Stihl MS391 and MS311 Main Bearing Replacement with 6202

sawfixer

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I’ve finally decided to join this forum after a couple of years of great information on fixing chainsaws. I recently encountered an issue which I haven’t seen addressed in this forum and could help others in a similar position. I found a Stihl MS391 on Craigslist for $50, which the owner stated would stall out after the cut. Thinking this was going to be a quick flip, I scooped it up and soon discovered what a “great deal” this saw was. Upon dis-assembly of the sprocket cover I found the crankshaft had a lot of movement and when I rotated the saw, it made a lot of rattling sounds. You guessed it, the great plastic FAG manufactured bearing retainer and failed and was burnt up in the chamber. Fortunately, the P&C was in good condition but the shaft seal, oil pump and worm gear was trashed.

I soon found to my discouragement this bearing is special in that it has a 38MM OD instead of the normal 35MM OD on a 6202 bearing. Worse, the price for each of these 15x38x11 bearings is over $50 a piece from the local Stihl dealer or about $20 a piece from Echo (the same bearing dimensions is on the CS600). I decided instead to use the 6202 NTN C3 bearings I already had and was able to use a 1-1/4” 4” long copper straight coupler. This works perfectly as the OD is at 38MM and the ID is slightly larger than 35MM. I used a tubing cutter to make an 11MM wide cut and after application of red loctite they fit snugly on the outer race of the 6202 bearing. I was able to use the Stihl MS230 seal which is commonly available and shares the same OD as the lower clamshell and cylinder OD for the MS311/391 and shares the same shaft OD of 15MM. Everything fit perfectly in the case and after application of Permatex Motoseal 1 on the lower cylinder and lower case, I torqued down the bolts.

So far after several cycles the saw runs great and checks out after both vacuum and pressure testing. The only potential issue is whether the bearing will spin in the copper ring over time but since I applied the loctite and there was a small amount of crush to get the cylinder and lower case to come together during the bolt tightening, I think it should be fine.

I wish I would’ve documented this via pictures but I was interested in getting the saw back into operation. Hopefully, other members can use this tip to fix their MS311/391 saws using lower cost 6202 bearings and should be able to do it for under $20 in parts, like I did.
 

tree monkey

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oem bearings are around $12.oo
if you squeeze the bearing to mutch it will fail, not enough it will spin

good luck
 
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