Stripped knob thread

[Voxbox]

Hello,


I have been rebuilding a 76 MV Twin and the grub-screw thread in one of the knobs has stripped.
I'd rather not replace it with a new one so all the knobs retain the same patina of wear.


Any ideas about saving the knob?
I've tried using PTFE (teflon) tape around the screw threads but no joy 


Cheers,
VB

[sluckey]

Pull the setscrew out. Fill the hole with JB weld. Tomorrow redrill and tap new threads.

[Voxbox]

Pull the setscrew out. Fill the hole with JB weld. Tomorrow redrill and tap new threads.

Cheers, Sluckey do you know what gauge the threads are?
Is JB Weld similar to Araldite?
VB

[sluckey]

Probably 6-32. Remove the setscrew from a good knob and see if a 6-32 screw fits.

There is no substitute for JB Weld. Widely available, easy to use, cheap.

[Voxbox]

Grand, thanks Sluckey


Is that the Putty or epoxy adhesive you would use? I've never seen JB Weld before here in the UK but I can get it locally as it turns out.

[sluckey]

This is what I'm talking about. 2 part epoxy...

     https://www.lowes.com/pd/J-B-WELD-Original-Twin-Tubes-2-Pack-Gray-Epoxy-Adhesive/3377874

I've actually repaired a Fender knob with this stuff. I removed the setscrew and put a 1/4" dowel into the knob just to block the bottom of the setscrew hole. The knob was held securely in a small pin vise with the setscrew hole facing up. Then mixed a small amount of JB weld and used a toothpick to fill the hole. The next day I removed the dowel and put the knob back in the vise. Then drilled a new hole through the original hole (now filled with JB weld) and finally tapped the hole for correct threads. Sorry, I don't recall what the thread size is but I think it's 6-32. (Definitely not metric!).

If you don't have a drill press, pin vise, correct drill bit size, and correct thread tap, you may want to take it to a buddy that does have this stuff. Or... buy a new knob and distress it a bit. 

[PRR]

+1 for JB Weld. It can repair engine blocks.

There are other metal-filled fillers but JB is epoxy based, and not the cheap epoxy in 5-Minute blends. It will fail in an engine valve or conn-rod, but sticks water-tight to a well-cleaned block or carb. Drilling is not much easier than mild steel. Mix exactly to instructions, a uniform grey (white streaks never harden).

[acheld]

JB Weld can work, it'll be a mess.   I use Helicoils in this situation, which are designed for this exact application.

https://www.mcmaster.com/threaded-inserts/

[EL34]

Knob threads can be several thread pitches
I have knobs here that are 4/40, 6/32, 8/32, M4 and M5

best to see what nut fits on the set screw
Lowes has the thread finder things in the nut/bolt dept.

[jjasilli]

Maybe another alternative:  go up 1 or 2 sizes in set screw size , or use metric to do the same.  To do this, drill & tap to enlarge the size of the existing set screw hole for the larger size set screw.  Or, make an entirely new set screw hole. 


JB Weld is also a good idea.  I wouldn't have thought of that.  In that vein, you could also use 2-part plumber's putty/epoxy to fill & re-drill the existing hole.  Same basic idea, but it's got the consistency of Play-dough & doesn't run like JB Weld. 

[Voxbox]

All good ideas, I just need to pick one. I've never heard of helicoils, but they do sound good!


Here's a thought, open out the old threaded hole a bit, grease the screw, coat in Araldite/JB Weld, insert into opened out hole. When it sets unscrew the greased screw. There should be a newly threaded hole created by the Araldite and screw?
Yes, I know its a bodge.....


Otherwise I think I'll go with Sluckeys experience on this one. Or bodge it if I cant find out the thread type and a tap.


Cheers Guys,
Love this forum 

[TitaniumValhalla]

I have a similar problem - I am working on a ‘60s Gibson with the top hat knobs and wondering about the best solution for a loose one. They don’t have set screws though, just split shaft pots. Any ideas on using JB Weld or something else to repair the knob? I’m considering finding a solid shaft knurled pot that I can use as a form, maybe waxing it or something so the knob can be pulled off, and putting the JB-welded knob on it to set up. I’m not sure if it would work or not.

[sluckey]

Use a flat screwdriver to spread the shaft split a bit.