No1 Mark 111 Breaching up

[Bruce_in_Oz]

Rastis:

SMLEs and all before them, martinis included, use an "Enfield Special" thread.

Whilst it may be 14 TPI it is neither Whitworth, BA or UN in form.

Details are as follows:

Outside (Major) diameter: 0.9955"
Root (Minor) diameter: 0.9024"
Effective (Pitch) diameter: 0.9574"
Pitch: 0.0714" (14TPI)
Angle (included): 49deg 40 min.
Root radius: 0.00804
Crest Radius: Unknown (I will load all that lot into my Cad system and do some creative geometrry)
Note that the crest radius is defined as being 0'01949" from the nominal crest height.
I have seen "sporters" with No4 barrels "fitted" to SMLEs and vice versa. Not sure which vise is versa, though.

Given the available metrology of the times, keeping to 5 decimal places must have been interesting, especially in mass production.

EVERY other thread on a SMLE is a "gunmaker's special". The only one close to "industry standard" is the stock bolt, which is listed as to 7/16 Whitworth but may be slightly different in form.

On a No4, every screw is "industry standard" of the day. Almost all are various BA sizes. The front triggerguard screw is 1/4" BSF.

No4 Threads are DEFINITELY Whitworth form.

P-14s are also full of weird Enfield threads; most definitely not UNF/UNC.

[Bruce_in_Oz]

Posted too fast:

"No4 Threads are DEFINITELY Whitworth form." should read "No4 Threads are DEFINITELY NOT Whitworth form except for that 1/4"BSF job

Going with "general engineering" threads made sense when involving thousands of subcontractors in wartime production. Managing the "Peddled Scheme" for SMLEs in WW1 must have been a nightmare to provide all the gauging right down to simple thread level. Specialised tooling would have been nececessary to make even the most basic screws in the rifle. How many basic lathes can cut 37TPI without a lot of fiddling in the gear train? However, every man and his dog would have had a set of BA tools and gauges. These days, CNC lathes can easily make any weird thread pitch you like; it is sometimes a struggle to get the operators to actually grind the correct form tool.

[Rastis]

Bruce,
I wonder how many barrels have been threaded either 55 or 60 degrees because that was the tool the gunsmith had in the lathe at the time. 49deg 40 min is certainly an odd one, Its not even 47.5 degrees that BA is though.
Apparently a No 1 barrel when screwed into a No 4 action indexes 180 degrees out.

Thank You,
Rastis.

[Bruce_in_Oz]

The thread form on the barrel drawing looks a bit like a BA, but it ain't.

What is also amusing is that Mauser 98s have a Whitworth form (55 degree) breech thread and breech up at the end face of the tenon, just like a SMLE. Type 30 and 38 Arisakas are full of Imperial threads; Type 99s are completely metric, as near as I can tell. I suspect that there was a LOT of British tooling in German and Japanese arsenals at that time. Funnily enough, the Japanese ship building industry kept on with British threads through and well past WW2. You can find Admiralty, Whitworth and BA in their vessels.

ALL of the commercial aftermarket barrels I have seen for Mauser 98s seem to have 60 degree UN form threads and folks tend to breech them up against the front face of the receiver. I have never whacked a thread gauge into any of the post war / commercial 98s though.

[tbonesmith]

Bruce on Oz, I'm always blown away by your knowledge on these technical matters. I was just looking through some of your old posts looking for some bit and pieces, all very interesting.

[Rastis]

Threads are certainly a interesting subject. Over the years I have cut alot of threads, mainly on CNC machines, most of which were BSW, UNC, Metric, and pipe. Started off using die boxs in threading machines before CNC came along.
Thanks for the interesting info,
Rastis.