1/2" x22tpi tap?

Sorting through some odd taps and came across one with the above dimensions. It was made by LAL Ltd England.

Tracy do two taps of this size, one with a Whitworth form and the other being UNS.

Looking up the Whitworth sizes, 1/2" x22tpi isn't a standard thread. Likewise for the UNS family.

So is this simply a special, or is it a member of some long since defunct series of threads? Just curiosity on my part.

Andrew.

Edited By Andrew Tinsley on 18/03/2019 13:26:37

Interestingly a Google search brings up references to 1/2" 22tpi used on Model Ts and other old US cars - but even there it is seen as non standard.

Edited By Nick Clarke 3 on 18/03/2019 14:02:08

Hello Nick,

Yes I noticed that too when researching this oddball sized tap. I have come across special tap sizes before, but there were always some sound reason behind their use. With many threads near this one, I can't see much logic to its existence.

Andrew.

One suggestion I have seen is that a "bastard" thread helps manufacturers to lock users in to their spares operation.

I work on Bugatti's and they use M7, M9, M11, M15 etc!

I have read on the web that the GPO used odd sized BA screws (3ba, 5ba, 7ba etc) on exchange equipment, supposedly to avoid pilferage.

Just ran it past an ex Garratts Green, Birmingham GPO Phones guy and he said that this was true!

I have a few oddball taps with non standard pitches, from memory one is marked up as an Admiralty spec.

Somewhere in the 60's or 70's, Drapers were selling bulk job lots of assorted surplus taps, a vast assotment right up to 1/2" BSP, many were by made American manufactures. Needless to say my cousin and I, who were running the family ironmongery business, got first pick, the customers got the leftovers!

I can't find 22tpi on a ½" diameter as a standard thread anywhere, either on the internet or in any of my ancient engineering books. In theory this thread doesn't exist.

Very odd then that taps and chasers for this size are on sale as if 22tpi ½" Whitworth was commonplace. Somebody must be buying them! My guess is it's something akin to the tripod thread used on cameras - a survivor retained for compatibility reasons on a particular bit of kit. No idea what though - it's a mystery!

Dave

I put 22 tpi x 1/2" for Ford Model T in Google, and came up with the flare nuts on the Rambler Mechanical engine oiler. someone on that site suggested Tracy Tools list the tap on stock. That was 2008, but might be worth investigating.

Ian S C

Hello Ian,

Yes Tracy list the 1/2" x 22 tpi in both Whit thread form and UNS form (whatever that may be!)

Andrew.

UNS = Unified Special, 1/2 x 22 still isn't a standard size though...

Just out of curiosity I checked AN (US Army / Navy) flare fittings, and SAE flare fittings, closest sizes to 22 TPI are 20 and 24 TPI. Maybe the Rambler oiler for the Model T engine that Ian SC mentioned used a proprietary special flare nut thread, maybe just for Old Henry himself.

Just on the outside chance that the tap has been labelled wrongly (typo?) have you checked that it is 22 TPI and not something more familiar?