Repairing Bren Mk 1 barrel

[Flying10uk]

If you are cutting a metric thread on a lathe that has an imperial lead screw, in my experience, you will have to keep the lead screw engaged until you have completely finished cutting the thread. Likewise if it was the other way round, cutting an imperial thread on a lathe with a metric lead screw you would also need to keep the lead screw engaged until the thread cutting is completed. I think that there is a formula for working out the change wheels required which I can't remember but will have a think.

[Richard-]

Joe H, I got the mill test certificate for my barrel blanks.

Fortunately they were heat treated and have a minimum proof stress of 96,000 psi!

As others have mentioned I needed to get oversize blanks - I went with 35mm from memory.

Fortunately a local supplier could make these for about $350 each from memory.

Surprisingly in Australia we have quite a few barrel blank makers.

So ideal for 7.62 NATO.

Thanks very much for the comments on your repair method Woodsy.

I think I'll try that with the Mk 1 barrels, and see how I go!

[Peter Laidler]

Within an hour or so in a decently equipped workshop I'd have had that/those two(?) barrels with a hole through the chamber up and running. We seem to be doing the 'don't mistake motion for action' scenario with hypothesis and a fair bit of what diplomats call waffle!

A bit of BAR and JoeH common sense and my notion of '...the bleedin obvious' plus a good helping of 'K.I.S.S.' is all that is required.
Accurately end-mill out the chamber of the cross drilled Bren barrel to .020" short of the case length to a suitable diameter. OK so far?
Machine the length of No1 barrel chamber exactly to suit. External dia to be what we engineers that haven't forgotten what it's like at the working end of the workshop call 'a stomping good interference fit'. Heat chamber end of barrel and freeze insert. Align and press FULLY in until tight fit.

That is it. 2 hours work.

The chamber you have new fitted is the same as the chamber you have milled out. The fact that the new chamber is .020-.025"" or so short of the existing chamber means that the last .020-025" or so of the cartridge case will o9verlap or cover the joint line between the two. Round is fired, cart case expands and effectively seals the joint line. Radial strength of existing Bren barrel remains.

Nope........ 90 minutes simple work............ Let's stop talking and get on doing..........

[browningautorifle]

Accurately end-mill out the chamber of the cross drilled Bren barrel to .020" short of the case length to a suitable diameter.

The fact that the new chamber is .020-.025"" or so short of the existing chamber means that the last .020-025" or so of the cartridge case will o9verlap or cover the joint line between the two. Round is fired, cart case expands and effectively seals the joint line.

Sounds good. That would work. As long as the gasses aren't allowed to push on the division of the two it should stay.

[Flying10uk]

Did the British/Canadian manufactured Brens retain the metric thread to the barrel and nut or, as has been suggested switch to an imperial thread?

[Joe H]

Did the British/Canadian manufactured Brens retain the metric thread to the barrel and nut or, as has been suggested switch to an imperial thread?

Flying10uk,

I believe the thread stayed the same, 7mm. All Bren barrels are interchangeable, at least as far as I can tell.

I checked out my lathe owners manual. To my surprise the lathe will cut a 7mm thread (coarsest metric available) using a 127 tooth gear doubled up with the normal 120 tooth idler. I have the 127 tooth gear because at one time I cut some much smaller metric threads. I do need a 30 tooth change gear. I have a 30 tooth but not with the Colchester spline drive. I'll have to modify it or buy a new one from a UK supplier. Cutting my own threads may still be in the realm of possibilty

Joe

[Peter Laidler]

I don't believe that it is any particular thread as such, just an acme type thread form on a diameter. The idea of making the lugs an interrupted thread, that could be used to draw the barrel tightly rearwards was a British idea that was incorporated into the design at our request. Previously to that the lugs were just that - interrupted lugs. And in any case, there is absolutely no need whatsoever to mess about with, re-cut, fix, etc etc the barrel locking thread. They are an intergral and important part of the geometry and alignment of the barrel. LEAVE THEM ALONE

Once again, as I mention in thread 23, this has no bearing on the subject of fixing a knackered chamber.

I had thought about the prospect of gas leeching into the radial joint BAR. That's why I suggest a slight overlap by the cart case. If the joint was forward of the case end, that'd be an entirely different matter. But they did it with the stellite lined GPMG barrels didn't they - and it worked there. (but failed for an entirely different reason).
Additionally it'd take a LOT of gas to move an insert that was a stonking good hot/cold press fit in my opinion.
Additionally (again.....) remember that while that high pressure explosive force is acting, the flat breech face of the BB outside the rim recess is also facing the rear face of the new insert. And the BB is locked absolutely solid AND absorbing this extremely high pressure we're talking about.
Additionally (and again......) Better still, the BB face remains there, locked up solid until the high pressure has completely dissipated to atmospheric pressure. Only THEN does it unlock, primary extraction...........onwards and upwards

Why are we waffling on and on and on and not just getting on with the bloody job. I'd have that gun rock and rolling this afternoon. Frustrating or what.......

[browningautorifle]

Additionally it'd take a LOT of gas to move an insert that was a stonking good hot/cold press fit in my opinion.
Additionally (again.....) remember that while that high pressure explosive force is acting, the flat breech face of the BB outside the rim recess is also facing the rear face of the new insert. And the BB is locked absolutely solid AND absorbing this extremely high pressure we're talking about.
Additionally (and again......) Better still, the BB face remains there, locked up solid until the high pressure has completely dissipated to atmospheric pressure. Only THEN does it unlock, primary extraction...........onwards and upwards

All that is correct, as I proved a couple times in practice already.

But they did it with the stellite lined GPMG barrels didn't they - and it worked there. (but failed for an entirely different reason).

Yes they did and yes it did...at least the theory worked.

[Flying10uk]

Slightly off topic, but another item on the Bren which I happened to notice recently that remained metric is the tripod attachment pins @ 12mm diameter.

[Peter Laidler]

We were not permitted to change anything from the moment the UK spec was approved by ZB when we were licensed to make the gun. We could change it but the financial penalties were large. We did change certain things for the Mk2 and 3 guns as wartime expedients but paid dearly. In a roundabout way, it is this licensing agreement that saw the heavy out-of-date and almost obsolete Mk1 gun continuing in production plus royalties which we honoured to the letter, right up until the war ended.

Same applied to the GPMG