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Discussion Starter · #1 ·
So I was riding my 700 last weekend and it started running poorly on one side. I was thinking I had issues with my exhaust valve, but when I got home I found the jug was cracked. I had to limp it home for about 2 miles where it was mostly running on one cylinder. Has anyone seen anything like this before? Any ideas on the cause? I'd like to avoid doing this again.

I'm thinking the cracks may have been there for a while. I noticed it ran very poorly the previous two rides until it was fully warmed up. I'm thinking that maybe these hairline cracks were causing a loss of compression that disappeared when the engine got hot and the metal expanded. Eventually they got large enough that even the expanded metal couldn't hold the compression up enough, which is when I limped it home running on one cylinder.

Right side is the side with the cracked Jug

[attachment=276656:Heads.jpg]

This is the piston from the left

[attachment=276657:Good_Side.jpg]

This is the right piston (cracked side)

[attachment=276659:Broken_Jug.jpg]

Here are the cracks in the jug. They are very hairline. When I took it apart, the right side of the jug fell off but the left is still attached and not cracked all the way through.

[attachment=276658:Cracks.jpg]
 

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The cylinders, head and exhaust manifold should ideally not be fully torqued down until all are attached so as to not put stress on the cyl while it is aligning itself.[/b]
That's a good point - I never thought of attaching the manifold before torquing the head and jugs down.
If the machine was smashed before, a hit to the exhaust could also cause a crack like that.
 

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Discussion Starter · #7 ·
Not impacts on this sled (except the suspension). I have a freind that used to own a polaris shop, and according to him this is something they have seen before. Not sure of the reason it happens but 700's and 800's from 02 on sometimes have issues at the base of the cylinders like this.
 

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Clayton,
I would be interested in seeing the fracture surface. Is it completely broken in two? After you take it apart, see if you can get a good close up macro photo showing the fracture. Part of my job is failure investigation.
 

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Discussion Starter · #9 ·
Sure. It's already apart. I'll try to get a pic. The front crack is not all the way through. That piece is still attached to the cylinder. The back piece was partially attached still, but only by the last pieces of aluminum down by the base gasket. It's now all the way off of the cylinder. If I can get some photos before I swap it in as a core, I'll upload them here.
 

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you sure they will take that as a core? i tryed turning in a cylinder once that had a nick in the top by the o ring grooves and they said nope.
 

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Discussion Starter · #13 ·
No they didn't take it. I do have the pics for you Polaris Dave...they are just on my camera and I haven't taken them off yet.
 

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check your fuel pump. the pulser line is in the MAG side of the motor. if the diaphrams are ripped, the pump will push raw fuel up the pulser line and into the crankcase. this leads to piss pour performance and heavily flooded cylinders, possibly a hydro-lock situation like yours. seing your head is wiped clean, no carbon, it looks very gas washed to me. it should still have compression since the cylinder did not pop off, but pumping that much gas through it would have a fouled cylinder and poor performance for sure. it's worth checking out before re-assembly. also check your needle and seat in the float system of your carbs. if you have some junk in the needle and seat, or they are worn, it will also flood your motor. usually the float seats fail as a pair, not one at a time, with wear. of course debris will fail one at a time. lastly, check your crank seals. if it truely was a hydro-lock, it also over pressured the bottom end and most likely damaged the crank seal.
 

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upon farther inspection of your pics, i'm an idiot. the PTO side cracked out, correct?? i kinda got lost in your left and right description, i usually describe left and right from sitting on the seat. when the PTO cracks like that, you are getting crankcase flex. i've seen about 20 sleds that have had it happen. the torque of the motor is more than crankcase can support. an engine baseplate is a good idea. since you are already working the motor over, it is a good idea to tear it all the way down and inpsect the run-out on the crank. with that much case flex, the crank is taking a beating.
 

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Discussion Starter · #16 ·
Didn't tear it all the way down to the crank. Put in a new jug and new pistons. I've got about 250 miles of riding it like I stole it miles on it so far and no issue. Here are the close ups I promised


[attachment=286429:IMGP1667.JPG]

[attachment=286431:IMGP1669.JPG]

[attachment=286433:IMGP1670.JPG]
 

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Discussion Starter · #17 ·
upon farther inspection of your pics, i'm an idiot. the PTO side cracked out, correct?? i kinda got lost in your left and right description, i usually describe left and right from sitting on the seat. when the PTO cracks like that, you are getting crankcase flex. i've seen about 20 sleds that have had it happen. the torque of the motor is more than crankcase can support. an engine baseplate is a good idea. since you are already working the motor over, it is a good idea to tear it all the way down and inpsect the run-out on the crank. with that much case flex, the crank is taking a beating.[/b]
Yeah you are right, I labeled them from the front not the seat. Oops!
 

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Very interesting fracture. The fracture appears to have initiated at the corner of the gasket surface and the aluminum 'sleeve' and propogated to the outside. The darker area is likely the initiation side where fuel was coming through from the cylinder bore. If the base of the cylinder/gasket surface is not machined properly and the radius between the 'sleeve' and gasket surface is not large enough, this will cause a stress riser and will be the likely place for a crack to initiate. Once the crack starts in a casting like this, one single overload event can cause failure. The overload could have been the hydrolock that someone mentioned or even related to cyclic flexing of the crankcase.
 

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Discussion Starter · #19 ·
Very interesting fracture. The fracture appears to have initiated at the corner of the gasket surface and the aluminum 'sleeve' and propogated to the outside. The darker area is likely the initiation side where fuel was coming through from the cylinder bore. If the base of the cylinder/gasket surface is not machined properly and the radius between the 'sleeve' and gasket surface is not large enough, this will cause a stress riser and will be the likely place for a crack to initiate. Once the crack starts in a casting like this, one single overload event can cause failure. The overload could have been the hydrolock that someone mentioned or even related to cyclic flexing of the crankcase.[/b]
If you look to the right on the bottom picture you can see where the other crack (shown in the original pics) had developed but the aluminum was still attached.

I would have thought I had pretty high miles for a machining issue to expose itself. Wouldn't something like that show up in the first 500 miles or so?
 
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