Inspectors Bolt Gauge for ShtLE

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   Remember, the old non standard engineering specs were a standard at one time, for somebody.
At the time that Enfield was getting into tooling for mass production (circa 1856) and needed standardisation, there was no universal standard of measurement so they stuck a stick in the sand and established their own. The Imperial inch standard came later. A difference of 0.0004 inches, I believe. It caused all kinds of initial problems with tolerance stacking for Lithgow.

Some of the strange thread pitches might seem bizzare to some these days. Why a thread pitch of 26-1/3 per inch??  Simple answer actually, the change gears for thread cutting on the lathes just worked out that way and was actually an easy gear change to do on that equipment. Most modern machinery does not have these screw cutting gear ratios and combinations, they have different sized gears. 

The Enfield thread form is a little different in angle and has rounded crests and valleys. But cutting tools can be made, little problem. Enfield thread forms work well and are robust.

So back in 1856 a screw 0.181 inch in diameter and 26-1/3 thread pitch was straight forward to make, where as a modern common Unified threaded screw might have been a challenge with special tooling needed.

One thing that I don't have is a set of Enfield thread taps and dies. If somebody ever had some made, I am sure that they would sell well. 