Bolt HeadcFitting Question

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Originally Posted by Bruce_in_Oz

It is "plug and play", all the way. According to ALL of the original technical literature I have seen, for No 1 Mk 111 rifles, there was ONE "standard" bolt head, with a "qualified" thread and a set distance (with the usual tolerances) from face to the shoulder forward of the thread. No "Shake and Bake" assembly here.

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My understanding from the instructions to armourers manuals is that the No1 MkIII bolt heads were anything but 'plug and play' with every one needing to be individually fitted to each rifle to ensure correct headspace and overturn. This is further supported by a post by Peter Laidler on May 5th 2008 where he states :

Spare bolt heads issued from the factory were actually oversize and marked with a small ‘S’….., but nobody can tell me by how much! Other Armourers of the period have told me, only yesterday over a frantic phone call, that this is incorrect but they WERE all to the longest specification.
Whatever it is/was, there should be room to stone to size. And THIS is where Armourers were always taught DON’T OVER CHS. Or in this case, should that read don’t UNDER CHS. If your rifle closes on the .074” NO GO gauge, this is what you do. Go to the No1 bolt head drawer and select half a dozen bolt heads that don’t overturn by more than 10 degrees (later, 15 degrees was permitted to make best use of remaining spare parts stockpiles), the bolt face is not ringed sufficient to allow the escape of gas past the primer and the striker hole is not greater than .084” dia. Try them all until you get the best fit. If necessary machine or stone the bolt head square and true until it closes over the .064” gauge and doesn’t close over the .074” gauge. The point at which the bolt doesn’t close prior to the .050” limit is academic because so long as it doesn’t go/close, it’s passed the test.

Now, how you shorten the bolt head it is up to you. You can machine it in a lathe if you like but some are quite hard, or surface grind but I was taught that the best way was to rub the face down on a sheet of ‘400’ wet and dry carborundum paper on a sheet of glass, just covered in slow running water. Go round and round with equal pressure, rotating the bolt head slightly every so often, taking a gnats knacker off at a time for several minutes and trying it again and again. Every so often, smear a smidgin of engineers blue on the rear of the .074” gauge and close the bolt head lightly against it to ensure a crisp round witness mark on the face of the bolt. This is the acid test of it being perfectly square to the bore. Be sure to remember these old Armourers technical words such as ‘gnats knacker’ meaning something too insignificant to be measured and ‘smidgin’, indicating a quantity equivalent to a gnats knacker.


Then, of course, there were the two different (non-interchangable) designs of bolt heads - the Mk1 and the Mk2
(photo from member 'Bear')

I have to say some the writing here is the best on any forum I have seen in quite some time.

"Plug and Play" was probably a poor choice of terminology, especially with regard to the No1 rifle and its predecessors. The fitters had trays of bolt-heads that had passed metrology at a previous stage. At Factory level,they also has stands of bolt bodies..

The trick was to meet a number of "requirements", more or less, simultaneously. .

The overall length of the bolt head had to work with a striker to produce striker protrusion within tolerances. Pretty basic.

Then that bolt head had to thred onto the bolt body with "zero" underturn; (or it could not be fitted into the rifle "body") and a VERY small allowance of "overturn, so that the firing load was transferred to the interface of the bolt body and bolt head "shoulders".

The bolt body / bolt head assembly then had to produce correct headspace (within tolerances) in the designated rifle body/ barrel assembly.

Meanwhile, the striker had to fit FIRMLY into the cocking piece. it is, effectively an interference fit; the firmness of the alignment is NOT determined by the thread, but by the short parallel sections forward of the thread in BOTH components. If this fit is NOT seriously firm, the cocking piece will "float" and engagement with the sear and safety mech will range from "annoying" to "BLOODY DANGEROUS".

The safety catch and locking bolt had to work to inspection standard with the cocking piece and the trigger and sear. Additionally, the cocking piece also had a very tightly specified set of angles and travels.

How many variables can dance on the point of a firing pin (striker)??.

Just for giggles, the barreled a bodies had a "different" chamber; slightly undersized.

The entire mechanism (sans wood) was fired with a "special" proof round. This was intended to "shock" fit the barrel to the body and "final seat" the bolt locking lugs into the shoulders in the body.

The 19th Century was an "interesting" place.The myriad changes to virtually EVERY component, (apart from the stock bolt in the butt) to produce the no4 basically HAD to happen, otherwise the rapid, mass transfer of production to "subcontractors could not have happened as smoothly.

The No1 rifle was essentially the product of a very "upmarket" Khyber Pass system, the No 4 borrowed somewhat from the automotive industry. No1 rifles are full of weird "Enfield Special" screw threads. No4 rifles went with "British industry standards like BA and BSF, thus the components could be 'farmed out" to a myriad small engineering shops. The tooling used to form the threads used pre-existing cutting tools and more importantly, "industry standard" precision gauges to CHECK tool wear and the threads cut with these tools, at factory level.

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Originally Posted by Bruce_in_Oz

"Plug and Play" was probably a poor choice of terminology,

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All of the above is true, it was simply the choice of the words 'plug and play' I was questoning, as a newbie to LEs would take that as meaning 'take a bolt head out of the tray and it'll be the correct fitting, working perfectly', which we all know, is not very likely.

Somewhere in that tray of a 100 bolt heads is (maybe) the 'plug and play' bolt head, but finding it is not easy.

The bolt head chart that was on here a few years ago think ADe was updating it (Think he was the OP of it).

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Originally Posted by CINDERS

The bolt head chart that was on here a few years ago think ADe was updating it (Think he was the OP of it).

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Yes it was mine.

As you can see you can have a number '0' that is larger than a number '3' so specifying I want a 'number 3' is pointless, it is important to specify the actual measurement you need.

This was part of the original post I made :

Very few of us will have a bucket-full of bolt heads to work with so we need to have an idea of what bolt head size we need to overcome the headspace problem, headspace gauges will tell us it fails but not by how much.
A simple way, that gives a close approximation is by using a case and a primer :
Since the 303 headspaces on the rim, its headspace is the distance from the front of the rim of a seated case to the face of the bolt. To determine that distance we:
• Measure the rim thickness of a new case. A fired case will have expanded and closed the headspace. You must use a new, i.e., unfired case. Measure around the rim and use the highest value obtained. For an example, we'll say it measures .062".
• Take a fired primer and start it into the unfired case's primer pocket. Seat it just enough that it doesn't fall out.
• Hand chamber the new case, and slam home the bolt, seating the fired primer with the bolt.
• Remove the case from the rifle's chamber, and measure the distance the primer protrudes from the case head. For an example, we'll say it protrudes .014".
• Add the rim thickness measurement to the primer protrusion measurement and you have your rifle's headspace, at least you have a close approximation of its headspace.
In our example, 0.062" + 0.014" = 0.076"


Thus if your headspace comes out at 0.076” and your bolt head is currently 0.630” then you’ll need a bolt head a minimum of 0.002” (two thou) longer but ideally somewhere between 0.002” (to give the 0.074” max) and 0.012” (to give the 0.064” min).
Instead of specifying “I want a number 3 bolt head” you can now look for a bolt head between 0.632” and 0.642”

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Originally Posted by CINDERS

The bolt head chart

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That was the one I was referring to. I've used it myself in past. The information is very clear.

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Originally Posted by Alan de Enfield

Yes it was mine.

As you can see you can have a number '0' that is larger than a number '3' so specifying I want a 'number 3' is pointless, it is important to specify the actual measurement you need.

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Just to clarify slightly Togor, if that's needed: the reason bolt heads can be found with such widely varying or inconsistent measurements relative to their marked lengths is mostly due to previous grinding and lapping by owners or armourers, rather than poor QC in manufacture.

Generally speaking you can be pretty confident that the 0,1,2,3 size marking does indeed reflect a longer bolthead, but by exactly how much? The original size difference was 0.003" between each number.

Civilian owners including myself, tend to want the bolt to just close over the "0.064" GO gauge, but anything between 0.064" and 0.074" was considered acceptable so there was little need to grind boltheads unless as the manual says, the bolt would not close over the 0.064" GO gauge.

There's really no substitute for the correct gauges.

Yeah it all makes sense now.

In my case, both the original and "shooting" bolt head gauge between .628" and .629". And the replacement bolt head times up right on the rib. Just waiting for an extractor screw and better weather.

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Originally Posted by Surpmil

Just to clarify slightly Togor, if that's needed: the reason bolt heads can be found with such widely varying or inconsistent measurements relative to their marked lengths is mostly due to previous grinding and lapping by owners or armourers, rather than poor QC in manufacture.

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That would certainly account for 'undersize' bolt heads but not 'oversize' (outside of the expected) bolt heads.



What Size Should The Bolt Heads Be ?

It seems to be generally accepted that No4 / No5 bolt heads should fall in the following range :
Size 0 = 0.620” – 0.625”
Size 1 = 0.625” – 0.630”
Size 2 = 0.630” – 0.635”
Size 3 = 0.635” – 0.640”

Whilst I don’t question that this information is printed somewhere, I have not been able to find the original source, the information I have found does not actually state the ‘starting’ dimension, only that the sizes are incrementally larger :

a) From “Parts Identification List Rifle No4 Mk1 and Mk1*” dated 1945
Bolt head 0 part number BB8584
Bolt head 1 part number BB8585
Bolt head 2 part number BB8586
Bolt head 3 part number BB8587

“Number 0 to 3 increase in length by increments of 0.003 inch”

b) From Canadian National Defence Manual “First Line Maintenance Instructions Rifle No4, all marks” dated 28th June 2002

Bolt head size 0 NSN = 1005-21-103-1143
Bolt head size 1 NSN = 1005-21-103-1144
Bolt head size 2 NSN = 1005-21-103-1145
Bolt head size 3 NSN = 1005-21-103-1146

“…….. the sizes being progressively larger in increments of 0.08mm (0.003”)


What I cannot find in any document is the 'starting size' of the #0 bolt head such that the 0.003" incremental increase per number can be referenced.