54R Bren Issues

[Joe H]

Peter,

You guys don't need any slack, you are the experts on the real thing. The real problem is that Djandj's problem just can't happen if he was firing the original Bren, its too well designed.

I also just added a bunch of info to my previous post.

[tankhunter]

Give us Brits a bit of slack Joe as we (me and Tankie) are talking blind here. As he says, we're just bit part amateurs here when it comes to the semi autos!

Yes, it's quite difficult, 'walking' amongst Bren's. When you have a white stick out in front of You!.......

[Vincent]

Also, It hasn't been explained, if any/ one of the return springs has been cut down/ Omitted either. To overcome the Differing pressure of the STRIKER Block Operating spring.

That’s a good question. The striker spring is absorbing some of the energy of the recoiling mass. But that energy is not released to close the bolt. Instead the energy is stored in the striker spring until the trigger is pulled.

The carrier and breach block must still be pushed into the locked position (and now held there) by the return spring(s). So, the man with the white stick sees the physics of the semi-auto quite clearly, I would say.

There has to be a balance between the striker spring and the return spring(s). The return spring(s) must have enough energy to close the bolt. And the striker spring needs enough energy to drive the striker. If too much energy is stored in the striker spring, there might not be enough energy put into the return spring(s) to close the bolt. If not enough energy is stored in the striker spring, the result would likely be light primer strikes.

Finding the right balance between the springs is what we call “tuning the springs.” It’s basically the same process for a lot of machine guns that are converted to semi-auto only. Some are striker fired and others are hammer fired. The energy for the striker or hammer spring is often taken from the return spring by shortening it or replacing it with a weaker spring.

One very nice thing about the Bren is the adjustable gas system. It lets you easily regulate the amount of energy being put into the springs to a degree. The MG42 is recoil operated. To make adjustments you have to change the return spring and/or the booster. It’s not difficult, but you have to have the parts with you. With the Bren you just change the gas setting.

I am guessing “light ball” needs a higher gas setting than “heavy ball”?

H/Spacing is insufficient If this is the case, then it is NOT correctly locked up when firing. & the pressure generated combined, WILL indeed cause this problem.

The locking shoulder should arrive today. The flooding we have been having here might have slowed it down a day or two though. If it doesn’t cure the blowouts, the springs might need tuning.
The return spring rod passes inside the striker spring on this design. If the striker spring is contacting the return spring rod and creating friction, a stronger return spring might be needed.

[Vincent]

Attachment 63182

Is it normal for locking shoulders to have different heights? The one I sent was too high and blocked the bolt path. It never occurred to me that some would be taller than others.

[Peter Laidler]

I didn't know they had different heights. Strange! How much different?

[Vincent]

The one I sent is .006" taller than the one in his gun.

[djandj]

The one I sent is .006" taller than the one in his gun.

Yep. Vincent is the greatest. I'm sending it back to have him trim it down. The question is since his shoulder didn't allow for much, if any, room for bolt movement when installed, (still couldn't get the .005 feeler gauge between the bolt and the new shoulder) should he pair back the edge by another .005 while he has it. Also, he is sending another 54R barrel to try so I will report back on that as well.

---------- Post added at 08:26 PM ---------- Previous post was at 08:20 PM ----------

Attachment 63182

Is it normal for locking shoulders to have different heights? The one I sent was too high and blocked the bolt path. It never occurred to me that some would be taller than others.

Yeah Vince, the pertinent measurement is the thickness in the channel. The sides were fine, it was the center cut that stopped the bolt.

---------- Post added at 08:28 PM ---------- Previous post was at 08:26 PM ----------

That’s a good question. The striker spring is absorbing some of the energy of the recoiling mass. But that energy is not released to close the bolt. Instead the energy is stored in the striker spring until the trigger is pulled.

The carrier and breach block must still be pushed into the locked position (and now held there) by the return spring(s). So, the man with the white stick sees the physics of the semi-auto quite clearly, I would say.

There has to be a balance between the striker spring and the return spring(s). The return spring(s) must have enough energy to close the bolt. And the striker spring needs enough energy to drive the striker. If too much energy is stored in the striker spring, there might not be enough energy put into the return spring(s) to close the bolt. If not enough energy is stored in the striker spring, the result would likely be light primer strikes.

Finding the right balance between the springs is what we call “tuning the springs.” It’s basically the same process for a lot of machine guns that are converted to semi-auto only. Some are striker fired and others are hammer fired. The energy for the striker or hammer spring is often taken from the return spring by shortening it or replacing it with a weaker spring.

One very nice thing about the Bren is the adjustable gas system. It lets you easily regulate the amount of energy being put into the springs to a degree. The MG42 is recoil operated. To make adjustments you have to change the return spring and/or the booster. It’s not difficult, but you have to have the parts with you. With the Bren you just change the gas setting.

I am guessing “light ball” needs a higher gas setting than “heavy ball”?



The locking shoulder should arrive today. The flooding we have been having here might have slowed it down a day or two though. If it doesn’t cure the blowouts, the springs might need tuning.
The return spring rod passes inside the striker spring on this design. If the striker spring is contacting the return spring rod and creating friction, a stronger return spring might be needed.

If I remember the builder correctly, I believe that he said that he used the 8mm recoil spring (to the front of the butt stock). The ones in the stock are definitely not OEM either.

[Peter Laidler]

.006"...........! That's got me puzzled everyone.........

What exactly is causing the breech block to hang up on the 'step' between the l/sh and the friction surface of the interior top surface of the body? The ONLY part that bears against that friction surface and could possibly encounter the .006" 'step' is the front part of the rear locking lug on the breech block. Think about it...... And even then, this front surface is chamfered and polished so that any minute obstruction, such as a tiny .006" 'step' in its path WOULD cause the breech block to simply depress slightly to clear the 'step' and carry on.

Nope, that apparent step isn't the cause here............... There is something more fundamental. A simple .006" step cannot stop a Bren working. I've just tried it by increasing the height of the l/sh by that amount, using a .008" (nearest I could get) shim and it still works perfectly.

Even if the l/sh screw protruded a tad, the breech block is chamfered appropriately to cater for it too. Clever lads these Czechs!

[Vincent]

There is something more fundamental.

Yes. I wonder if it has something to do with one of the carrier rails being ground off?

If am understanding correctly the feed horns on the breach block are hitting the rear surface of the locking shoulder. Like this:

Attachment 63204

I exaggerated the amount of overlap for the picture. It's only .006" in his gun.

Does the whole channel in the locking shoulder need to lowered or will a bevel suffice?

I read somewhere that some of these HA guns had the feed horns ground down. I will see if I can find it again. There might be some helpful info.

[Peter Laidler]

Nope...... got a live gun stripped in front of me. This might be different from your semi rebuilds but in the case you illustrate Vince, it'd be the advanced feed horns causing the BB to hang up. Or they might/could foul on the protruding l/sh screw - if it does. If so, then just grind a small chamfer around the threaded end of the screw!

However, I CAN get this feed horns hang-up situation to arise but only artificially.