Annoying British Screw Size

[JohnD]

I am trying to match a pre-metric screw in an oldish English lock. Probably from the 1980's though it was designed in the 1960's or 1970's, and the factory probably used their existing tools.

It is almost M4 (later versions of this same model lock have an M4) but an M4 binds and will not go beyond around half a turn, so the thread is obviously wrong although the size is about right.

I thought it was UNC #6-32, which enters, and screws home tightly, so the thread must be about right, but is a very loose fit and wobbles (until it is screwed tight enough for the head to be tight)

So what can it be?

 

[big-all]

you have the thread pitch as well ??
1/8 off an inch would off course be 3.175mm

 

[raykf]

4BA ?

 

[JohnD]

The screw is only about 5mm long, so quite difficult for me to count TPI.

I believe #6-32 is about 3.5mm major dia, so the size I need must be nearer 4mm, but in a non-metric size. #6-32 has 32TPI and threads easily into the parts. M4 just about fits, but will not go more than half a turn as the thread is wrong.

I don't think I've got any 4BA, though I have some 2BA for a different purpose. I think of BA as an electrican's thread. 4BA is 3.6mm major, so I think would still be loose.

Funnily enough the old screw is marked as Pozidriv, so it must be "transitional" screw they used before going over to metric.

 

[JohnD]

you have the thread pitch as well ??
1/8 off an inch would off course be 3.175mm

Maybe it's 5/32" ?

just under 4mm

 

[JohnD]

I reckon it's probably Whitworth

5/32" BSW
32 TPI

that would explain the #6-32 UNC screwing in at 32TPI

 

[big-all]

arnt the screws in electrical sockets or switches about 3mm ??

 

[JohnD]

Modern electric socket and switch faceplate screws are M3.5

but old (very old) UK switches and sockets had, I think, 2BA screws to hold the fronts on (and probably other sizes in other electrical equipment).





5/32" BSW Csk Stainless machine screws are hard to find....

 

[Harry Bloomfield]

but old (very old) UK switches and sockets had, I think, 2BA screws to hold the fronts on (and probably other sizes in other electrical equipment).

No, those were smaller than 2BA, maybe 4BA. 2BA were used in conduit boxes.

 

[JohnD]

2BA were used in conduit boxes.

Ah yes, that's what I was thinking of.

 

[JohnD]

just found that 5/32 BSW is size of Meccano screws.

But no use to me, I need CSK stainless.

 

[dontbelieveawordofit]

3/16 whit normally in old locks.

 

[JohnD]

ah, but that has a major diameter of 4.763mm, so my M4 screw should have dropped into it... but was a tight fit and would only go about half a turn before binding

TPI of 3/16" is 24, but a UNC of 32TPI threaded in easily (but was loose)

 

[JohnD]

well, I ordered some 3/16" BSW and some 5/32" BSW.

The 3/16" arrived but are definitely too big and the thread is too coarse.

Hoping the 5/32" will arrive and be the one.
Didn't manage to locate CSK stainless yet.

 

[dontbelieveawordofit]

cant you retap the holes to suit a slightly larger thread.