Gap between barrel and receiver

[jmoore]

That looks like the gap ought.

Your Mk.V is the suspicious one.

[5thBatt]

That looks like the gap ought.

Your Mk.V is the suspicious one.

Been trying to find my feeler gauge to measure the gap on my MkV, if i dont find them over the weekend i will buy a new set.

[jmoore]

Found some pull offs. (Found more after taking these photos.)

The significant feature is the lack of a thread relief at the front face junction. Quite mad actually! All of the load (90% or so) is taken by three-four threads. So, losing a thread or two here would mean nothing! Having to run threads to a hard stop means that the cutter was quite fragile. Even if some sort of die was used. So it would be quite easy to have an oversize condition in an area that's hard to measure. Which could easily cause the barrel to stop rotating early. The natural reaction would be to remove metal from the rear face. It's where contact is supposed to occur, after all. Trim off a bit and try again. And so on. If cutting the rear face didn't work, the next most likely step taken by a hurried factory fitter would be the front face. Redoing the thread? Out of the question for that particular machine operator.

But it seemed to work in these instances anyway. The ones that loosened prematurely were rebarreled long ago. The "thread lock only" works partly because not much force is required to secure a barrel for shooting purposes. Just ask the benchrest crowd. It was helped by the fact that the No.1 had the bayonet attachment separate from the barrel, so no external torque loads would be applied except during front sight adjustments.




This barrel pictured above and below is one that made both front and rear face contact, BTW.




But even when rear face contact is made, too much can be bad. Here's a badly galled example:



Here's a scrap No.4 barrel that shows a much better threading design with a nice relief cut that makes both threading and setting the front face dimension easier . (Disregard the extra step cut into the shoulder during removal.):



Muffet.2008 might be able to 'splain all this better. (He already knew the drama.) Or any number of our fine mob...

[muffett.2008]

I don't think I need to JM, your pic's show it all, but to add a simple explanation for the uninitiated, by cutting a thread relief (removing the last two threads near a shoulder) allows the component to be locked up against a face with no binding of the threads, or step cutting (cutting a relief in the female face) to achieve the same thing.
This relief or undercut assists the machinist when thread cutting also, it would appear by those pics that the barrels were actually threaded on a die rather than a lathe, I guess that would certainly speed up production, it may also explain the misaligned thread on the third barrel from the left, or is it just my eyes?

Thanks JM, that's a great result and certainly brings the answers to the original posters question

[Bruce_in_Oz]

Receivers and barrels, especially in Lee Enfields are relatively "soft" and thus ductile.

Therefore as the barrel is wound in past he point of initial "seating", the threads will start to take up the load. It doesn't much matter whether the barrel is internally (SMLE) or externally (No4) breeched. Because of material yielding the load will be distributed. In high-grade tthreaded material (Grade 8) for example, there is pattern of load distribution on consecutive turns of the thread, along the lines of 35%, 25%, 15%,10%,9%, 7% and so on.

Essentially, approximately 75% of the load is taken on just three turns of the thread. In more ductile material, like SMLE receivers and barrels, the load will be distribured over more threads because of material crush. If you want to achieve better load distribution in harder materials, you need to produce exceptionally accurate and finely finished threads. The ductility of a material actually HELPS surface engagement, It just may not like being flogged repeatedly at 55,000 PSI plus.

There is NO significant improvement in shear strength if the thread engagement length is greater than the screw major diameter. An engagement length of 1.5 times diameter MUST, in ALL cases, provide at least ONE good turn, which will take up all of the load. Further axial stress will cause thread crush and start more thread engagements. (Up to a point).

Basic rule is that an engagement length equivalent to 80% of the nominal diameter will mean that the screw (internal fastener / barrel thread) will fail in TENSION before the receiver (female thread) fails in thread-stripping; assuming similar material properties.

Lee Enfield tenon threads are a little shorter than their nominal diameter, however, the "thread-loading" percentage rule applies. Basically, you will blow the bolt out sideways (because of the nature of the stress paths between the barrel and the locking shoulders) before you shoot the barrel off the front!

[Son]

basically, you will blow the bolt out sideways (because of the nature of the stress paths between the barrel and the locking shoulders) before you shoot the barrel off the front!

OH NO WE ARE ALL GOING TO DIE

hahaha... can't wait until somebody comes across that line in a google search!

[muffett.2008]

Savage shooter, alias LE owner, will have it up in the next post.

[Surpmil]

Surely.................. It's a case of what I call in mechanical engineering terms, the bleedin' obvious! If there's NO gap between the front face of the body and rear face of the nocks form, how will you know whether the rear face of the barrel is breeched up against the corresponding face of the body? I say, let's cut out the crap and stick with the facts!

Just happened to see this thread, and maybe comment is pointless, but I have an ENVOY which is breeched up this way: tight against the front or outer face of the body and against the inner face of the body at the bottom of the barrel thread recess. I'm not sure how the smith set it up, but he's done quite a few. I suppose he could measure the distance from the front face to the inner face with a depth mic if cutting the threads/shoulder on the barrel. Or if it was an already threaded barrel, screw it until it sits tight against one face and use feeler gauges to determine the distance between the other face and the body face, then remove the necessary amount of metal from the 'prouder' of the two, until the distance between all four faces is identical. The barrel should then screw in with both faces meeting at the same time, and compressing to the same extent as the barrel is tightened.

I assume!

[jmoore]

Generally both faces engage on Mausers as well. With the primary datum being the rear face against the internal collar.