Stihl MS 390 carnage - disassembly.

[MacAttack]

New P/C ~0.002-0.0025". Double that and it's getting sloppy, much less and it may seize.

My P/C clearance and ring gaps seems good. The replacement cylinder has a "B" cast into it. Piston does not have any visible markings except a part number cast inside. Is there a way to know if it's the right Piston other than checking the clearance? (Sorry if this is a dumb question)

 

[Automan]

 

[stihltech]

There was a time when you ordered pistons by A and B sizing. Have not seen it since the mid 80s.

 

[MacAttack]

There was a time when you ordered pistons by A and B sizing. Have not seen it since the mid 80s.

The cylinder appears to have a year 2000 casting date, if that means anything?

I got the piston and rings installed in the cylinder and it feels perfect.

 

[MacAttack]

Ready for the clam shell to be assembled:

 

[Gord404]

nice

 

[MacAttack]

Look very closely at the impulse hose. They love to crack and create air leaks at the bend radius. Causes lean condition. See it on most I have repaired.


View attachment 1242383

I took another look at my impulse line and guess what.... the sumbitch is about to split!!

Thank you for telling me about this.

 

[MacAttack]

Engine assembled and installed in the housing. Now I just need a new impulse line and the patience to put it all back together.

 

[Automan]

Trash the spark plug

 

[MacAttack]

Trash the spark plug

Yeah I'm not going to use that plug, it's just in there to keep anything from accidentally getting inside the cylinder. I'll get an NGK plug for this saw.

 

[MacAttack]

Hey Stihl experts...can you watch this youtube video I made and let me know if this little bit of drag on the clutch drum seems normal or not? I can't find an issue, it seems to be assembled correctly. I did apply a generous amount of grease to the needle bearing.

 

[Bob Hedgecutter]

Is the clutch drum slot lined up to catch the oil pump drive arm?

 

[Bob Hedgecutter]

If that is aligned properly- is the large flat cover washer on the oil pump- below the clutch proper- on the crank correct way up?
Correct is with the side marked TOP facing out towards the clutch.

 

[MacAttack]

Is the clutch drum slot lined up to catch the oil pump drive arm?

I think this was my problem! I reassembled and could feel the drum "drop" into place while turning it. Now seems like it should be. Thanks!

 

[MacAttack]

If that is aligned properly- is the large flat cover washer on the oil pump- below the clutch proper- on the crank correct way up?
Correct is with the side marked TOP facing out towards the clutch.

Yep I paid careful attention when I installed that washer. The problem was the metal hook for the pump not in the right spot.