Bren Gas Cylinder

[Joe H]

Guys,

Anyone have the specs for the I.D. of the Bren Gas cylinders. I found the spec for the piston head .626"-.6245". I'd like to be able to check a used cylinder before it is installed.

My interest is in rebuilding Brens into semi-autos in the US, particularly getting the gas system functioning properly. The semi's have an additional striker spring to compress so it's important to have full gas pressure.

I've read the thread about replacing the cylinder, no surpises there

Thanks,

Joe

Information

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[Joe H]

Guys,

I found the cylinder spec. So I'll answer my own question in case it would be useful for someone else . The gas cylinder ID .634" +.003". The piston is scrapped if the head is .618" or less.

Joe

[Peter Laidler]

I read your original request Joe and wondered whether to come in and say something. But at our big Base level workshops we didn't have a gauge criteria for the gas cylinder or pistons beyond that they should be in good condition and not pitted etc etc - you know the sort of thing. BUT, we also had a depression test where the gun was fired vertically and on automatic with a full magazine in two or three bursts into a sand filled depression pit. It should function flawlessly and fully. THIS was the acid test of a worn gas cylinmder and/or piston. Obviously we'd change the piston first, Not necessarily with a new one as we'd have a large tray of good used/perfect to select from. Then if these were failing, then new. Only if that failed to cure the gun would we revert to the gas cylinder. Then you just KNEW that there was a busy morning ahead of you because changing a gas cylinder on a Mk1 or 3 gun was never a simple job. Mk2's were a bit easier as we had a 2-part collared hydraulic puller that we could use!

But the acid test was the depression pit test where the gas on gas port 2 had to lift the working parts, feed AND cock the gun on the sear. That is REALLY making the gun work and when it's passed that test, you know that there is plenty of gas to spare should you need to use gas port 3 or 4.

[Joe H]

But the acid test was the depression pit test where the gas on gas port 2 had to lift the working parts, feed AND cock the gun on the sear. That is REALLY making the gun work and when it's passed that test, you know that there is plenty of gas to spare should you need to use gas port 3 or 4.

Peter,
What a great insight into the realities of checking the gas system. In the US our Bren semi builds use a rewelded demilled receiver. The striker fired versions which are the most common require an additional striker added to the recoiling parts plus the additional striker spring has to be cocked. These added components make the proper functioning of the gas system critical. With the added components I've found the semi won't function below the #3 setting and in many cases the #4 is required for 100% reliability. I doubt they would pass the depression pit test as they probably have little or no reserve. Some guys have enlarged the gas holes to get reliable operation. IMO the hole enlarging is probably a result of some binding as a result of welding the receiver together. It's a delicate balance going from an open bolt FA system to closed bolt (required) semi automatic .

I got the specs from the "Bren Manual" on the Weapons Guild Board. I think it did mention a depression pit test but I really didn't understand the significance of it. I was looking for some way to check the cylinder so that if necessary it could be possibly bored and sleeved while the receiver was still in pieces. I've had one experience with replacing a MK II cylinder It wasn't fun.

Joe

[tankhunter]

Joe,The bottom line is this. Any Bren is a GAS Operated Weapon Sysytem. If you have eliminated any possible Binding of the working parts. A simple check by allowing the gas piston extention to run back & forth. Lightly held at the back of the ejection slot at the rear with a finger. It should move freely with no binding at all.

The do the SAME, WITH. The breech block mounted on the piston extention afterwards.

When this condition has been reached, & if you are sure that the extra strain of compressing a striker spring Etc is still a difficulty.
Then there really is only one thing you can do. Enlarge the gas port in the barrel & gas block.
Do this CAREFULLY, a tiny bit at a time. As you can remove metal as required. But you cant put it back, so to speak! Ream a little at a time, reassemble & test fire at port 2. Until she functions flawlessly!

If you are STILL unable to get it to function correctly. Then there is only one component left. The Gas Regulator! Spares are plentiful your side of the pond I believe? You MAY also have to ream the gas port/s a LITTLE to get her to function at setting 2. Thereby leaving you plenty of 'Reserve' power should you need it.

To give you an example of how this was done. The now Obsolete (Sadly) 7.62mm L1A1 (FAL) Semi-Auto Rifle. Had a mod if required in this same direction. The first stage was to ream out the gas block & barrel gas port at the same time. Stage one was: use a 2.6mm Reamer, straight through till it entered the barrel. Then ream to 2.7mm. IF Required. Stage 2 was: Ream the same, to 2.8mm, then ream to 2.9mm.

You see how LITTLE metal was required to be removed to get the rifles to function correctly when 'Tired'?

In THEORY: The gas port in the barrel SHOULD already be large enough to enable a sufficient reserve of gas. Should you need to move the gas reg. The CHEAPEST method of the above suck it & see method. Would be to FIRSTLY ream the gas Regulator ports. As THIS is the CHEAPEST & SAFEST method of modification. A Gas regulator is cheaper & easier to replace than a Barrel, should you make a mistake!...

If you think about the Bren's design. there is also a Buffer mech at the rear of the Butt slide. This was to cussion the recoiling effects on the piston extention assembly. If the designers at BRNO put this setup in the original Bren design. It was there for a reason. As well as a buffering effect (Slightly) on felt recoil on the gunners shoulder. It should be borne in mind. That it also was put there to cussion the effects of the working parts hitting it. Why? because it was designed in the first instance with a gas port of sufficient size orginally. To drive the working parts rearwards with a gas port of maximum diameter.

It's a GAS OPERATED Weapon. If it does not function/cycle correctly after curing any binding. And also, The Breech Block clears the ejector Etc. It is a GAS Problem. (Use known GOOD Ammo as well!)

[Peter Laidler]

I go along with Tankie there - to a degree....... The gas port in the barrel is already at its largest/max size and can't be altered (- welll, I suppose it could.....). As is the gas port in the nozzle part of the barrel (the bit past the gas regulator that sits inside the gas cylinder)

The problem is in getting EXTRA gas through the gas regulator and believe me, these are HARD and TOUGH and will test any reamer that you are foolish enough to attempt to ream the hole/holes slightly oversize.

To give you some idea of how hard they were/are. On the DP guns, the original drawing specified that the outer circumference, across a width that covered the 8 gas vents, there had to be a .010" (?) deep groove machined. This would dissipate any gas should anyone attempt to use the DP gas cylinder. Anyway, the machine shops found the gas regulator soo tough and hard and so hard on the tools specified for the job/available that this part of the DP conversion was abandoned.

Anyway, that's the reason that you might see a small shallow groove around some Bren gas regulators

[tankhunter]

The Gas regulator could be softened by heat BEFORE reaming any of the gas ports. With modern metals, it might be possible to use a carbide drill to open the ports out as an option? As mentioned by Peter, & myself in my original post. The gas port in the barrel SHOULD be of sufficient size in it's present state before needing to touch that. It is the GAS REGULATOR you should tackle first. Though to reiterate what Peter has explained. The Reg's ARE very hard! But I look at it from this viewpoint. They Were drilled/machined originally. WHY therefore, can they not be modified today?.........I would go the heating up & letting down route in the first instance to soften the metal. Before attempting to ream/drill any of the gas ports in the regulator. (Don't forget to push out & remove, the retaining cross pin/spring at the rear of the regulator. BEFORE applying heat to it!!!)

[Joe H]

The Gas regulator could be softened by heat BEFORE reaming any of the gas ports. With modern metals, it might be possible to use a carbide drill to open the ports out as an option? As mentioned by Peter, & myself in my original post. The gas port in the barrel SHOULD be of sufficient size in it's present state before needing to touch that.

Tankhunter,

I was thinking the same thing. I have a small lab heat treat furnace which I use for heat treating small parts, so it would be easy to anneal the regulator. After reaming and testing it could be treated back to hard. As was noted the regulators are availble in the US at very reasonable cost. It would be alot easier to experiment with some HSS drills than expensive carbide drills and reamers. I would like to know my semi had a reserve gas capacity like the original Bren. Im also working on some caliber conversions so flexibility on the gas system may be very useful.

As with most parts that were hardened to this degree, the regulators, like the bolt etc. must have been machined before hardening.

I certainly appreciate your's and Peter's insight into the gas issue.

Peter, back in my much younger days I had a British Racing Green (Morris Garage) MGB, wire wheels, wood steering wheel. What a great fun car!

Joe

[jmoore]

The Historic Arms (Len Savage's conversions) SA Brens have had the barrel and regulator ports opened hugely. Even with no receiver binding, they have little power reserve. Don't have the sizes in my head, but it ought not be too hard to source the numbers.

[Joe H]

Guys,

I checked the gas hole sizes. The Bren barrel hole is .150" as is the #4 port on the regulator. The exit nozzle on the gas block is .161". The regulator #1 port is .101", #2 - .116", #3 - .136", #4 - .150"

Since I'm using #3 and mostly #4 I'm going try to redo the gas system to work on #2. Hopefully that will give me some measure of reserve.
Below are all Number (wire gage) size drills.
Trial Modified Gas System for semi-auto
Barrel hole - .185" #13
Gas block exit - .196" #9
Reg #1 - .1285" #30
#2 - .147" #26
#3 - .1695" #18
#4 - .185" #13
Barrel hole & gas block exit were no problem with HSS drill bits. The regulator was a different story so I annealed it and am sure it now can be drilled with normal tooling. I'm hesitant to use carbide since it chips easily and I'd have to buy all the drill sizes. I've got an old spare barrel & regulator to experiment with. I doubt it will be necessary to reharden the regulator after it has been drilled for semi operation.

Joe