Rebuilding The Rotax 399cc

[fowler]

Need to find specs for ring groove clearence and bore

Then measure them up

And whats the obsession with digital ****
Get a standard micrometer

Learn how to read it and your away

 

[exenos]

A general a rule of thumb with two strokes is if its apart, replace it. Personally at a minimum I would be replacing both pistons, no matter how they look, and the crank seals for sure along with the normal gaskets that go along with a top end rebuild.

Do you have any idea why the piston went? Figure that out before you do anything else because if its not fixed then the piston will probably go again.

 

[firemanjim]

Looks can be deceiving...... Can you spot a hairline crack by eye? That piston needs a thorough inspection before its used again, by someone who knows what they are doing. (no offense) Because you have to ask, is all the more reason to have some one ELSE look at it......
BUT in all honesty.... Buy 2 new pistons..... Have the bores gone over. Not by you, by a professional....Do you want to keep spending money and time on mistakes? Do it right, now, while you have the opportunity. Make it run like a swiss time piece. Spend the money and do it right...... And you will never have to worry about the engine again..... (with proper maintenance ) If you don't have the bread now, grab a lawn mower and get to cutting.... Quick easy way to get cash..... You can pull off $100 a day easy, around here, and not even work but 5-6 hours.....

 

[rushking19]

This wouldn't have been too expensive to rebuild until I found out I cracked the cylinder

 

[exenos]

That's just the sleeve. You should be able to replace it. Heat up the cylinder and push it out, freeze the new one and it should just slide in. Its a little bit more involved but not much more.

 

[rushking19]

That's just the sleeve. You should be able to replace it. Heat up the cylinder and push it out, freeze the new one and it should just slide in. Its a little bit more involved but not much more.

I'll have to look around for a cylinder sleeve

 

[n3480h]

I'd replace both sleeves. Same reasons as replacing both pistons. Looking at the inside of the cylinder in the pic above, it appears to be trashed, regardless of the crack. Exenos's advice about replacing the crank seals is very good, because these types of engines are known for having problems related to their crank seals.

 

[rushking19]

I'd replace both sleeves. Same reasons as replacing both pistons. Looking at the inside of the cylinder in the pic above, it appears to be trashed, regardless of the crack. Exenos's advice about replacing the crank seals is very good, because these types of engines are known for having problems related to their crank seals.

If I replace the seals that means I have to tear the whole thing and mess with timing And i don't have the tools to time

 

[exenos]

You shouldn't need to replace both sleeves, just the one that is cracked and have both bored to the next oversize piston. Replacing the crank seals could mean messing with timing but you don't need to change the timing. Just make a mark on the stator and one on the crank case that line up.

 

[rushking19]

I spent today tearing apart the snowmobile engine, removing the pistons and there no signs of leaking from the seals.

 

[rushking19]

Turns out I need to replace both sleeves because I looked at the good cylinder and there are indents in the bottom of the cylinder

 

[exenos]

You may not need to replace that sleeve. Personally I would figure out what caused the damage then just take off any high spots. That part of the sleeve is not too critical and it should be fine.

 

[rushking19]

You may not need to replace that sleeve. Personally I would figure out what caused the damage then just take off any high spots. That part of the sleeve is not too critical and it should be fine.

Ok thanks

 

[rushking19]

Do y'all think that this is worth the price,or a good idea to buy for 55 doldollars and 26 dollar shipping

 

[Denny]

From the looks of the cylinders that engine should never have been started let alone run with all the rust I see in there. Always replace the pistons in pairs or you will throw off the balance of the engine and to a small degree (very small) the port timing. 2 strokes are pretty finicky do it right the first time and save your self a bunch of . My buddy and I tried to as cheaply as possible rebuild a 440 rotax I think it was (he didn't want to put the money into it) we ended up rebuilding it twice I was . Remember used engines are just like a woman you never know exactly how they work until you are married to them!

Good luck,
Denny

 

[rushking19]

From the looks of the cylinders that engine should never have been started let alone run with all the rust I see in there. Always replace the pistons in pairs or you will throw off the balance of the engine and to a small degree (very small) the port timing. 2 strokes are pretty finicky do it right the first time and save your self a bunch of . My buddy and I tried to as cheaply as possible rebuild a 440 rotax I think it was (he didn't want to put the money into it) we ended up rebuilding it twice I was . Remember used engines are just like a woman you never know exactly how they work until you are married to them!

Good luck,
Denny

I am going to replace both pistons, that was the plan but I was wowondering if I could buy and use this cylinder and piston and also buy anew Mag side piston

 

[Denny]

Use the cylinder but not the piston. But a new next oversize piston and ring set and have the pistons fitted to the cylinders and then don't mix them up keep matching pistons and cylinders together. Also remember air cooled snowmobile really don't like warm temperatures (above 30 degrees f.) so make sure there is extra room for piston and ring expansion.

Denny

 

[rushking19]

Use the cylinder but not the piston. But a new next oversize piston and ring set and have the pistons fitted to the cylinders and then don't mix them up keep matching pistons and cylinders together. Also remember air cooled snowmobile really don't like warm temperatures (above 30 degrees f.) so make sure there is extra room for piston and ring expansion.

Denny

But that piston is for that cylinder

 

[BirdFanatic]

Hey looks like the original rotax engine that was in my Malibu.They used a rotax a snowmobile engine (ie my 440) and the vanguard

 

[exenos]

But that piston is for that cylinder

Is that a new or used piston? It looks like its been used and the seller has cleaned the top, also there is no crosshatching visible in the cylinder which would normally mean that it has been well used.

As far as for denny saying that fan cooled sled engines not running well above 30*, well it not the heat that does it. It the resulting lower density air that is causing the engine to run pig rich. Dropping a few jet sizes will fix that problem as long as you don't go to low and cause it to go lean.