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Enfield Headspace, Measurement and Bolt head Replacement

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Alan De Enfield
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Enfield Headspace, Measurement and Bolt head Replacement

#1 Post by Alan De Enfield » Tue Jun 13, 2017 8:03 am

Background:
A few years ago, having always wanted a ‘bit of history’, I purchased a Savage No4 Mk1*, it turned out that this rifle had a No3 bolt head and whilst it ‘passed’ the headspace test I thought I’d better get a bigger bolt head for when I wore out the size 3, I started looking for a size 4 bolt head which are only available in the land of ‘unobtanium’ (In the last ten years I have only ever heard of 3 examples).
During my quest I was informed by several ‘old hands’ that the bolt head size (number) was a guide only and in many respects was a meaningless number, “Cannot be right” – I thought, why put a number on it if its meaningless, and thus started my research into bolt head numbers / sizes.

I measured my (slender) supply of bolt heads and had a good response from members of various forums, which gave me a good basis for the evaluation, with these additional ‘numbers’ I feel the results give a good representation of the actual dimensions of 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 Are The Results Of The Survey ?


Firstly points for consideration :
a) Used bolt heads will be worn and should be slightly shorter than new bolt heads.
b) The measurements were taken by many individuals, with varying measuring equipment. In quality control circles this would be totally unacceptable but for the purposes of this survey it is the best we can do, and does give results that demonstrate the point of the survey.
c) There have been instances where a bolt head falls way, way outside the expected (ie a size 0 measuring o.641”) and it turns out that some enterprising individual has ‘peened’ over the rear face of the bolt head by as much as 0.010” (10 thou) to tighten up the headspace, these measurements have been discounted from the results.

The numbers x1, x9 etc are the number of samples reported. (x1 = one sample, x2 = two samples and so on) Any further contributions will be gratefully received and the table updated periodically.

How often do we see the advert “Wanted size 3 bolt head” – hopefully we can now show that its not a size 3 bolt head that is needed – just a slightly longer bolt head.
Example (with sizes taken from the above survey) you have a size 2 bolt head that measures 0.630” but fails headspace by 0.003”, you actually need a bolt head that is 0.633”+ and this could be anything from a size 0 to a size 3

I don’t want to get into a headspace debate but we need to clarify what headspace we are looking to achieve.
The Enfield is a British Military rifle and it is they who set the headspace (if others want to set alternatives to achieve their own aims, then so be it). The headspace is specified at 0.064” min and 0.074” max.
So when measuring headspace ensure you use the correct gauges – the American SAMMI ‘field’ gauge is only 0.070” (The Sporting Arms and Ammunition Manufacturer's Institute (SAAMI) .SAMMI specifications call for a Minimum of .063" and a Maximum of .070" headspace in 30 calibres.) so you could be condemning a perfectly good rifle by using the wrong gauge.

The following are notes from Peter Laidler on proper use of headspace gauges and bolt head sizes :
Now that the gauge is into the chamber and the bolt is being closed, this is where the trouble lies. It’s important here to remember the often said phrase among Armourers of ‘DON’T OVER CHS’. Here’s another thing to remember during this. Because there is a camming action operating while OPENING the bolt, called ‘PRIMARY EXTRACTION’ this action also operates when closing the bolt. And the same primary extraction forces that will enable the infantryman to force the bolt closed and unlock and extract a possibly distorted shell case, covered in wet silt and mud in the saltwater Magrove swamps of Johore in Malaya that’s caused the case to stick hard to the chamber walls will also enable the butchers or bubbas to close the bolt hard against the gauge. NO-GO on the .074” gauge is when, using the lightest finger and thumb action on the knob causes a slight feel of resistance.
The next question you’re going to ask is ‘…where during the bolt closing movement is this slight feel of resistance acceptable --- Is it almost closed or, hardly closed or in between’? Good question and the answer in the bible reads thus: ‘…with a 0, 1 or 2 bolt head, there must be resistance onto the .074” gauge prior to there being a minimum of .050” from the underside of the bolt lever to the contacting point of the body socket’. Then it goes on to say ‘…….With a No3 bolt head the left edge…………..’ But I want you to forget this because the statement was too ambiguous. I suggest that you use my maxim of ‘WITH ANY SIZE OF BOLT HEAD FITTED THERE MUST BE RESISTANCE OF THE BOLT ONTO THE .074” GAUGE PRIOR TO THERE BEING A MINIMUM OF .050” FROM THE UNDERSIDE OF THE BOLT LEVER TO THE CONTACTING POINT OF THE BODY SOCKET. I want the master Masons among you to learn that by rote!
Using this criteria, if a No3 bolt head starts to resist half way closed and a No2 resists at .051” from the body side, then use the No2 bolt head. Remember DON’T OVER CHS
There are a couple of reasons for this. Firstly, it is from a point when the underside of the bolt handle is approx .15” from the sharp curve between the top of the body and the side of the butt socket, that the bolt effectively ceases to move any further forwards and is effectively locked.
The second is that prior to the point of fully locking, a feature called ‘mechanical safety’ comes into operation and (it’s getting technical now……….) the stud on the cocking piece will strike the stud between the short and long cam groove at the rear of the bolt causing a diminished force of blow to the striker, resulting in a miss-fire! Phew!
You will understand that this is a highly condensed précis of events taking in months of learning, investigating and examining undertaken by apprentices and I’m trying to cram it into a 40 minute lesson!
This brings me to another point………. It’s very basically this. If you cannot get headspace, even using the very relaxed criteria I have discussed here by using a new bolt and a No3 bolt head, then I’m afraid that it means your body is knackered. This quaint old Armourers technical term indicates that it’s seen better days ….., it’s xxxxed! You COULD allow yourself up to, say, .078” headspace but the technical opinion reached during research into this during the 50’s is that it’s a palliative and not a true cure.

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”

Finally when changing over bolt heads consider ‘bolt head over-rotation’.

Check the bolt head to ensure it does not overturn by more than 20 degrees when assembled to the bolt.
The 20 degrees may be measured as 4.76mm (3/16”) from the left corner of the bolt head lug to the left corner of the bolt column.
An over-rotating bolt head allows all the recoil to be taken on the threads, with in few rounds your headspace will be back to where it was.
A picture speaks a thousand words :
Bolt Head Wear 3.jpg
Bolt Head Wear 2.jpg
Attachments
No45BoltHeads001-1.jpg
"When a well-packaged web of lies has been sold gradually to the masses over many years,

the truth will seem utterly preposterous and its speaker a raving lunatic". Dresden James

72 usmc
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Re: Enfield Headspace, Measurement and Bolt head Replacement

#2 Post by 72 usmc » Mon Dec 23, 2019 10:13 pm

Thank you for reposting your past post. A great post.
To old to fight and to old to run, a Jar head will just shoot and be done with you.

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Re: Enfield Headspace, Measurement and Bolt head Replacement

#3 Post by 72 usmc » Tue Dec 24, 2019 6:25 am

Mods, for your consideration, please consider making this a sticky in the Enfield section.
To old to fight and to old to run, a Jar head will just shoot and be done with you.

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Re: Enfield Headspace, Measurement and Bolt head Replacement

#4 Post by DaleH » Tue Dec 24, 2019 9:01 am

Great info, thank you! I learned that fired primer trick a while ago and like it for rimmed cases.

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Re: Enfield Headspace, Measurement and Bolt head Replacement

#5 Post by revjen45 » Mon Jan 20, 2020 12:21 pm

Does this apply to the Ishapore also, i.e. does it take the same bolt heads??

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Re: Enfield Headspace, Measurement and Bolt head Replacement

#6 Post by 72 usmc » Mon Jan 20, 2020 2:30 pm

Ishapore SMLE .303 Enfield and the 7.62 Ishapore 2A/2A1) are not the same, the later is a 7.62×51mm NATO. I never messed with the .308 dog.
Now for the Ishapore .303 Enfield (see https://www.allaboutenfields.co.nz/smle/ishapore-mkiii/ ) it's the same specs. The Ishapore Rifle Factory, began manufacturing Indian SMLE rifles in 1909 using British machines and the same specifications as that followed by the Royal Small Arms Factory. Bolt heads are the same, but have different markings. Headspace is the same on the .303 GRI Enfield. I dislike them, I favor British made & marked specimens.
Here is some interesting information on different bolt heads. More info than I know, so this is good stuff:
https://milsurpafterhours.com/bb/viewtopic.php?t=6576
I like this statement from the above source:
Re: SMLE & Lee Enfield Bolt Heads
Quote
Post by brewstop » Tue Sep 01, 2009 4:29 am
Ishapore 2A/2A1 boltheads are identical to No1 MkIII items - in fact many of them are either recycled No1 boltheads (ie with visible older British or Indian inspection marks) or otherwise indistinguishable from the concurrent .303" No1 production items from Ishapore.

[BTW, so far there is no evidence that 7.62mm 2A1 rifles are made of any different materials or processes than normal Ishapore .303 rifles. The few tests that have been conducted under proper laboratory conditions seem to indicate that the 7.62mm receivers are identical to .303 receivers.]

The problem you have is that there is manufacturing variation in the bolt & bolthead thread starts. Enfields were never designed to be serviced by civilian owners buying one or two boltheads and trying to fit them - in service the armourer would have test-fitted a dozen or so boltheads from his stock of a hundred or so, in order to find one which (a) lined up within about 10 degrees past the rib (b) provided the correct headspace.

Whilst No4/5 boltheads had a rough size increment indicated by numbers 0-3, No1 boltheads not only came in unmarked size variations, but the armourer was expected to grind down the face of a "long" bolthead if necessary to achieve the headspace.

The 2A1 boltheads will fit a No1 rifle (including the .303 extractor & spring), but you may need to try several in order to find one that lines up correctly. Also be aware that new boltheads were often intended to be force-fitted by use of a special clamp (a spanner with an aperture in the shape of the bolthead) and filing to the threads - ie the bolthead starts off under-aligned to the rib, but the threads are adjusted to give a tight line-up.

I would caution about the need to change boltheads. Due to internet hysteria about headspace, a lot of owners seem to be compelled to go out and fit a "longer" bolthead. Whereas slack headspace in an Enfield makes little difference to rifle safety or performance (and this was exhaustively trialled), poorly fitted boltheads can lead to bolt damage (ie if the bolthead over-rotates too much, the recoil force is transferred to the bolt threads, and not directly to the bolt body), and over-tight headspace quickly causes wear in the locking lug surfaces - the quickest way to scrap the receiver. Most Enfields have lasted up to 100 years perfectly well on their original bolth

With bolt heads there is always a timing problem and correct thread problem. Not all are equal. Back in the day when bolt heads cost $5 we had cigar boxes full of them for fitting. Also not all SMLE bolt heads are equal, you have to measure each one and write down its actual size. Most just fire form and reload the that specific.303 cartridge on somewhat out of spec chambers and headspace with SMLEs. :shhh: If an enfield is miss matched, sometimes it is easier to just switch out complete bolts (that are correctly timed and matched) to gain a little headspace. But the day of $25 spare, complete bolts is also gone. Most of the old timer, Enfield guys got maybe 10-15 spares. SMLEs were a dime a dozen. I am not sure an Ishi is even worth fixing at the cost of todays parts.
To old to fight and to old to run, a Jar head will just shoot and be done with you.

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Re: Enfield Headspace, Measurement and Bolt head Replacement

#7 Post by 72 usmc » Mon Jan 20, 2020 5:33 pm

Two great references from the reference library at www.milsurps.com



Anyone for CHS? By: Peter Laidler
http://photos.imageevent.com/badgerdog/ ... 0_CHS_.pdf

Fitting rifle bolts. By: Peter Laidler
http://photos.imageevent.com/badgerdog/ ... theads.pdf
To old to fight and to old to run, a Jar head will just shoot and be done with you.

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