Rebuild?

Yaeger34 · May 10, 2018

Got my questions answered about what changes I can make on the sled and I appreciate the response

Since the my 18 has 3200 miles I am considering just throwing in a top end even though compression feels strong. What is compression on these. Pair of oem piston kits are $200. So I'm sure under $300 with all gaskets and o rings. I haven't done a sled rebuild since the 90s and 2000s sleds. A few banshees and about 20 fourstroke motocross bikes. Anything specific in need to watch for or replace while I'm at it? Is it worth it if compression ia good? Again not sure what compression ia. Thanks again

summ8rmk · May 10, 2018

Learn about the 3valves per cylinder before u take it apart.
I guess u dont have to remove them but i would , to make sure they're all clean.

kiliki · May 10, 2018

It's a relatively new motor so nobody knows how far it'll go but on a 2-stroke generally two to three thousand miles is a good idea

dunatyk · May 10, 2018

Country Cat has the full service manual on their website available to view

Fish10782 · May 10, 2018

Generally you'll feel the engine loose power in a long pull. That's a huge sign it needs a new top end. It will even show decent in a compression test but things will change when it's wide open under 100% load for 4 minutes straight.
3,200 miles IMO is to long with out a top end. Very least head would come off for inspection.

The Suzuki engine here where I mostly ride will need a top end about 2500 and then entire rebuild about 5500 ish. Long pulls, heavy loads, and in air density in the 14,000 ft + most days.

Suzzy-Q · May 10, 2018

Things may look fine but with those miles you will be getting near the thin piston skirt and possible engine failure that I’ve seen with high/hard miled cat engines. Granted I’ve seen higher miles before failure but it’s just a good idea to change pistons now to save a bigger bill later

Big10inch · May 10, 2018

I think people who just do top ends are crazy. It is hardly any more work to do the bottom end crank seals. Maybe you can get away with it but I wouldn't. M1000s used to lose the pto side all the time after a top end. I have had two apart at 1700-2000 miles AFTER someone else did a top end. On both there was dirt inside the seal. If dirt can get through, air can and that is what burns down a lot of 2 strokes.

If you are going to do it, I would do the whole job.

Suzzy-Q · May 10, 2018

Big10inch said:
I think people who just do top ends are crazy. It is hardly any more work to do the bottom end crank seals. Maybe you can get away with it but I wouldn't. M1000s used to lose the pto side all the time after a top end. I have had two apart at 1700-2000 miles AFTER someone else did a top end. On both there was dirt inside the seal. If dirt can get through, air can and that is what burns down a lot of 2 strokes.

If you are going to do it, I would do the whole job.

M1000s were harder on cranks but you can do 2-3 sets of pistons on an 8 before worrying about the crank, most 800 crank issues I’ve seen came from failed pistons

kiliki · May 10, 2018

Suzzy-Q said:
M1000s were harder on cranks but you can do 2-3 sets of pistons on an 8 before worrying about the crank, most 800 crank issues I’ve seen came from failed pistons

^^^^this I've had many of thousand cranks and the 800 definitely go a lot further than the thousands do

sno*jet · May 10, 2018

id give a quick look down the exhaust ports, if the pistons look good, minimal scratching (tiny ones), run it.

Big10inch · May 11, 2018

Suzzy-Q said:
M1000s were harder on cranks but you can do 2-3 sets of pistons on an 8 before worrying about the crank, most 800 crank issues I’ve seen came from failed pistons

I didn't say there was an issue with the crankshaft. I stated that it is a VERY good idea to replace the crank seals. They distort and wear with mileage. If it creates a bottom end air leak you will lose a piston, not the crank.

How do you know what caused the piston to fail? I think bottom end leaks kill more pistons that you realize.

Yaeger34 · May 11, 2018

While we are on the subject of piston failure. On a 4 stroke I know more often than not a piston failure takes the whole deal (most often). On a 2 stroke dirtbike odds are good it will just be a piston and or cylinder, maybe head, but not often the crank in my experience. So on a sled when the piston fails what are we typically looking at most the time. Piston and cylinder mostly? Or does it take the head, crank, etc?

kiliki · May 11, 2018

sleds run way more RPM than dirt bike do and when you lose a piece of piston it passes between the crank and the case leaving a large mess. when this happens you only have one good jug and a 5 gallon bucket full of recycle. if you get lucky you have one smear and stick.

boondocker97 · May 11, 2018

kiliki said:
sleds run way more RPM than dirt bike do and when you lose a piece of piston it passes between the crank and the case leaving a large mess. when this happens you only have one good jug and a 5 gallon bucket full of recycle. if you get lucky you have one smear and stick.

That's not really true on the RPM. The two stroke bikes run similar RPM ranges, but you'll have to find someone on a MX track running it hard to see similar high RPM pulls to what someone riding a sled hard sees. The 4stroke MX engines run higher RPM than a sled does.

In a two stroke the same type of piston failure should result in similar carnage in either type of engine. However, if you loose a cylinder on a sled it has the other one to possibly continue propelling it to further destruction (crank and possibly knocking a hole in the block). In a 4 stroke you can get lucky and just have a crown crack and loose compression without any further damage (happened to me once). If it comes apart though, you'll be going through the entire thing.

Also, I was just thinking since the Ctec pistons are slotted, I wonder if they will be more prone to skirt fatigue earlier than the Suzuki was?

Yaeger34 · May 11, 2018

Ya I was going to say my Ktm sxf350 hits around 13,000 rpm. Although I imagine the lines on snow where you are on the pipe wot for a significant amount of time can really where it out. On the other hand hitting whoop sections on a bike has to put some serious stress on a crank.

I think to play it safe I am going to do a top end. Probably skip the crank seals to be honest. Get another 1,000 to 1,500 out of the sled before it gets sold.

Big10inch · May 11, 2018

Yaeger34 said:
I think to play it safe I am going to do a top end. Probably skip the crank seals to be honest.

That is how most people do it and, with a pretty good degree of success.

One advantage you have is that it has not burned down, it is still running well.

Good Luck

jim · May 18, 2018

It is a good idea. Yeah, total rebuild is probably better but not necessary. Top end can have a lot of stuff waiting to fail. Pistons fatigie and can drop a chunk of metal into the bottom...which gets bad fast. Plus a lot of heat cycles. Also overlooked is the top wrist pin and bearing...hot pistons certainly heat and dry the top bearing more. You can also have ring landings growing in size due to cycles...this makes the metal weaker. Also consider that the low end doesnt see the hard cycling a top end does. So, lots of variables. Can you go another 2k? Maybe...could be fun to see what it can do...but is expensive to find out. For the cost in gas for a few rides, you can have a fresh motor. Also, top end is not a great indicator of a worn top end...low to mid is. Top can be strong still due to the air being compressed so fast...not so much with lower rpms and throttls response situations. So, yes, do it. Great insurance and a performance maintainer.

Big10inch · May 20, 2018

jim said:
It is a good idea. Yeah, total rebuild is probably better but not necessary. Top end can have a lot of stuff waiting to fail. Pistons fatigie and can drop a chunk of metal into the bottom...which gets bad fast. Plus a lot of heat cycles. Also overlooked is the top wrist pin and bearing...hot pistons certainly heat and dry the top bearing more. You can also have ring landings growing in size due to cycles...this makes the metal weaker. Also consider that the low end doesnt see the hard cycling a top end does. So, lots of variables.

Your last claim about the low end not seeing the stresses of the top end demonstrates a real lack of understanding about the engine.

All of that "hard cycling" heads on down to the crank which takes that energy and changes its direction. Then the drivetrain of the sled pulls on the crank in another direction with massive leverage from the clutches. There is WAY more stress on the bottom end, no contest.

What I recommended doing is replacing the crank end seals to protect the new top end. I do not think new bearings and complete crank disassembly is needed but those seals wear and leak.

You are all correct on 2 stroke pistons failing though, some much quicker than others (Polaris). I bet they fail more quickly when bottom end air is creating lean conditions on your top end. It is a good idea to go in and replace them. I just do not understand the resistance to splitting the case, replacing the seals and moving forward with a better chance of survival and longevity. Skipping this step is like playing Russian roulette, five will live, one wont. Good Luck

89sandman · May 20, 2018

Just to be safe I'd rebuild it after every ride

Heck maybe do it when you break for lunch...

If you're one of the people who think you need to ride it into the ground every time you get on it then I'd be doing the top every thousand.

If you're a little smarter and know how to actually use it at a normal level I've pulled apart the Suzuki motor with more than 5k that have never been touched that still had hatching throughout the cylinders.

Kind of like people who can't keep a belt on it for more than a few hundred miles, if you're not smart enough to tell when your belt is overheating and needs a rest it will cost you.

Big10inch · May 21, 2018

89sandman said:
If you're a little smarter and know how to actually use it at a normal level I've pulled apart the Suzuki motor with more than 5k that have never been touched that still had hatching throughout the cylinders.

More fundamental lack of understanding... Unless you gall a piston up you will always still see the cross hatching. It doesn't wear off of nikasil coatings like it does in cast iron. It is harder than the rings and pistons. The fact that you could still see the original hone after 5000 miles means nothing. It certainly doesn't mean the pistons are still good or the rings are sealing well.

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