How to headspace?

[Peter Laidler]

Worth bearing in mind that locking shoulders and barrel nuts were not issued as spare parts originally. They only became spare parts from about 1942 or 3 as I recall due to the dire need from several fronts

[tr63]

Gravity,

Virtually all US Bren Semi's were made from demilled receivers so that the internal dimensions of the receiver may not be in spec., as a result of welding or incorrect initial alignment. Changing bolts , barrels and locking shoulders may help on a spec receiver but if it easily closes on a field gage and you are blowing the shoulders on .303 brass cased ammo the problem may be larger. Brass cased .303 is very tolerant of excessive headspace.

You can get locking shoulders from BRP, ask for the largest he has. The difference between the smallest and largest is less than .015". The difference between the GO and FIELD gage is .010" . the barrel shims and armor's Locking Shoulders were never meant to correct for incorrect receiver dimensions. They were made to correct for normal tolerances and wear & tear on the receiver as a result of usage, not initial out of spec tolerance on the receiver.

As you noted the first thing is to find how much it is out.

Strip the frame, carrier and bolt. Remove the firing pin and extractor from the bolt. Take your GO Gage. Slide the gage in the barrel. Now only install the bolt without extractor, no carrier springs etc, only the Bolt. Push it into the locked position. Push the bolt forward with your finger. The case should be snug under the bolt. If it isn't check to be sure the bolt isn't resting on the lugs on the receiver. You can't headspace with lugs interfering with the bolt resting on the head of the gage. With your finger still pushing on the bolt and using a feeler gage you should have between .002" and .005" between the bolt and locking shoulder. If its .010" or better you may have problems. If its .015" plus IMO you definitely will have a problem. You probably will have to replace the locking shoulder with a longer one. The feeler gage check will tell you how long the LS should be. Spec is a GO gage (.064") and .002" between the bolt & LS.

If you can get a longer LS replace with the longer one and recheck. If it is way out swapping barrels, shims etc probably won' get you in spec.

Other options:

Make a new LS

TIG 4140 rod on the face of the short LS and grind to size. Preheat the LS before welding. It does work, I've done it.

Joe

What can you do if the bolt is making contact on the lugs on the receiver? Is the bolt front angled , closer at the top and tapers away at bottom ?

[Joe H]

Tr,

The bolt contacting the lugs is usually the result of a weld up on the "MSG" type of demill (a result of the location of the demill cut). I have firsthand experience. Use a pencil grinder or dremmel and grind the lugs back until they have .005" clearance with the closed bolt.

The bolt is vertical in the front and angled in the back to match the Locking shoulder



Joe

[gravityfan]

Joe H, thanks for the info, that's going to be useful.

"You can get locking shoulders from BRP, ask for the largest he has."

For BRP, Google gives many hits, but none sound like the right place for a LS.

Can you point me to the right place?

Many thanks.

[Joe H]

Gravity,

Click on link below:

Home

Go to Parts / Bren then on to the parts group. Many Bren parts reasonable pricing.

Joe

[gravityfan]

Thanks Joe.
A great resource to bookmark.
Some good No.4 parts too.
Much appreciated.

[tr63]

Has anyone a picture showing where the feeler gauge is used in head spacing the bolt ?

[Peter Laidler]

You don't need feeler gauges TR, you just need the two GO and NO-GO gauges. I always used to CHS with the actual gun breech block LESS the extractor assembly. Gage into chamber gun nose down on the bench without the butt slide fitted, breech block fitted into piston extension*. Slide extension slowly forwards under finger control. It must fully lock-up and 'fire' on the go gauge. Same again and it should NOT fully close and as such will definately NOT fire on the no-go gauge. As simple as that

* we had a load of slave items used for these sorts of tests in the big workshops and had a slave piston extension without the piston for these sorts of tests. But it MUST have the piston post as the striker must indent the wax filling in the end of the CHS gauges. This shows actual FPP in real time.

[stencollector]

gravity and TR: I have a reasonable pile of locking shoulders for Brens. If you are not able to find what you need from BRP, give me a PM.