MS390 Worm Gear (038 Super pro.)

[Jack Russell]

The worm gear has threads but the oil pump does not. Is this the way that Stihl made the MS390? Then why the threads on the worm gear.

I have this 038 Super Pro. that has a rattling in it when pulling the starter rope. The piston is tight. There is some lose movement or play when working fly wheel back and forth . The saw will start on first most of the time and runs good. Why the rattling?

Jack

 

[Lakeside53]

Huh? The pump has a piston that turns as its "threads" engage with those of the worm..

"rattling" can be many things, but it's most likely a worn piston skirt (often the inlet side from bad filters) and what you are hearing is piston slap. Other possibilities are the piston pin bearing, or the crank big end bearing, but I doubt it.

 

[Fish]

The pump has a little "gear" [or should] on the end of it, that ride in
and are turned by the "threads" of the worm gear. If the pump is missing the little gear, then you need a new one.