661C Cylinder/Muffler Repair Option

[Wood Doctor]

I was given a Stihl MS 661C that somehow vibrated its two muffler mounting bolts loose, so much so that they wrecked the threads in the piston, gouged them out, and ruined the cooling plate. The shroud even has a hole burned into it from the exhaust leak, so it will have to be replaced. I tried threading the cylinder holes larger to accept an M6 screw but they were so messed up that an M6 thread would not take. So I went up larger and tapped M8 threaded holes into the cylinder because nobody even sells M7 machine screws.

Therefore, I now I have two options for fastening the muffler to the cylinder: (1) use a pair of M8 screws in the threaded holes that I tapped onto the cylinder or (2) use a pair of M8 to M6 threaded adapters and use M6 screws. The adapters would allow smaller holes drilled into the gasket, the muffler, and the cooling plate that were designed for M5 machine screws, but it looks like drilling larger holes for using the larger M8 screws is not a major problem. So, should I go with option (1) or (2)? Please advise. TIA.

 

[huskihl]

I don’t trust helicoils or inserts inside a muffler. I’d send it to srcarr52 or michpatriot and have the holes welded shut and re-tapped to m5

 

[Wood Doctor]

I don’t trust helicoils or inserts inside a muffler. I’d send it to srcarr52 or michpatriot and have the holes welded shut and re-tapped to m5

I agree about the helicoils, but it seems to me like the M8 machine screws will work also. I have a drill press and can center the drill outs on the gaskets, the muffler, and the cooling plate accurately. I really hate to give up on those holes I drilled and threaded in the cylinder. And, I would not have to remove the cylinder.

 

[huskihl]

I agree about the helicoils, but it seems to me like the M8 machine screws will work also. I have a drill press and can center the drill outs on the gaskets, the muffler, and the cooling plate accurately. I really hate to give up on those holes I drilled and threaded in the cylinder. And, I would not have to remove the cylinder.

I don’t believe there will be much left of the flange once tapped to m8

 

[Wood Doctor]

I don’t believe there will be much left of the flange once tapped to m8

Actually, there is quite a bit left on the flange. Say, here's the threaded adapter I was referring to in Post #1 that has more beef than a typical helicoil:

This would take the M8 I already machined down to an M6, which is getting there. Another thought I had was to take an M8 bolt and drill/tap an M5 female thread right down the center, assuming I can find an M8 bolt that isn't casehardened. I have some old Stihl studs that mght work for that idea or I could use an ordinary M8 bolt. The end result resembles what you see in this Pic but the M5 female thread would be in the center rather than an M6. WDYT? Is it worth a try?

 

[Wood Doctor]

It was worth the try. It worked. I found two used Stihl chainsaw studs, and drilled them down the center about 3/4" and threaded them to accept the original M5 muffler bolts. I cut off 15 mm lengths and inserted that into the M8 threads I had already drilled and tapped into the cylinder. The muffler is now back in place and tight as drum, good to go.

I did run into a case hardened stud that would not accept any drilling, so I had to reject it. This mission would have been impossible without a drill press, but I was able to drill and tap the M8 holes in the cylinder with a hand drill without removing the cylinder. In the future and if I run into his again, I may just order the threaded adapters shown in post #5 and use M6 muffler bolts. That, of course, would have required enlarging the holes in the gasket and cover plate. The ideal threaded adapters would have been M7 to M5, but I could not find them anywhere.

 

[Dave P 71801]

Actually, there is quite a bit left on the flange. Say, here's the threaded adapter I was referring to in Post #1 that has more beef than a typical helicoil:
View attachment 971568
This would take the M8 I already machined down to an M6, which is getting there. Another thought I had was to take an M8 bolt and drill/tap an M5 female thread right down the center, assuming I can find an M8 bolt that isn't casehardened. I have some old Stihl studs that mght work for that idea or I could use an ordinary M8 bolt. The end result resembles what you see in this Pic but the M5 female thread would be in the center rather than an M6. WDYT? Is it worth a try?

I have had good luck with that style thread repair I used them on a 200t years ago and they are still holding strong. I used JB weld to secure the insert and have had zero issues

 

[Automender]

There are a few things on the internet on sort of the same fix for TS400 cutoff saws that had a habit of loosening up. They retap the holes and use bolts. The adapters are a simple fix.

 

[Bob Hedgecutter]

Wonder what might have happened if you had skipped 6mm and gone straight to a 1/4 inch tap?

 

[Wood Doctor]

Wonder what might have happened if you had skipped 6mm and gone straight to a 1/4 inch tap?

Bob, I agree that might have worked. Then I could have used a 1/4" machine screw and abandoned both the metric system and Torx screws. I failed to considered that idea, and yes, it might have worked. Somehow I elected not to try. Had that succeeded, I would have saved a day's work. Had it failed, I would have lost nothing. Maybe I was brain dead at the time. Frankly, I thought that 7 mm machine screws exist, but they apparently do not.

 

[Bob Hedgecutter]

Bob, I agree that might have worked. Then I could have used a 1/4" machine screw and abandoned both the metric system and Torx screws. I failed to considered that idea, and yes, it might have worked. Somehow I elected not to try. Had that succeeded, I would have saved a day's work. Had it failed, I would have lost nothing. Maybe I was brain dead at the time. Frankly, I thought that 7 mm machine screws exist, but they apparently do not.

7mm fixings are out there, they are just like walking sticks with Unicorn horn grips, rocking horse poop and hen's teeth.

 

[singinwoodwackr]

I agree about the helicoils, but it seems to me like the M8 machine screws will work also. I have a drill press and can center the drill outs on the gaskets, the muffler, and the cooling plate accurately. I really hate to give up on those holes I drilled and threaded in the cylinder. And, I would not have to remove the cylinder.

Use a reamer, not a drill bit.