Stihl MS290 Farmboss chain trouble

[Mr Mower Man]

I have a Stihl MS290 Farmboss in the shop that's doing something I've never seen before. The owner uses the saw for about 15 minutes only to find that the bar nuts are vibrating loose and the chain gets extremely tight. He adjusts the chain tension, tightens everything up, and it does it again a few minutes later. I'm guessing that the bar nuts loosening allows the bar to fall (since you're supposed to hold the nose up when tightening the nuts), which makes the chain tighter. But what's making the nuts come loose? The chain is far from new, but I can't see any obvious flaw in the chain that would cause an issue. Then again, I'm somewhat of a novice when it comes to saw chain.

Anyone seen this phenomenon before? What would you do?

[bgsengine]

- Cracked crankcase where bar studs mount
- broken anti vibe mounts
- damaged bar stud threads
- sawdust build up between sprocket cover and bar plate (or the bar plate and bar mount)
- wrong bar for the application
- bent chain adjuster screw
- stripped bar nuts
- worn out sprocket (especially rim sprockets)


Basically you need to strip it all down to the bare bar studs and clean everything so you can check for cracks and worn sprocket and then check bar plates, bar mounting, bar adjuster , bar studs and threads, bar nuts, chain, and the sprocket cover and replace whatever ain't kosher.

[KE4AVB]

Basically you check everything as BGS suggests.

A few additional things to check while you at it.

I would also make sure the oiler working and that bar grooves are clean. Lack of oiling will cause saw chains to tighten up from overheating. With old chains they do wear and stretch to out of pitch causing a lot excess vibration. This is just like any roller chain that wears and stretches to point where it tries climb on top of the cog teeth.

Should have right bar, chain, and sprocket but verify it anyway. I do get several saws in every year with the wrong bar, chain, and sprocket combination.

Additional Note Added: If bar studs are loose, don't drill out mounting holes. Stihl should still have available the oversized studs they came out to repair this problem. Repair instructions are in the service manual if you need a copy.