MS 391 seized oil pump X2

[PoginyHill]

First issue I've had with bar/chain oil on this machine. It's maybe 10-12 years old. Bar was running dry. Removed the oil pump and it has seized. Could not turn the gear wheel and the plastic worm gear ring (that goes over the crankshaft and has the arm connected to it) was stripped. No problem. Purchased an OEM pump and worm gear. Installed and verified it pumped oil. First time I used it, no oil. Removed the replacement pump and same problem: seized pump and stripped worm gear. Instead of another $65 for an OEM pump, I have a cheap one on the way. This time I cleaned the tank and line from the tank to the pump. I'm thinking some gunk in the line may have lodged in the pumps causing them to seize. Unless I'm just unlucky with a new pump. Any other thoughts?

[doc henderson]

my 046-mag. pump bit it cutting firewood 40 miles from home.  it was packed with wood chips and dust, and oil.  it broke, but the replacement worked fine.  clean everything well.  I did not notice it till the drive sprocket broke and it had been running fine for half a tank, then it would die.  replaced the fuel line that had a crack at the bend, and the drive sprocket.  the oiler was found by my Stihl mechanic who put it back together after working through his lunch.  took it back apart and replaced the oil pump.  RIP Sylvan.

[PoginyHill]

Think I found the smoking gun. The last oil pump still turned freely, but the worm-gear drive (plastic) was completely chewed off. I noticed the crankshaft had significant slop. Perhaps 0.050-0.100". Don't know if it's just the crank bearing or something with the plastic case. Either way, I don't have the appetite now to repair it. Plan to pick up a MS 462C-M to replace it. Quite an upgrade. More hp and less weight. Paid under $600 for the 391 in 2014.

I had suspected a loose crankshaft, but wasn't able to feel any play in it the couple previous times I had the clutch removed. Perhaps the piston position impacts whether I can move the crank by hand. I had noticed a lot of noise running it - like something was loose or rattling around somewhere, but was never able to find a culprit. I guess getting a new saw will put all these unknowns to bed.

I will probably take the 391 apart someday - just because I'm curious. If it's an easy fix, I'll repair it and gift it to someone.