Can't keep muffler on saw

[bendjoseph]

The muffler on my Stihl MS391 will not stay tight.  I tighten them while the block was hot an they still came loose.  Can I use a heat resisit lock tite?

[T Welsh]

You can try it! Will not hurt a thing, but they will usually back out and you lose one of them on the ground somewhere. I found a full proof way of keeping them tight is to safety wire them in like on aircraft applications! Drill through the head of both screws and wire tie them to each other with stainless steel wire. Tim

[AdkStihl]

Mufflers.......heh.....theyre overrated   ;D

[Al_Smith]

A "star" washer will work better than a lock washer .If you could find a belleville type washer small enough it would likely work better .

Tie wire is a sure fired cure .You can drill a standard socket head cap screw but I doubt you have much success with a torx head ,hard like a rock .

[clww]

As I'm sure you are aware, be real careful not to over-torque. If that head breaks off, you'll really have a situation. >:(

[bendjoseph]

half the threads are gone in each hole!  Looks like a Heli-coil repair kit is in order.

[joe_indi]

half the threads are gone in each hole!  Looks like a Heli-coil repair kit is in order.

This might be a better fix, because its what I do on 460s and 660s.
The standard screw is a M5X16. You have no threads in the holes, so tap the holes to take M6 screws.
You dont need any fancy tapping tools. Just use a small grinder to shape the lower ends of the M6 screw to resemble taps. Taper the tips slightly and cut  4 slits on the threads 1/3 of the length.
You might have to use an old chain file to enlarge the screw holes on the muffler, nothing to it just a 5 minute job.
Use the M6 screws to tap 6mm screws in the mounting holes.Screw them in, back off a bit, screw them in further, back off a bit and so on till you sink the screws to the 'hilt'.
By the way, use M6X18  or M6x20 screws for this job.
I dont think you get M6 screws in these sizes, what I do is use readily available M6X25  for the job, they are the cylinder screws for the MS660.
I cut them to the required size.
The M6 screws are a major improvement when you come to think of it.They bear the muffler's weight much better.
That is why Huskys have M6 screws on their mufflers.

Joe

 

[Al_Smith]

You can punch right through a wallered out M5 tap hole with an M6 spiral point tap .

Going back ,under torqued fasteners fail faster than over torqued .If for example you snug up a screw on a hot engine the aluminum is at full expansion .Once it cools the fastener torque will lessen because the material shrinks .

To get full thread strength in a material on steel the depth of the hole has to equal the thread diameter .On cast iron it's double ,on aluminum it's 4 times minimum .So you'd need nearly 1" of thread depth on M6 .

So if you're going to retap it ,first a spiral point .Then blow the chips out with air and use a bottoming tap .Tap a bit,blow etc .

[AdkStihl]

You can punch right through a wallered out M5 tap hole with an M6 spiral point tap .

Going back ,under torqued fasteners fail faster than over torqued .If for example you snug up a screw on a hot engine the aluminum is at full expansion .Once it cools the fastener torque will lessen because the material shrinks .

To get full thread strength in a material on steel the depth of the hole has to equal the thread diameter .On cast iron it's double ,on aluminum it's 4 times minimum .So you'd need nearly 1" of thread depth on M6 .

So if you're going to retap it ,first a spiral point .Then blow the chips out with air and use a bottoming tap .Tap a bit,blow etc .

Perfectly put!!  ;)

[bendjoseph]

After a couple of e-mails from Stihl, I have decided to sell all my Stihl equipment and switch brands.  I work in the manufacturing field, the threads were crossed threaded during assembly.  Stihl feels since I am past my one year mark that it is my responisablity.  The saw has 43 hours on it.

[Al_Smith]

Now what's that going to  prove other than you can spend money?

Why not instead of selling a saw for half what you paid for it and spending more money on a new saw instead spend less than 20 on a couple taps and two M6 torx head screws .

That way instead of being the screwee you become the screwer in a manner of speaking . ;D

[clww]

I've gotten more than upset with a Stihl chainsaw more than a few times over the years, for one thing or another, but sell all my equipment from one manufacture? That's a bit over the top IMHO.

[joe_indi]

Al has it right.
Just two screws should not be the reason for you to remain the screwee forever.
Use  two  6mm as a fix and become the screwer, because that way they dont get to replace the cylinder, which is what they might do, at your expense.

Joe

[bendjoseph]

I work in aerospace manufacturing.  When I see workmanship at such a low level, I get scared about the rest of the saw.  If that was a Husky that I had bought from the Box store, they would have replaced it at no charge.

Stihl has no respect for their customer's. 

[beenthere]

Stihl has no respect for their customer's.

I've not had that experience at all. But each of us look at problems differently.

I for one, get tired going back to the box store to return the cheap goods. After a few trips, they are not so cheap any more. When they repackage the returned items and put them back in stock without testing them, the next unsuspecting customer ends up with an item that doesn't work.  :

[joe_indi]

I had a MS460 in today with a stripped muffler thread.
These pictures show the method I use. It takes less than 10 minutes for this job
Joe
Thread on the left is stripped

I converted a M6x30 screw into a tap using a hand held grinder


I tap a M6 thread using this screw

I cut the screw to size

The muffler is tightened down

[JohnG28]

Stihl has been pretty good to me also. I'd also say that other companies would probably leave you in a similar situation given the same circumstances. Not the company, just the nature of the world today IMO. They make a good product but there will be problems. I have had issues with low hour pro equipment, things happen. I'd make the quick fix and get back cutting if it were me. 

[joe_indi]

The muffler on my Stihl MS391 will not stay tight.  I tighten them while the block was hot an they still came loose.  Can I use a heat resisit lock tite?

I read this first post again and this is what struck me.
At times, we (read as I) look for complicated solutions for simple issues.
The original Stihl spline screws are designed to be self locking.That is why the screws have teeth on the lower side of their heads.
In critical areas such as the muffler and cylinder, it is advisable to use new screws because those teeth might not do their job of staying locked in position.
Probably, your muffler will stay in place if new screws are used, that is, if the threads are still there.Otherwise, you need to use new oversize self locking screws.
Joe

[Al_Smith]

Generally speaking Stihl makes a good product .I get a little miffed at the prices for repair parts but that just comes with anything you order OEM .Doesn't make a difference if it's Ford ,Caterpillar or parts for your dishwasher .

If you had if for over a year and a dealer didn't look at it while  under warranty  then obviously the warranty  period has expired .Simple as that .

I can understand how a person to get kinda miffed with a new saw falling apart .I get rebuilders myself and tweek and twitter them so such matters are just common place with me .

What happens if you get a Husqvarna and it falls apart ? Another good brand but they're not perfect either . ;)