Had this beauty come into the shop. Pulling it over rewarded me with a sharp clacking sound. Pulled the muffler and the cylinder wasn't scored but looking through the spark plug hole I found an odd mark on top of the piston. Pulled the jug and found the transfer port cover on the clutch side pretty much blown off and the piston skirt cracked on the intake side. New Meteor cylinder kit going on now.
Stihl MS391 failure
- Thread starter Opelman
- Start date
Boy I have an MS361 it has just been flawless knock-on wood. Its about ten years old.
- Joined
- Jan 6, 2005
- Messages
- 10,255
Those are nice saws. What do you think caused this to happen?
I wonder if it was just a lot of hours. Heat cycles, metal fatigue.
4wd
$50 site donor 2026
Looks like it was used too much when it was new 
The W or M mark on the piston top would've been made by a foreign object that entered the cylinder, including broken spark plug or other part of the engine broke up and ended in the cylinder.
Interesting, there is no signs of overheating or blow by.
Stihl uses magnesium pistons in some engines, not sure if that one is.
Interesting, there is no signs of overheating or blow by.
Stihl uses magnesium pistons in some engines, not sure if that one is.
It looks like part of the transfer cover channel broke free. Look at the bottom of the cover in the last pic.
Last edited:
Fuel/ oil unknown, it did have some signs of being hot or possibly low grade oil as there is a layer of burnt oil on the underside of the piston crown. No scoring anywhere though. The bearings all look and feel good so it will be getting a new top end and crank seals this morning. I will be having a chat with the customer about 2 cycle oils. I'll probably toss in a small bottle of Stens branded oil with the repair. It is FD rated.
Pics of the piston. Appears to be oil breakdown to me.
Signs of being too lean of a fuel mixture. Piston top damage, blued piston pin and oil burnt under the piston head. Piston top got hot enough to start to melt and coke the oil under it.
Lean is mean, melts pistons more severely if bad enough.
I've blown up many a 250cc and up 2 stroke snowmobile motors in my youth in the power quest.
Lean is mean, melts pistons more severely if bad enough.
I've blown up many a 250cc and up 2 stroke snowmobile motors in my youth in the power quest.
This one went home today, it ran strong with the new top end. I ended up also replacing the broken oil pump drive, chain sprocket and bearing.
- Joined
- Jan 6, 2005
- Messages
- 10,255
What's the $$$ damage on a repair like that?
That one was right at $400.00 out the door. It is about a $700.00 saw new. I was on my quote of $350-360 until I found the oil pump drive broken from a worn sprocket and bearing. No extra labor since those pieces were already off for the top end replacement.
Last edited:
