It must be disassembled to proper measure. And at that point, if you have stock pistons, why would you put them back in? I guarantee they have collapsed at least some. (that guarantee does not apply to the 800HO in the Axys sleds because I haven't had one apart yet, though I highly suspect it will be the same. The design flaw is still there)

Anyway, I know I didn't write out the design flaw, because Im sure it will open a can of worms. Maybe if the OP has all the info he wants Ill post it in here.