No 4 Mk 1 markings confusion

[Alan de Enfield]

Presumably somewhere there is a drawing the Gauge, Inspector's Bolt was patterned after, but I can't see why it would matter whether the body lugs bore evenly against that gauge as long as they bore evenly against the bolt fitted to the rifle!?

That makes sense.

The Inspectors bolt was primarily used to check that the body / locking lugs were not worn to such an extent that it couldn't be headspaced :

From another 'lecture' by PL

GAUGE, inspectors, selected breech bolt......... Haven't seen one of these for a few years. Yes....! Only used by the most senior examiner, this bolt was a calibrated bolt made to the EXACT size. It was kept in an oily drawer along with the other gauges. It's main use was as the final arbiter as to whether a body was worn out. I seem to recall that the No1 gauges came with a matched bolt head which your are missing (oh no they're not, I've just looked properly......!) The locking shoulders were specially hardened so NEVER use one of these bolts as a true bolt and the distance between them and the face of the bolt was the datum. When it was impossible to get CHS, using the old bolt and boltheads then a new bolt, you'd use the gauge bolt do the final check. If this failed then the rifle was scrapped as this calibrated bolt had deemed that the body locking lugs were worn through.

The No4 bolt didn't come with a bolt head as a matched set BUT there was a special bolt head to be used with it that was a special measurement that I forget at the moment. This did the same thing.

This gauge bolt also dictated wear in the boltway. For example, the smaller left locking lug MUST clear the nose of the sear when loading and unloading. If it does foul, then the gauge bolt is used to verify that the body is worn and not the new bolt. This test should be carried out with the bolt lightly pressed down towards the sear nose to give an accurate assessment of the condition of the body. Obviously, it could also be used as a gauge to test other worn bolts too but in those cases, we had the 'inspection standards' specifications to refer to.


There is a little more you might need to understand before you can appreciate the whole picture. The No4 rifle was a very precisely made piece of gear. The very fine production tolerances achievable meant that every rifle could be assembled with any parts from production within set tolerances and be correct for headspace with either a No0 or a No1 bolt head fitted. To leave the factory, that was between .064" and .068" headspace. If a rifle failed headspace in service (failed the "field" gauge at .074") Then it was sent back for repair. If it could be headspaced with the next size bolt head (either a 1 to replace a 0, or a 2 to replace a 1) it was deemed good to go. If a rifle did not pass headspace with a No2 bolt head, then it was passed up the line to the senior inspector who would apply the Gauge, Inspectors, Selected Breach Bolt. If the receiver passed the test, it could be fitted with a No3 bolt head and put back into service.
NOTE HERE. A NO3 BOLT HEAD WAS ONLY TO BE USED ON THE SAY SO OF THE SENIOR INSPECTOR.
If it did not pass the test, it would have been sentenced Z for return to the factory, (even if a No3 bolt head would pass headspace)
Interestingly, I wonder how many rifles are out there with No3 bolt heads in them that the owners have fitted to keep them in heasdspace, when actually the receivers may be worn beyond reasonable limits.
Also I wonder about the amount of rabid buying in the past of number3 bolt heads and the owners who have fitted them because their rifles failed the SAAMI spec field gauge at .070", which is a good .004" below the manufacturers spec...
I would suggest, short of panic, anyone who has a No3 bolt head in a No4 or No5 rifle should own a .074" headspace gauge and check the rifle every couple of hundred rounds. If the hardness has gone (or becomes too thin from ongoing use) from the locking shoulders, the headspace will increase as the softer metal is extruded by the forces of firing. Checking and finding it failing with a No3 bolthead that passed a hundred rounds ago will tell you things are not right.. time to retire the rifle. NOT REPLACE THE BOLT BODY!

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