Bolt head thread size

[scharfschutzen63]

Does anyone know the thread size used on the No4 bolt head/bolt body connection?

[Alan de Enfield]

Does anyone know the thread size used on the No4 bolt head/bolt body connection?

0.4375"dia x 20tpi

[Alan de Enfield]

Just for completeness (whilst I have the information to hand) here is the No1 Mk3 bolt head :

.4175dia x 20tpi (75-78deg, incl flank angle. Enfield form).
This was produced using the Enfield inch and thread form which is not the same as the standard Imperial Inch

Extract from from the original blue- prints.

https://i.postimg.cc/sDhXRgk6/Screenshot-2511.png

[Flying10uk]

(75-78deg, incl flank angle

It seems like a weird angle to choose for a thread form when one considers that for Whitworth thread forms the angle is 55 degrees, which covers quite a number of different threads, and for metric thread forms the angle is 60 degrees.

[Alan de Enfield]

It seems like a weird angle to choose for a thread form when one considers that for Whitworth thread forms the angle is 55 degrees, which covers quite a number of different threads, and for metric thread forms the angle is 60 degrees.

Agreed - but (simply a guess) maybe Enfield didn't want any other manufacturers screws used.

Remember also that the No1 Mk3 was the last of the 'Victorian engineering' rifles & these screw sizes / threads go way back into Victorian times when there was little or no standardisation, for example the Martini-Henry / Enfield had an ever weirder screw size for the "Screw, Swivel, Nose Cap" of 0.181dia x 26-1/3tpi

Who'd design a screw with a 26-1/3rd tpi ?

The late 19thC and early 20thC were a mish mash of engineering designs and sizes, exemplified by :

In 1919, Morris Motors took over the French Hotchkiss engine works which had moved to Coventry during the First World War. The Hotchkiss machine tools were of metric thread but metric spanners were not readily available in Britain at the time, so fasteners were made with metric thread but Whitworth heads

[CINDERS]

It's in here Alan not the measurements but the conundrum !

[Alan de Enfield]

It's in here Alan not the measurements but the conundrum !

Indeed a well knowm problem.

There are / were some 30 odd 'inches'. the Enfield inch is so close you wonder why they bothered.

[Mk VII]

Luger threads are Whitworth form, but they are not British Standard Whitworth sizes.

[Flying10uk]

are Whitworth form, but they are not British Standard Whitworth sizes.

Like I said there are quite a number of different threads that use the Whitworth thread form. The most obvious example which uses the Whitworth thread form and is NOT British Standard Whitworth is British Standard Fine. Both threads have exactly the same thread form, i.e. 55 degrees, but the teeth per inch is completely different between the two threads. Obviously a B.S.W. thread will not fit onto a B.S.F. thread, without causing damage, because although the thread form is exactly the same, the teeth per inch is completely different.