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Suppose you have a stripped thread in a palm key threaded post.
I see quite a few options for bringing it back to normal ranging from what commonly known as 'band aid' to a good solid repair.

I accidentally found that one thread in my Martin's LH palm key post is stripped. I wouldn't know that unless I needed to unscrew it!
Yes the rod worked as intended and it didn't came out in the process of playing the horn.

- So, the first (not so)valid option is leave it as it is. With time as I understand it may result in the worn out key post holes since the rod will mostly likely rotate in the posts and not in the hinge tube.
- Next "soft repair" would be to accurately swedge the threaded post head so that the rod's thread begins to catch the post threads. It might work and probably it can work as long as the horn will last.
- Next is running low-temp solder in the threaded post hole and then making a new thread.
- The most common repair as far as I understand is making a "bushing" - drilling out a larger hole in the post head and soldering in a brass insert and threading it.
- Making a new rod with larger thread size. It can actually work without even touching the post head thread.
- Replacing the post with a new one with the good unstripped thread :) (Well, kidding...)
 

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The "quick and dirty" way I have used on occasion is to "flash" the inside of the threaded post with soft solder and use a tap to cut a new thread. One might get away with making a new hinge rod the same diameter and cutting the next larger size thread and then tapping the post to that size. That depends entirely upon whether the diameter of the rod is such that it can be given threads the next larger size. Another option I have used is to go to the next larger size rod and cut the next larger size thread on that. This method usually requires making a reamer from the same diameter rod and using that to open the unthreaded post and the hinge tube of the key. If you use this method, it is important to use a bit of oil when cutting the hinge tube and cut a short distance and then back the tool out repeatedly until it goes completely through. If the hinge tube fits too snugly to the new size rod, one can chuck the rod in a bench motor, put a dab of metal polish inside the hinge tube, and spin the rod inside the hinge until you feel it "relax".
 

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Simso, what do you do about pivot screw in the similar state. I recently serviced an old Mk VI where someone had obviously lost the pivot screw, put one in with a finer pitch and cross-threaded it slightly off angle...it had gouged the inside of the rod and was very sloppy. Customer needed the horn right away so I packed the end of the rod with oiled felt, and screwed a proper pivot screw in after chasing the original threads...put a dab of Loctite 222 to stabilize the thing temporarily. What would you do when he comes back for a proper fix? It is the lower pivot on the bis Bb key...awkward place to run a tap in and the post is part of a long rib. In reality, my view is that this is now about as good as some of the early Borganis that had flat-bottomed holes in the end of the solid rods with Selmer pivot pins jammed into felt. Seemed to work well and hold up pretty good if you packed the felt tight and oiled it!
 

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I make a bigger pivot screw and re tap the post. A tap for brass is as simple as a long piece of drill rod with the desired thread with a small angle ground across the threads.

Steve
 

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I simply cut an oversized thread and make a new rod, does not need to get more complicated than that

Steve
Steve, since this is your area of expertise let me ask you a question. Suppose the rod diameter is .1055 - .106 and the thread is 3-48. To cut that thread with a die the recommended rod diameter is .0995 so you reduce the .1055 diameter by .006. If the threads inside the post are stripped, the next larger size thread is 4-48. To cut that thread, the recommended diameter of rod is .113 which means that your .1055 diameter rod is .0075 too narrow. My question then is will the 4-48 threads cut on a .1055 rod have enough of a "crest" to the thread to work when screwed into a post?
 

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Steve, since this is your area of expertise let me ask you a question. Suppose the rod diameter is .1055 - .106 and the thread is 3-48. To cut that thread with a die the recommended rod diameter is .0995 so you reduce the .1055 diameter by .006. If the threads inside the post are stripped, the next larger size thread is 4-48. To cut that thread, the recommended diameter of rod is .113 which means that your .1055 diameter rod is .0075 too narrow. My question then is will the 4-48 threads cut on a .1055 rod have enough of a "crest" to the thread to work when screwed into a post?
My Machinery's Handbook says the range of major dia. for a 4-48 thread is from 0.1113 to 0.1068 (2A class, which is good enough for this work). If the minimum dimension of the rod is 0.1055, you're only losing 0.0013 on the diameter, or 0.00065 on the radius. For comparison the minor dia. is nominally 0.0857 so the theoretical thread height is (0.1113-0.0857)/2 to (0.1068.0.0857)/2 = .0128 to .0105. It looks like you lose about 6% of the male thread height. The internal thread minor dia. spec is 0.0894 to 0.0968. With the major dia in spec, your diametral interference is 0.0100 to 0.0219. With the major dia. at 0.1055, the minimum diametral interference to the female thread is 0.1055-0.0968 or .0087, so you've lost 13% of your diametral interference.

Given that all the loads are side loads, I would say you are OK, but be super careful in tapping out the post because you don't want to wallow out the hole. I would certainly advise strongly against using anything but a good quality tap, certainly not a screw with a slot cut down the side. You might want to use a tap drill that's a wee bit smaller than standard. My chart shows a #42 drill at .0935 for a 4-48 tap. That's 2.375 mm, so you might even try a 2.3 mm drill. And make sure to use a taper tap, not a plug tap, so you have the lowest cutting torque on it.
 

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Suppose you have a stripped thread in a palm key threaded post.
I see quite a few options for bringing it back to normal ranging from what commonly known as 'band aid' to a good solid repair.

I accidentally found that one thread in my Martin's LH palm key post is stripped. I wouldn't know that unless I needed to unscrew it!
Yes the rod worked as intended and it didn't came out in the process of playing the horn.

- So, the first (not so)valid option is leave it as it is. With time as I understand it may result in the worn out key post holes since the rod will mostly likely rotate in the posts and not in the hinge tube.
- Next "soft repair" would be to accurately swedge the threaded post head so that the rod's thread begins to catch the post threads. It might work and probably it can work as long as the horn will last.
- Next is running low-temp solder in the threaded post hole and then making a new thread.
- The most common repair as far as I understand is making a "bushing" - drilling out a larger hole in the post head and soldering in a brass insert and threading it.
- Making a new rod with larger thread size. It can actually work without even touching the post head thread.
- Replacing the post with a new one with the good unstripped thread :) (Well, kidding...)
don't forget Heli-coils.
 

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Suppose the rod diameter is .1055 - .106 (2.68mm-2.69mm) and the thread is 3-48. To cut that thread with a die the recommended rod diameter is .0995 (2.43mm) so you reduce the .1055 (2.68mm) diameter by .006 (0.15mm). If the threads inside the post are stripped, the next larger size thread is 4-48. To cut that thread, the recommended diameter of rod is .113 (2.87mm) which means that your .1055 (0.68mm) diameter rod is .0075 (0.19mm) too narrow. My question then is will the 4-48 threads cut on a .1055 (0.68mm) rod have enough of a "crest" to the thread to work when screwed into a post?
Edited to make it easier to understand (rounded a fraction) :)

Turf3 already gave pretty good info. Some other things worth mentioning...
According to several charts, your sizes are slightly larger than usual max major diameter. 0.0990" for 3-48 and 0.112" for 4-48, and that's for the largest diameter fit which is unnecessary for this. So that's already about 0.001" less of a problem :)
Those are general max major diameters and as turf3 mentioned, there is a range that is considered a good fit. In many uses the threads are purposely cut slightly smaller to whatever degree. Musical instruments, more often than not, have smaller than max major diameter threads.
The minimum major diameter for this thread size is 0.1068" so consider that instead of your 0.113" measurement (maybe you just forgot one of the 1s in the 0.1113" measurement?).

Although charts show a 42 drill size (0.0935"/2.375mm), the minor diameter for the 2A fit is 0.0857" (2.18mm). Assuming you use all precautions (good quality tapered tap, etc.) then basically you can use the smallest drill that your tap minor diameter passes through. you can choose that depending on whether your drill and way of drilling produces larger or smaller hole than spec. Pretty sure 2.3mm like turf said would be fine. 2.2mm might be ok... not really familiar with numbered and inch sizes but maybe there's a size that would work better.

Assuming you have control of the hole diameter of the post threads, you might prefer 4-40 over 4-48 in this scenario. Smaller minor diameter and significantly decreasing the issue you described. The minimum major diameter is 0.1061" much closer to your rod size. Since finer threads are generally stronger than coarse threads, maybe someone more knowledgeable can calculate what the difference in fit has to be to completely counter the difference in the threads themselves. Intuitively I would go for 4-40 with the smallest minor diameter possible.
 

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don't forget Heli-coils.
Are they available in such tiny sizes? Having only seen it done, never worked with them, I have no idea, but I've only seen them used on significantly larger size threads.
Also, do they work well on through holes? I think I've only seen them used on non-through holes.
 

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1. You almost certainly don't need to go the next size up, from 3 to 4 when making a new rod. Make the thread with a split button die that has been opened to be a bit oversize. Isn't that the reason they are split?

2. For quick, budget-determined work, I have found bushing non-threaded posts with CA glue amazingly successful. I can't see why it would not work for a thread:
Clean inside the post with solvent. Apply grease, as thin as possible - only a trace, to the rod's thread. Insert the rod - gently so as to rub off as little grease as possible.. Apply thin grade CA glue to wick inside the post. When it is set, unscrew the rod. A new thread should exist inside the post. The demands of a rod's thread in a post are not great, so cured CA glue should be sufficient. If the glue sticks to the rod too well, use heat to remove it, and try again, perhaps with a little more grease.

3. Another viable option, if there is any hint of thread remaining in the post, - rather cheap and nasty, but exceedingly quick, is to crush the thread on the rod to slightly oval, using say the Knipex, smooth, parallel-jaw pliers. Just enough that the thread takes purchase.
 

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1. You almost certainly don't need to go the next size up, from 3 to 4 when making a new rod. Make the thread with a split button die that has been opened to be a bit oversize. Isn't that the reason they are split?

2. For quick, budget-determined work, I have found bushing non-threaded posts with CA glue amazingly successful. I can't see why it would not work for a thread:
Clean inside the post with solvent. Apply grease, as thin as possible - only a trace, to the rod's thread. Insert the rod - gently so as to rub off as little grease as possible.. Apply thin grade CA glue to wick inside the post. When it is set, unscrew the rod. A new thread should exist inside the post. The demands of a rod's thread in a post are not great, so cured CA glue should be sufficient. If the glue sticks to the rod too well, use heat to remove it, and try again, perhaps with a little more grease.

3. Another viable option, if there is any hint of thread remaining in the post, - rather cheap and nasty, but exceedingly quick, is to crush the thread on the rod to slightly oval, using say the Knipex, smooth, parallel-jaw pliers. Just enough that the thread takes purchase.
I thought about recommending the Loctite space filling products (similar to what you're recommending on CA glue), but it seems to me that if you inadvertently fail to coat the thread effectively enough, and the CA product bonds to the rod, you've got a hell of a mess.

As far as ovaling the thread, wouldn't you oval the post, not the rod?
 

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Are they available in such tiny sizes? Having only seen it done, never worked with them, I have no idea, but I've only seen them used on significantly larger size threads.
Also, do they work well on through holes? I think I've only seen them used on non-through holes.
Here’s all you would ever need to know on heli-coils. https://www.stanleyengineeredfasten...hash=BD368D6014A78C96FBDA8B6E09BF3AC19C76F55B
I’ve used them plenty for field repairs at the racetrack. Cast iron,steel or aluminum. Through holes are no problem. Put some thread locker on it. Smallest size appears to be #1-64unc or m2x0.4
 

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Never seen helicoils that small, really good link and good to know, purchasing some now for future use.

Steve
Glad this is something you can use and hopefully benefit from. Personally I’ve never used anything smaller than 5mm. Overhead Camshaft bearing caps in a high-performance race bike running alcohol. They never failed. If you’ve never installed one before,practice on something that’s not a live job is my best suggestion. Would love to see a repair executed with one.
 

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For a specified M2x0.4 helicoil I assume that is the inside thread.
Therefore in order to accept the helicoil I would have to drill and tap a larger diameter in the work-piece, and the pitch of the thread would have to still be 0.4mm.
Does a suitable M?x0.4 tap come with the helicoil?
 
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