Chainbrake screw on 500i keeps disappearing!

[Alan Smith]

Hi guys, don't know if anyone else has experienced this before but it's driving me potty.  My Stihl 500i , now coming up 2 years old keeps vibrating out the screw that anchors the flywheel side end of the chain brake handle/guard.  As originally fitted this is a Torx head M5 x 20mm screw. I have tried cleaning the threads with acetone followed by Loctite.  I even cross drilled the head of one screw and lock wired it admittedly the wire was on the thin side and eventually broke and again the screw was lost.  

 I used to service and repair small engine kit and have plenty of spare screws, used and new to replace lost screws. Best result so far has been changing the M5 screw for the Stihl proprietary coarse thread, brutal I know but this screw has lasted well till this morning when it disappeared during cross cutting some large Ash logs, long bar and of course plenty of vibration.

 Anyone come across this issue and have a solution?

[Muddy42]

- Does either the male or the female thread look stripped or worn after cleaning? Is there any way a screw has been used that is M5, but too long?  I've seen this cause damage before.

- You could try the stronger more permanent Locktite red. which can be removed with a bit of heat.

- or try epoxy if you think the screw is a bit loose.

You could try to repair the thread with a tap or drill it out to M6 and re tap. Note ideally for blind holes you need a square/flat ended tap.  Or if that fails use a helicoil.  I have done all three of these techniques in sequence to various Stihl saws (not MS500i)

- I think the hole is blind so you cannot get a nut in behind, but have a look.

 

 

 

[Alan Smith]

Cheers,   yes I upgraded the Loctite level this morning. Will wait and see what happens next. 

 

 

I don't believe the female threads are stripped and I have not seen any of the screws that have fallen out.

 

 Good idea on using a bolt, I'll investigate that further. Equally re-tapping etc. I have a fully equipped engineering workshop on site so I'm happy dealing with this sort of thing it was just that I have never encountered this sort of problem over 40 years of using chainsaws of all sorts of sizes.

[Muddy42]

I come across this problem quite frequently on second hand saws and all over the place - bar nuts, saw dogs, vibration mounts etc.  I blame the bell ends who use impact drivers to scrunch in the screws - its fine to undo screws with an impact driver and maybe get the screw started on the way back in, but its best to tighten the last bit by hand. Also as said, often you get two sizes of screw that are the same thread.  Its easy to mix the long and short ones up.