Stihl MS 390 carnage - disassembly.

[MacAttack]

A very kind member sent me a Stihl MS310 (or an 039, it's a bit of a mystery) with a blown engine. I'm in the process of disassembly. It seized so abruptly it actually sheared the key off the flywheel. I have a good cylinder and piston but at the very least will probably need a crank / rod assembly.

I need to remove the clutch, any recommendations of a specialty tool that you've had good experience with?

 

[chainsaw-57]

MS 390

 

[MacAttack]

MS 390

How can you tell?

This is my first Stihl.

 

[chainsaw-57]

"49" 0n cylinder description 49 mm
Flip caps 039 had screw caps

 

[Chainsaw Pete]

A very kind member sent me a Stihl MS310 View attachment 1240840

What is that tray under the saw? I like it! I've used a cookie sheet but things roll around and get mixed up and even roll off.

Sorry I cannot help you...

 

[Gord404]

the worst thing I have seen in those saw s 290-390 is the bearing keepers go- some sort of black crap they use- doomed to fail- I think by the looks of it might be able to take a normal 6203-c3 with quality- they could have done better for the price of those saws- imo

 

[ZeroJunk]

If it is seized it should just screw off with a 19 MM wrench. Righty loosey.

 

[MacAttack]

If it is seized it should just screw off with a 19 MM wrench. Righty loosey.

Is there a hex head on the clutch? The clutch drum is held on with a clip, and then the clutch assembly requires a special tool (I think)

Edit: you were absolutely right, I just got the clutch off. Engine is actually not seized...but cylinder still needs to come off.

 

[MacAttack]

Guys....I'm stumped, need input. Turns out, engine is NOT seized, and the bore and piston look GOOD!

BUT...it turns over with some resistance, like a bearing is dragging or something. Are these known to have main bearing seize / partially seize??

How did it shear off the flywheel??

 

[Chainsaw Pete]

It could shear off the flywheel if it was running and the piston/crankshaft suddenly stopped, or if it was running and something suddenly stopped the flywheel. I've never heard of keys just breaking on their own.

Do I see a chamfer on the piston - left top edge? Or is that simply reflected light in the photo? I think the cylinder still has to come off so you can inspect inside. The piston rings look to be pushed into their grooves. I've not seen that on the engines I have taken apart.

Full disclosure - while I've worked on motorcycles and car engines, I've not spent any time inside my yard equipment's two strokes, so you need to wait for more knowledgeable folks.

 

[MacAttack]

It could shear off the flywheel if it was running and the piston/crankshaft suddenly stopped, or if it was running and something suddenly stopped the flywheel. I've never heard of keys just breaking on their own.

Do I see a chamfer on the piston - left top edge? Or is that simply reflected light in the photo? I think the cylinder still has to come off so you can inspect inside. The piston rings look to be pushed into their grooves. I've not seen that on the engines I have taken apart.

Full disclosure - while I've worked on motorcycles and car engines, I've not spent any time inside my yard equipment's two strokes, so you need to wait for more knowledgeable folks.

Cylinder definitely has to come off, there's a bad bearing or something.

The workmat I'm using is a Grypmat, gift from my wife, they work very nice!

https://toolboxwidget.com/collections/tool-mats

 

[MacAttack]

the worst thing I have seen in those saw s 290-390 is the bearing keepers go- some sort of black crap they use- doomed to fail- I think by the looks of it might be able to take a normal 6203-c3 with quality- they could have done better for the price of those saws- imo

Are they prone to seizure?

The bottom end of the engine seems to have some serious resistance.

 

[slowcar281]

remove the cylinder, separate the engine pan and have a good look at the bearings. Although I’ve never seen it happen, it’s possible that the big end rod bearing could have failed which means a new crankshaft is needed.

Was the flywheel nut loose at all? Maybe it could have rattled around a bit and broke the key ?

 

[MacAttack]

remove the cylinder, separate the engine pan and have a good look at the bearings. Although I’ve never seen it happen, it’s possible that the big end rod bearing could have failed which means a new crankshaft is needed.

Was the flywheel nut loose at all? Maybe it could have rattled around a bit and broke the key ?

It was completely loose! Came off with my finger. But I have no idea of the history of the saw...the previous owner might have pulled the flywheel.

 

[ZeroJunk]

Has my curiosity up.

 

[slowcar281]

It was completely loose! Came off with my finger. But I have no idea of the history of the saw...the previous owner might have pulled the flywheel.

Possibly. But either way that cast flywheel key just acts as a reference point. The taper and bolt torque hold the flywheel in place.

Curious to see what you find when you pull the cylinder

 

[Chevboy0167]

I say complete teardown..... these look concerning.

 

[MacAttack]

I say complete teardown..... these look concerning.


View attachment 1240957

That white fleck is a piece of the rope that I accidentally fed into the exhaust port and cut off with the piston when I used it as a piston stop to remove the clutch.
The other spot I have no idea.... but it's coming apart.

 

[MacAttack]

Possibly. But either way that cast flywheel key just acts as a reference point. The taper and bolt torque hold the flywheel in place.

Curious to see what you find when you pull the cylinder

Yep me too!