No32 Mk3 Thread question.

[Roger Payne]

I am currently getting ready to rebuild several No32 Mk3 scopes, but I am short of some of the 'cap & plug' screws that locate the brass shoes which in turn locate the grat block for the purposes of eliminating backlash. I had assumed they may well be the same as the thread on the screw that fits into the bottom of the clicker plunger housing on Mk1 & Mk2 scopes (1/4" Model Engineer), but it isn't, it's a little larger. At a WAG I think it could be 9/32" ME, although even then there is both a 32 TPI & a 40 TPI version.

It is in effect a screw within a screw, although I don't think the inner thread is too crucial, & I'm sure I can sort that, but the outer screw thread is frustrating me!

I don't have a problem making them, once I have the correct info. I checked the 1971 EMERS for the L1A1 scope, & whilst brief reference is made to the component, the thread is not mentioned. It's pretty nerdy stuff, so I doubt very much it'll be in most texts, although I shall continue looking. Anybody out there know off the top of their head??


I would be eternally grateful for any assistance!

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[Flying10uk]

Are you sure that it is not a brass thread size, being as the thread is going into brass? T.P.I. is easy with brass thread because it's all 26 T.P.I.. 1/4" B.S.F. and 1/4" Brass Thread are identical threads, although labeled differently, i.e. a 1/4" B.S.F. tap is identical to a 1/4" Brass Thread tap but labeled differently.

If I have something that is old and British, that is brass with threads in it, I normally consider the possibility of the threads being a Brass Thread and if this is the case I know that the T.P.I. will be 26 T.P.I..

[DanL96a1]

Roger from memory they were OBA, although 5mm would “fit” it was not a correct “fit”.... will look up in EMERS too, but I’m pretty sure when we made them it was OBA.

[Alan de Enfield]

I would be eternally grateful for any assistance!

I can find nothing specific, but, PL has stated (over the years) that many screws on the MK3 are BA threads.

[Flying10uk]

0 B.A. is almost the same as 6 mm x 1mm pitch thread.

[Roger Payne]

I never thought of 0BA! I want coal cracking over my head! I'm pretty sure I have both taps & die so I'll cautiously try it. I didn't know that 1/4" brass thread was the same as 1/4" BSF Flying 10, many thanks. The thread sizes used on No32's are nearly all BA, mostly 8, & odd ones 10, & even one 12 BA, with the exception of the ME thread used on the MK1 & 2 for the screw that fits into the bottom of the clicker plunger housing. Over the years I've built up a stock of all of these, fortunately.

Many thanks everyone. Much appreciated.

[Roger Payne]

Bugger! That would have been too easy..........tried a 0 BA plug tap in a couple of Mk3 scope tubes & it is too small by a margin.......just drops in the hole & doesn't even touch the sides (where have I heard that before?). If Daniel's is definitely 0 BA that raises a question; were there different sizes used, were some modified?? Any road up my holes are bigger than 0 BA!?! I bit the bullet & ordered two carbon steel 9/32" ME dies (one 32 TPI & one 40 TPI - my money's on the 40 TPI) from Tracey tools. They're only three quid each & carbon steel will be perfectly good enough for brass.........if they fit! We'll see.

Thanks for the suggestions offered so far anyway.

[Bindi2]

Would the bigger holes be from a rebuild.

[Roger Payne]

Yes, that's a point.....I suppose it could be.......thread stripped so open it out to the next size up. Out of interest I'll try checking the threads on another couple of scopes when I get a minute.

[BillPredeaux]

In my box of bits I have a tap marked 1/8 GAS, this looks to have 32tpi and I think it is the same as 1/8 ins bsp. This is bigger than 1/4 in and approx 5/16 in.
Could this be the thread? Undeniably British and usually used on brass gas fittings.