No.1 Mk3 Bolt Cracks

[Surpmil]

I'm sure I've posted pic's of fractures around the small lug before...................that's my excuse to visit the local aircraft maintenance boys and use the crack testing gear.

As have others here. The granular structure at the breaks in those photos was commented on IIRC. I don't know if that was within the normal range or reflected progressive embrittlement due to stress.

The recoil lug in the body that was in closer contact with the bolt lug would have taken more load than normal obviously. Whether that would cause deformation of the lug in the body I don't know, but are they not surface hardened? Sub-surface deformation? Seems likely that the lugs would already have already "compressed" from prior loading as much as they ever would?

As for bolt "set-back" during proofing I will leave that to the experts to explain as well!

Another virtue of the design seems to be that if the bearing is unequal the worst that can happen is this kind of failure, rather than ejection of the bolt, because no matter what happens, that large recoil lug isn't coming off the bolt body.

[RobD]

Peter, have I understood you correctly?
Do you say that
(a) the Lee Enfield action body never stretches [or at least, never stretches significantly];
and
(b) this is because the forces produced on firing compress rather than stretch the Lee Enfield action?
Rob

[Pzkw38]

Yes, that does not make sense, the bolt will be compressed by the charge, react on the action which will be in tension from the bolt engagement to the breech. Else that bolt is moving rearwards.

[30Three]

The pressure on the bolt is directed on to the lug surfaces at the rear of the receiver. There is no load to stretch the receiver from the barrel to those lugs; just the felt recoil of the rifle moving rearward.

[Peter Laidler]

RobD, Post 22, yes. There is nothing for the body to be in tension against. The bullet is travelling up the barrel while the cartridge case is pushing the bolt rearwards. Equal and opposite and all that. I wrote this all up as a paper. Bren, L1A1, they're all the same. As students it was a very simple and easy to see classroom project.

PZKW, cvan't fathom out whether you agree with the laws of physics or disagree. You'll have to elaborate. There's nothing in tension..........., the round is moving forwards, pretty-well frictionless* up the barrel. I even explained this in the original paper too. I wonder if the paper might be in the L1A1/FN thread somewhere when that related to the mythical 'stretching' bodies.
* there is friction oif course but nothing to cause sufficient tension in our lifetimes sufficient to 'stretch' a body!!!!!

[CINDERS]

What I alluded to with the original OP's dilemma was the spare bolt is not just a drop in fit and away you go, I could swap the bolts on my wifes 308 Barnards and they will work just as well in either that's because Barnards are held to ultra tight tolerances built for precise breeching up by the gunsmith (My G/S is excellent at his craft when he does a barrel its in a locked up workshop so no numpties will annoy him from the shop)
Dropping a random bolt into a random Lee Enfield without the required fitting of the bolt & bolt head up correctly is just asking for trouble or you may just jag it and be as happy as Larry just do not shoot next to me........

[Maxwell Smart]

Peter's discussion about action body stretching is the FN FAL forum in the thread "FN distruction tests", post #23

[RobD]

Peter, I am still lost here ...
in your original post on the subject which is here: https://www.milsurps.com/showthread.php?t=50730&page=3
You correctly say" Put simply, to stretch something, you need to pull it apart between two fixed points."
Now, I know nothing about the FN/FAL. But to me it seems obvious that when there is an explosion in the L-E breech, the forces are equally+oppositely directed forward [bullet] and rearward [ bolt face]. The bolt transmits these forces to the back of the action through the locking lugs, and hence through the back of the action to the shooter through the stock [recoil].
BUT the sides of the action body in front of the locking lugs and behind the barrel ring are, surely,"being pulled apart" between those fixed points? Or am I missing something? As a thought experiment, if you sawed through the sides of the action directly above the magazine well, and reassembled the rifle, and fired it, it seems to me the rifle would separate into two parts at that point. So, it seems to me that everything behind the locking lugs is compressed on firing - but the action side walls do experience a "stretching" force. [Whereas in a rifle with locking lugs at the front of the bolt, like a Mauser, virtually the whole action is compressed on firing].
Am I completely mistaken here? Indeed I thought there was a special L-E Armourers' bolt-type gauge which could be used to look for the stretching effect - am I bonkers?
Rob

[Terrylee]

An interesting thread. Of course, if the receiver does not stretch, the bolt must obviously be shortened under compression. Otherwise why do we require bolt heads which progressively need to be lengthened over the rifle's life?

[rcathey]

Rob,
I think at first blush I would have felt the same way about it as you. But putting on my engineer cap (my business card folds into one), I find it helpful to think of the path of least resistance. If one were to strap the action right at the barrel threads to the theoretical immovable object, then yes, it probably would deconstruct itself as you suggest.
But as a shooter, we’re not doing that. We’re placing the butt on our shoulder. So what seems more likely, the forged steel stretching or your shoulder moving and absorbing the shock?
Path of least resistance.
Now maybe I’m missing something too but that’s how I see it.