Compression differences between cylinders

[ac600sp]

The cfi 800cc Polaris engines need the " Fix it kit" installed for long term durability and reliability. The stock piston skirts are to short and the wrist pin location is in the wrong place. Both problems make for a bad situation over some miles and use, basically a ticking time bomb if you ask me. The Pistons slap around in the cyclinders and the excessive side force with crack the skirts at some point. Everybody I ride with has done the updates and no problems since, they will self destruct if not fixed.

 

[Cirrus_Driver]

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.

God you guys are some mechanical geniuses - a wealth of information. Thanks. I'm seriously considering an 800 but certainly can't deal with a rebuild on my own. $206 is chump change for a guy that can do the labor on his own, but for a guy like me, that becomes $600. That's a deal breaker - at least for me.

 

[srt20]

The cfi 800cc Polaris engines need the " Fix it kit" installed for long term durability and reliability. The stock piston skirts are to short and the wrist pin location is in the wrong place. Both problems make for a bad situation over some miles and use, basically a ticking time bomb if you ask me. The Pistons slap around in the cyclinders and the excessive side force with crack the skirts at some point. Everybody I ride with has done the updates and no problems since, they will self destruct if not fixed.

Actually, IMO, the cylinder skirts are what is too short. Thats the design flaw. And here is why;

Polaris makes the smallest and lightest 600 and 800 engines. This is a good thing on all accounts. Except for the cylinder skirts. In order to have the smallest and lightest engines, they had had to have the bottom 5/8" of the piston skirt to drop below the cylinder skirt. But the 2 bad things about this design is they lose case volume, which can lead to lower hp, and it obviously does in the 800, and the bottom of the piston is unsupported. Because the bottom of the piston is unsupported, it rocks ever so slightly. What happens over hundreds of thousands of piston strokes with the piston rocking even slightly? The piston collapses. The cylinder is harder than the piston. The early CFI 800s had issues with piston skirts breaking. Then in 09 or 10, poo had thicker skirts in the pistons. Then they started having cylinder skirts break. In 2013, poo put thicker cylinder skirts on the intake side, the side that always broke because thats where the force was. Since 13, the failure rate has been less.

IMO, there is no 100% fix for this engine design. Fix kit pistons with shims, Indy Dans fix, etc. To properly 100% fix this issue, the cylinder skirts must be extended. All the fix kits with shims do is lessen the rod ratio on the piston. This helps, plus it gives more case volume, but its a bandaid. A good bandaid, but still a bandaid. Same with Indy Dans. He weld braces on the cylinder skirts along with less rod ratio. Anybody have the Indy Dan complete engine? Ever open it? Of course not, because he will void your 3 year warranty if somebody other than him opens one of his engines. He claimed to use Poo OEM pistons. Well, somebody opened his engine. Guess what, he used SPI pistons in it. He got called out and made some excuses. If Im spending $3500 on a "bulletproof" engine, I sure as heck want to know whats up.
Anyway, the rod ratio in fact is as small as the AC 800 engine. Hmm, arguably the best 2 cylinder 800 ever built. So its clear, the rod ratio isn't the problem. So the wrist pin is in fact not in a bad or wrong location.
And if anything, the Polaris pistons skirts should be SHORTER. But they can't be shorter, because if the are shorter, the ports will become uncovered at TDC(top dead center). And making the piston skirts longer will make the problem even worse, and they will hit the crank and BDC(bottom dead center).

So its clear, the only proper 100% fix it a complete redesign of the engine.

Im perfectly fine replacing pistons. The RMK chassis, is IMO by far the best in the mtns.

BTW, I have 3000 miles on a 12 RMK. At 1500 miles, Wossner pistons were put in. The stock pistons were bad sloppy. At 26xx miles I opened the engine and measured the pistons and cylinders. The Wossners were .002 smaller than when installed. Thats pretty dang good. Remember, these are all mtn miles. The sled doesn't ever get ridden below 6500 ft. I have a 14-1 compression head on it as well. I can't run the engine at low elevation without race fuel or I will detonate the pistons to pieces.

So as you can see, even though I just dogged on the poo engine design, it can be made to live a happy life.

The 600 is exactly the same. Though because the mass of the pistons is less, they are less prone to breaking. What happens on the 600s is the pistons rock so bad after time that the top of the pistons start rubbing on the cylinders and they knife edge. Then what happens is you lose compression at an extremely fast rate. Thats best case scenario. Worst case is aluminum breaks.

All this is from first hand experience. This isn't something Ive read on the internet. This is from polaris engines Ive opened and "fixed" or rebuilt. All measurements that Ive seen with my own eyes.

 

[elf]

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.

I have all the information I need so go ahead and post away! I appreciate everybodies feedback, while not a great mechanic I do feel comfortable doing a top end, whether it be jugs and pistons or just pistons. The time I did the mag side it took more time to disassemble than it did to replace and reassemble. Thanks again!

 

[elf]

God you guys are some mechanical geniuses - a wealth of information. Thanks. I'm seriously considering an 800 but certainly can't deal with a rebuild on my own. $206 is chump change for a guy that can do the labor on his own, but for a guy like me, that becomes $600. That's a deal breaker - at least for me.

Actually you could deal with a rebuild on your own. I'd never done one before and it was surprisingly easy. Also, as you say, theres a wealth of information on this board that people are very willing to share.

 

[srt20]

I have all the information I need so go ahead and post away! I appreciate everybodies feedback, while not a great mechanic I do feel comfortable doing a top end, whether it be jugs and pistons or just pistons. The time I did the mag side it took more time to disassemble than it did to replace and reassemble. Thanks again!

well, i didn't really wait. lol sorry.

Good luck with it. And don't drop anything into the cases, i.e. the wrist pin circlips. Put rags over the case openings. And to get antifreeze out of the bottom of the cases, stick a rag down in there and slowly turn over the primary. Works great and rag will come right back out the other side of crank.

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oh btw, that is my belief on why polaris doesn't use direct injection yet too. Its heavy and add height to the engine. Look at the doo engine. I referring to having the smallest and lightest engines.

 

[ac600sp]

Sounds like Srt20 knows it all.

 

[srt20]

Sounds like Srt20 knows it all.

I was asked, I explained. If doesn't matter to me if you agree with me or not.

 

[ezra]

God you guys are some mechanical geniuses - a wealth of information. Thanks. I'm seriously considering an 800 but certainly can't deal with a rebuild on my own. $206 is chump change for a guy that can do the labor on his own, but for a guy like me, that becomes $600. That's a deal breaker - at least for me.

I cant believe if u have been ridding any time at all u don't know some one who can give u a hand after work some night .
that poo is a cake walk have done 1 of those in the woods below zero . the poo is coming with pretty cheep ext warranty so really ride it 3 yrs and dump it with a yr of coverage left for ok money . u know u will want a new one in 3 any way

 

[snoluver1]

Sounds like Srt20 knows it all.

It sounds to me like Srt20 took the time to offer a very detailed explanation of his opinion and also how he has come to that opinion. Right or wrong can certainly be up for debate, but I get the feeling he didn't copy and paste his "opinion" off Indy Dan's website.

 

[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.

I agree with you, I didn't give the skirt failure potential a thought. I know regardless of rings & pistons, I would do both sides.-Mezz

 

[Hoosier]

Actually, IMO, the cylinder skirts are what is too short. Thats the design flaw. And here is why;

Polaris makes the smallest and lightest 600 and 800 engines. This is a good thing on all accounts. Except for the cylinder skirts. In order to have the smallest and lightest engines, they had had to have the bottom 5/8" of the piston skirt to drop below the cylinder skirt. But the 2 bad things about this design is they lose case volume, which can lead to lower hp, and it obviously does in the 800, and the bottom of the piston is unsupported. Because the bottom of the piston is unsupported, it rocks ever so slightly. What happens over hundreds of thousands of piston strokes with the piston rocking even slightly? The piston collapses. The cylinder is harder than the piston. The early CFI 800s had issues with piston skirts breaking. Then in 09 or 10, poo had thicker skirts in the pistons. Then they started having cylinder skirts break. In 2013, poo put thicker cylinder skirts on the intake side, the side that always broke because thats where the force was. Since 13, the failure rate has been less.

IMO, there is no 100% fix for this engine design. Fix kit pistons with shims, Indy Dans fix, etc. To properly 100% fix this issue, the cylinder skirts must be extended. All the fix kits with shims do is lessen the rod ratio on the piston. This helps, plus it gives more case volume, but its a bandaid. A good bandaid, but still a bandaid. Same with Indy Dans. He weld braces on the cylinder skirts along with less rod ratio. Anybody have the Indy Dan complete engine? Ever open it? Of course not, because he will void your 3 year warranty if somebody other than him opens one of his engines. He claimed to use Poo OEM pistons. Well, somebody opened his engine. Guess what, he used SPI pistons in it. He got called out and made some excuses. If Im spending $3500 on a "bulletproof" engine, I sure as heck want to know whats up.
Anyway, the rod ratio in fact is as small as the AC 800 engine. Hmm, arguably the best 2 cylinder 800 ever built. So its clear, the rod ratio isn't the problem. So the wrist pin is in fact not in a bad or wrong location.
And if anything, the Polaris pistons skirts should be SHORTER. But they can't be shorter, because if the are shorter, the ports will become uncovered at TDC(top dead center). And making the piston skirts longer will make the problem even worse, and they will hit the crank and BDC(bottom dead center).

So its clear, the only proper 100% fix it a complete redesign of the engine.

Im perfectly fine replacing pistons. The RMK chassis, is IMO by far the best in the mtns.

BTW, I have 3000 miles on a 12 RMK. At 1500 miles, Wossner pistons were put in. The stock pistons were bad sloppy. At 26xx miles I opened the engine and measured the pistons and cylinders. The Wossners were .002 smaller than when installed. Thats pretty dang good. Remember, these are all mtn miles. The sled doesn't ever get ridden below 6500 ft. I have a 14-1 compression head on it as well. I can't run the engine at low elevation without race fuel or I will detonate the pistons to pieces.

So as you can see, even though I just dogged on the poo engine design, it can be made to live a happy life.

The 600 is exactly the same. Though because the mass of the pistons is less, they are less prone to breaking. What happens on the 600s is the pistons rock so bad after time that the top of the pistons start rubbing on the cylinders and they knife edge. Then what happens is you lose compression at an extremely fast rate. Thats best case scenario. Worst case is aluminum breaks.

All this is from first hand experience. This isn't something Ive read on the internet. This is from polaris engines Ive opened and "fixed" or rebuilt. All measurements that Ive seen with my own eyes.

Good info. Are the newer Poo's the same? I thought they re-did their engines in the past couple years, at least the 800.

 

[indy_500]

Good info. Are the newer Poo's the same? I thought they re-did their engines in the past couple years, at least the 800.

Pretty much the same exact motor... They added grooving to the piston for better oil distribution, 3 stage electronic exhaust valves, electronic oil pump, and a lightweight crank with some obvious changes to the intake sides of things but as far as the motor goes that's about all they changed parts wise. FWIW you can use the 2016 HO pistons and/or crank in a 2014 non-HO. The failure rate so far on the new 800 HO is SOOO slim compared to the years past. I've heard less than a handful of 800 HO's going down on snowest, and you know if they go down, you're going to hear about it! That's not to say they haven't had other issues... But I still think it's a big improvement, to make such a small, compact, lightweight motor put out the power, and deliver the response that it does.

 

[srt20]

Good info. Are the newer Poo's the same? I thought they re-did their engines in the past couple years, at least the 800.

The design is the same. The quality of Pistons used maybe better better? I don't know yet. Haven't had one apart. My bud has a 16 Axys RMK. He only got a couple hundred miles on it out west this year. So I doubt it will be opened this summer, but you never know. He really really likes turbos....

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I agree with Indy. The throttle response of the poo 800s is stupid good. The 800HO is even better. Idk how doo can compete with that throttle response. The new lighter clutch will help, but until I drive one I just don't see it happening. It's still a heavier clutch.

Not that throttle response matters much on trails, but out west tree riding, or even in UP tree riding, it's a pretty big deal.

 

[longtrack]

Wow, I just changed the Spark Plugs on my 1200 at 8000 miles and I thought they should have lasted longer.