"M" bolt head

[ohgunner]

Can someone explain the meaning of the "M" stamping on the bolt head of wartime Australian Lithgow #1 rifles. Should it be present on all examples? Thanks in advance

Information

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[Bruce_in_Oz]

According to the "specs" in my possession:

The original bolt heads were made from "malleable cast iron". If anyone is desperate, I may even have the "recipe" for that here.

ANY bolt head that carries the "M" stamp is made from MILD STEEL. However, this is subsequently case hardened for service use.

[ohgunner]

Bruce in Oz, Do you have any information about either type being better, stronger, or longer lasting? Were they interchangeable? By the way, your countrymen made an excellent rifle. Thanks again from Ohio, USA.

[Bruce_in_Oz]

Sadly, I have nothing further regarding service "life". Lithgow seemed to have thoroughly committed to the "M" version, (starting date unknown to me) and stuck with it to the end. Both types are prone to wear at the little "bump" / "hook" that runs in the groove on the RHS of the body, but this takes a LOT of "cycles" of the action and lifting the bolt head past the "Spring, retaining" every time the bolt is removed for cleaning, inspection, etc. .

One reference for the "M" (or lack thereof), is: Drawing no. A.I.D. 1529, which is a copy of the earlier R.S.A.F. 3095(1). This accompanied Specification S.A. 1117 (B) / R.S.A.F 3095 (1).
Viz:

Material: Wrought Iron or Mild Steel. Hardened and Polished.

Lithgow drawing C-643, (13January, 1921), has a bit more information:

1. Note: For Spares, qualify between 9deg and 13deg in advance of new components. (Thread is "advanced" to allow for wear of the thread in bolt bodies).

Thus, Lithgow DID make different bolt heads, "original fitting" ones and ones that had a built-in wear allowance. How these "spares" were to be identified in packaging is NOT noted on the drawings. The "tolerancing" on the length of the "cylindrical" part of the "factory new" bolt head is: .635" Accept, .632" Reject. (effectively, less than three thou). HOWEVER, post WW2, they produced and the "system" introduced EMEI paperwork for, a "range" of bolt heads in "one thou" length increments. This was to "tide them over" until the complete adoption of a "new" rifle, i.e., the L1A1.

2. "Face of Bolt Head and Hook only to be case-hardened to a Minimum depth of .01" "

Further note, with an arrow pointing to the top of the "lug": "Mild steel bolt heads to be marked "M" here".

Still ferreting for any additional information.

[mrclark303]

Here's a picture for the record chaps.
Attachment 84735

[breakeyp]

2. "Face of Bolt Head and Hook only to be case-hardened to a Minimum depth of .01" "

How do you measure this in production? Case depth requires the part to be cut in two, polished and dye etched and inspected with a microscope.

By today's standards, the specifications and materials listed are all over the place. I suspect heat treat performance between wrought iron and mild steel is very different. It took true craftsmen to main the quality that they did.

[mrclark303]

As far as I can tell, mine is original to my 1917 SMLE, still perfectly serviceable, so 100 years and counting...

[Bruce_in_Oz]

How do you measure this in production? Case depth requires the part to be cut in two, polished and dye etched and inspected with a microscope.

I would imagine that the process was developed by using just such a procedure as you describe, to arrive at a "recipe" that would achieve results in the desired range. A bit like firing "ranging shots", plotting the results, doing some fancy trigonometry, then going to "fire for effect".

And given the rather primitive nature of assessing the temperature of a component undergoing heat-tratment, before the advent of "modern" instrumentation, a fairly large "fudge factor" would have been employed. This may also partially explain the VERY specific note in the general specifications, that ALL steels used were to be "Carbon" steels, and that "alloy" steels could only be used after evaluation and approval by the "head shed" at Ordnance.