Tabatiere / Zulu retaining screw thread?

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theophys

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Hi All,

Got a project-style Zulu / Tabatiere shotgun and was wondering what thread this screw (retaining the firing pin) might be?
A bit too small for 6-48 or M4, hole measures 2.9mm or 0.115 in.

(Granted, I can re-tap the hole either 6-48 or M4 but wanted to avoid that if I could...)

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B_noser said:
I believe the major diameter of M 3.5 x 0.6 is 3.5 MM and the minor diameter of the female thread is 2.85 MM.
Click to expand...

Thanks! So do you think it's metric M3.5x0.6? The gun seems to be older than when the metric screws were standardized (1945's?) - although it looks like SAE-type threading is even more unlikely.
I'll try to visit some machine shop and see if an M3.5 fits :)
 
The dimensions seem to fit. I suggest you start by checking the pitch with a thread gauge. If the pitch looks to be correct, get an M3.5 x 0.6 screw and check the fit. You might want to start by carefully cleaning up the threads with an M3.5 x 0.6 tap. You should be able to tell by feel if the tap is following the threads.
A machine shop should be able to do all of that for you.
 
A thread on an old piece of gear could be almost anything.

Decide if you wish to restore it, or make it work easy.

Start with a set of thread gages for both metric and inch threads. Add Whitworth, at least.

Screw a tapered piece of hardwood, into the hole to pick up the thread pitch and diameter. Compare carefully, make your best guess as to which is the best fit. Research old thread forms, to see if what you have is one of those, or just a mangled version of a newer type.

Drinking heavily helps.

:)

In all seriousness, modify the new part to fit the old, not the other way around.

By carefully comparing the available thread pitch gages, you should get a good idea of what type of thread it may be. Old stuff rarely started out with modern threads in it.

If it is metric, then you can work out the diameter.

Same if the TPI matches an available spec.

Whitworth threads and BA (British Assosciated) were kinda oddball half inch, half metric, and have some specific other issues to overcome (round crests and roots, odd included angle, and a geometric progression of sizes) But taps and dies are still available.

Rinse and repeat a couple times with the hardwood wedge screwed in to the hole. Each pass can clean out more mung, and make it more like that you can sort out the actual thread size that was used.

There is probably an app for that....Would not know firsthand, though...
 
B_noser said:
The dimensions seem to fit. I suggest you start by checking the pitch with a thread gauge. If the pitch looks to be correct, get an M3.5 x 0.6 screw and check the fit. You might want to start by carefully cleaning up the threads with an M3.5 x 0.6 tap. You should be able to tell by feel if the tap is following the threads.
A machine shop should be able to do all of that for you.
Click to expand...

Thanks. (It seems easier to buy a M3.5 tap and die than the screws, so this is probably what I'll end up doing - the screw needs to become a pin once it clears the sleeve anyway, so time to test that newly bought mini lathe :)
 
trevj has given you good advice. The tapered piece of hardwood trick works quite well. I would at least check the pitch with a thread gauge before running a tap into it. I doubt it is BA or Whitworth, but one never knows.
By the way, M3.5 is available with more than one thread pitch. There is M3.5 x 0.6 and M3.5 x 0.35.
 
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theophys said:
The hardwood trick is brilliant, thanks indeed! (I was wondering how I could cram a thread gauge into a hole this small.)
Click to expand...

Welcome! I didn't invent it, it was shared to me, and I simply pass it on! :)

One good thing about cheap tools, is if you have the CTire tap and die set, (~$100, on sale) you can also use the dies to thread the end of an aluminum rod to check for fit as well. This avoids simply twisting a tap into the part and potentially making more work for yourself.
 
I have cut cheap thread gauges down for small holes with a good pair of snips, and cut with thin cutoff wheels on the dremial, but there is a limit on how small you can go .
I don't think I have thrown a small screw away in 60 yrs .
Over the yrs ,I have dismantled a lot of electronic hardware and lots of strange screws and bolts in that stuff
BTW if you are into old Stanley tools and wood planes as I am, they used their own thread pitch just to be a PITA, thou not too many different sizes on them
 
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marshall said:
I have cut cheap thread gauges down for small holes with a good pair of snips, and cut with thin cutoff wheels on the dremial, but there is a limit on how small you can go .
I don't think I have thrown a small screw away in 60 yrs .
Over the yrs ,I have dismantled a lot of electronic hardware and lots of strange screws and bolts in that stuff
BTW if you are into old Stanley tools and wood planes as I am, they used their own thread pitch just to be a PITA, thou not too many different sizes on them
Click to expand...


Indeed.
Making the gauge fit into a small hole is only part of the problem. The other part is how to determine if the gauge fits.
With a hole small enough, and the thread worn / rusted enough, the required feeling skills are on part with those of a professional lock picker. Not something I was ever good at.
 
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