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Author Topic: Stihl MS391 blew its muffler  (Read 537 times)

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Offline deminin

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Stihl MS391 blew its muffler
« on: November 06, 2017, 08:18:44 pm »
I was chainsawing some dead trees over the weekend, when suddenly my MS391 shot its muffler about 20 feet.  The saw still ran good, albeit with a lot more noise.  I said a couple of choice words, and quit for the day...picked up the muffler, and went back to my shop to assess the damage.  I appears that one of the mounting holes on the cylinder head broke a piece off, and the other screw quickly worked loose.  Today, I patched the mount with some JB Weld, and ordered an thread insert kit.  My plan is to drill out the cylinder head slightly, install a set of M5 inserts, then reinstall the muffler using some permatex loctite 271.  If this works, the muffler should be installed for life, if the permatex (red) is supposed to form a permanent bond.  A new cylinder/piston kit for this saw would run over $300...almost half the price of a new saw. 

If this doesn't work, I will be in the market for a new saw....and probably a slightly smaller saw....perhaps a Stihl 271. 

If anyone has any ideas for a better way to fix this muffler issue, or some insight on a new Stihl, Husqvarna, or Jonsered "homeowner" saw, I would welcome any input.  I have 40 acres of forest, and I generally cut about 7 or 8 cords of deadwood every Winter to feed my outdoor wood furnace.   

Offline Gearbox

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Re: Stihl MS391 blew its muffler
« Reply #1 on: November 06, 2017, 09:25:29 pm »
Loc Tite will not stand up to the heat . even the new green can be loosened with a bunch of heat ( maybe 600 degrees )
A bunch of chainsaws a BT6870 processer and not near enough time

Offline Crusarius

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Re: Stihl MS391 blew its muffler
« Reply #2 on: November 07, 2017, 07:05:55 am »
I agree with gearbox. Red is not permanent it can be released using heat. It may, or may not slow down the fastener from backing out but it will not prevent it.
I knew what I thought I meant.

Offline pineywoods

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Re: Stihl MS391 blew its muffler
« Reply #3 on: November 07, 2017, 08:47:01 am »
Use new screws, properly torqued. We assume when a screw becomes loose, it's because ir backed out. NOT true in most cases. What happens is the screw stretches in length. Once a screw or bolt has stretched beyond it's limits, you can never keep it tight...
1995 Wood Mizer LT 40, Liquid cooled kawasaki,homebuilt hydraulics. Homebuilt solar dry kiln.  Woodmaster 718 planner, Kubota M4700 with homemade forks and winch, stihl  028, 029, Ms390
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Offline deminin

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Re: Stihl MS391 blew its muffler
« Reply #4 on: November 07, 2017, 10:11:46 am »
Yup, I've got new screws....but the problem is that one of the "holes" in the cylinder head physically cracked.  I'm going to try the JB Weld/screw insert/Red Loctite rig, and if that doesn't work, I will just run this saw without a muffler, until it finally craps out.  It is very noisy, but I always wear my helmet/facemask/ear protectors when chainsawing, and the nearest neighbor is 1/4 mile away...so noise is a minor issue.  I've had this saw for 7 or 8 years, so I've gotten my use out of it...and at my age, 75, i need to think about getting a smaller saw, anyway. 

Offline Crusarius

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Re: Stihl MS391 blew its muffler
« Reply #5 on: November 07, 2017, 11:27:28 am »
won't you burn the valves without the muffler?
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Offline old guy

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Re: Stihl MS391 blew its muffler
« Reply #6 on: November 07, 2017, 11:47:28 am »
Yup, them valves will be warped the first day. (snicker)

Offline sablatnic

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Re: Stihl MS391 blew its muffler
« Reply #7 on: November 07, 2017, 03:12:08 pm »
won't you burn the valves without the muffler?

With a bit of luck it will be an old 2stroke without valves to burn. Only the cylinder and piston will end up being ruined.

Offline Texas-Jim

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Re: Stihl MS391 blew its muffler
« Reply #8 on: November 07, 2017, 06:13:12 pm »
I have never had a problem using heli-coils for just that problem.
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Offline Al_Smith

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Re: Stihl MS391 blew its muffler
« Reply #9 on: November 07, 2017, 07:02:54 pm »
Just for general info most Stihls use serrated belleville type washers on the mufflers .More times than not if the bolts are M5 and the hole is stripped it's easy enough to re tap it for M6 using a spiral  point tap . I have never even seen a 391 so I have no idea what holds the muffler on .Evidently  whatever did didn't do such a good job of it .

Offline ZeroJunk

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Re: Stihl MS391 blew its muffler
« Reply #10 on: November 07, 2017, 07:20:26 pm »
Haven't seen a 391 either. But, most of those home owner saws have bolts with a square head that hook in a slot in the cylinder and a nut screws on the threaded end that comes out the muffler cover.  Easy enough to replace. Unless, the cylinder slot broke.

Offline drobertson

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Re: Stihl MS391 blew its muffler
« Reply #11 on: November 10, 2017, 10:18:10 pm »
go with the 362 if you choose to get another, I've had both, still have the 362,  the 39 was liked by others ,, nuff said, but it was a gas hog, so, 362 will do the job,
only have a few chain saws I'm not suppose to use, but will at times, one dog Dolly, pretty good dog, just not sure what for yet,  working on getting the gardening back in order, and kinda thinking on maybe a small bbq bizz,  thinking about it,

Offline deminin

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Re: Stihl MS391 blew its muffler
« Reply #12 on: November 11, 2017, 04:40:01 pm »
Success...for now.  I repaired the cracked screw hole with some JB Weld, and let it cure in the heated garage for 24 hours.  Then I tapped the screw holes and installed a pair of M5 heli-coils.  I then installed the muffler with a liberal amount of Loctite 271 on the screws and heli-coils, and added a lock washer between the screws and muffler, and tightened it up.  I let it cure for 2 days.  Today, I fired it up, and took down 2 good sized dead oak trees...ran the saw for over 2 hours, and so far, the muffler is still tight.  The way it ran without the muffler makes me think about welding up a "header" and letting it run with no backpressure from a muffler....the RPM's and power were substantially higher without the muffler. 

Thanks for the tip on the MS362...the R-CM model sounds promising...with its Elasto Start function...at my age, that might be a plus.  If I blow this 391 up, I'll take a serious look at the 362 R-CM.

Oh, and BTW...I didn't have to fool with the "valves", and none of the screws were "stretched"...chuckle, chuckle....

Offline teakwood

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Re: Stihl MS391 blew its muffler
« Reply #13 on: November 12, 2017, 08:20:38 am »
The elasto start is just a built in bushing, damper in the handle that you pull to start the saw so its a little bit softer for you body. You can buy that handle separately and ad  to your saw if you want.

Offline deminin

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Re: Stihl MS391 blew its muffler
« Reply #14 on: November 12, 2017, 12:04:06 pm »
I looked up the Elasto start, and it wouldn't do much for this big chainsaw.  What I would like to see is a version of the Stihl "easy to start" on their chainsaws.  I have that on my trimmer and cultivator, and with just one easy pull, the spring loaded unit operates the cylinder with ease.  Now, That, would be a real plus on these higher compression chainsaws.   

Offline johnny newburgh

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Re: Stihl MS391 blew its muffler
« Reply #15 on: November 12, 2017, 01:09:00 pm »
Personally I think you should go with electric start option.

Offline ZeroJunk

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Re: Stihl MS391 blew its muffler
« Reply #16 on: November 12, 2017, 08:18:51 pm »
Stihl has Easy Start on chainsaws, just not the larger displacement saws.

Offline Al_Smith

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Re: Stihl MS391 blew its muffler
« Reply #17 on: November 13, 2017, 10:04:55 pm »
A former sponsor of this site had their own version of the noted elasto start.It was just a spring inside of a tee handle which worked well .Fact I had several given away by that  former sponsor at an Ohio chainsaw GTG .I kept one for a Stihl MS 200 and gave the other away to the owner  of another web forum .It did work well though .