No4 Bolt head question

[WanderingKiwi]

I am sure that, over the years, this question must have come up but I can't find it in the forums:
Is there a technical reason why an No4 bolt headspace/clearance can not be adjusted by adding a shim behind the bolthead?

[Alan de Enfield]

I am sure that, over the years, this question must have come up but I can't find it in the forums:
Is there a technical reason why an No4 bolt headspace/clearance can not be adjusted by adding a shim behind the bolthead?

1) Moving the bolt head forward means the striker will now not be long enough.

2) It is very poor engineering practice.

3) The official answer is :"If headpsace cannot be achieved using existing bolt-heads then it is the body that is scrap. Typically the hardness has worn thru' on the body locking lugs and every firing / recoil pushes the bolt locking lugs further and further back and no bolt head can accomodate that"

[Bruce_in_Oz]

The striker protrusion is set by the overall length of the bolt-head . (Given "consistent" striker dimensions)

Furthermore, when the action is closed for firing, the bolt-head is set to transfer ALL of the thrust od the cartridge head DIRECTLY to the front face of the bolt body, and thus to the locking surfaces. The bolt-head THREAD should play NO part in transferring this thrust.

This is why bolt-heads are "paired" to their bodies in the final assembly. If the bolt-head "under-turns", the bolt CANNOT be assembled to the body. Get another one out of the parts tray.

If the bolt-head OVERTURNS more than a few degrees, The threads will start being battered at every shot and headspace will thus start to grow.

Lee Enfields are NOT "shake and bake" rifles.

There are copies of the "official word" on this matter out there on these Inter-Tubes.

How many variables can dance on a Km7 bulllet meplat?

[togor]

I think in some cases the threads end up taking some of load initially but loosen up with repeated firing until load is taken by the bolt body.

This is just how it has to be for any bolt where the head can over-rotate slightly with the bolt out of the rifle. Some small over-rotation I am told is allowed by specification.

[Surpmil]

The striker protrusion is set by the overall length of the bolt-head . (Given "consistent" striker dimensions)

Furthermore, when the action is closed for firing, the bolt-head is set to transfer ALL of the thrust od the cartridge head DIRECTLY to the front face of the bolt body, and thus to the locking surfaces. The bolt-head THREAD should play NO part in transferring this thrust.

This is why bolt-heads are "paired" to their bodies in the final assembly. If the bolt-head "under-turns", the bolt CANNOT be assembled to the body. Get another one out of the parts tray.

If the bolt-head OVERTURNS more than a few degrees, The threads will start being battered at every shot and headspace will thus start to grow.

Do we have a consensus on the acceptable limits of over-turn? IIRC 12° was mentioned by Peter in connection with the L42A1?

When I mentally compare the interfaces of the female threads in the bolt body and the male threads on the bolthead I am reminded of the Ross 1910 action or an artillery breech block with interrupted threads: the (partial) recoil force is spread out between the surfaces thus giving the great strength of those designs. Is there any reason to think the same principle does not apply with bolt and bolthead threads?

If a bolt and bolt head are mated such that the head is aligned with the recoil lug and a feeler gauge slid between the two circular shoulders, one can determine the distance that the overturn if any represents, but when the bolt head is screwed in tight to the bolt body it is the forward faces of the threads on the bolthead bearing tightly against the rearward faces of the threads in the bolt body, so there is in fact no load-bearing contact between the two threads in that state, and all the thrust that is not contained by chamber wall adhesion (the vast majority of it), goes onto the mating shoulders of the bolt and bolthead.

Where there is extreme over-turn there is perhaps the potential for the the two thread faces to contact each other: the forward face of the female threads and the back face of the male threads, and in that case thrust could indeed be transferred onto the threads, but I can't see that being a problem given the thrust/load distribution mentioned above. What is a problem of course is that the greater the over-turn the more of the rearward motion of primary extraction is lost to the gap between the two threads. As the bolt handle is lifted, the bolthead is being unscrewed from the bolt body and therefore in principle moving towards the chamber, not away from it, though the travel is so short that the clearance between the two threads probably means there is no actual forward movement. The rearward motion is produced by the camming action of the front face of small recoil lug bearing on the forward face of its recess in the rifle body.

Stopping to think about the design overall, that is probably the primary purpose of the small recoil lug!

Comments or corrections welcome.

As for adding shims to the back of boltheads, people have tried though I'm not sure why! Probably a target shooter thinking a greater area of contact might improve accuracy "somehow"?