I have no lock nut area on the front.....
Going to use a piece from another build to replace.


The barrel socket and related holes are my 1st step.
Going to weld up solid and re-machine details.

I laser scanned a MKIII section for dimensions.
Much easier with real dimensions.

 

Relaxing day to do some welding...

 

About time to start cutting the weld I added to the missing sections. Copper slug in barrel socket did its job and had good fill out. My receiver has a socket insert where the barrel is inserted. They torched about 1/3 of it away so I welded it up on the face to hold it then build up the other missing areas. I have a cheap IR thermometer and weld until the temps hit 450'ish then give it a break then weld some more.

Left part of the original dust cover detent hole visible and the new one I drill should line up and clean it up.

 

Now back to some L4 stuff.
Starting to machine welded areas. So far looking good with no voids , nice welds that are machining clean with carbide tooling.


One set up to match the angle on the mag well , inside area that was torched and the slots for the dust cover.
Next set up will be machining the barrel socket face and receiver sides.

 

Worked on leaves for a few hours today , A flame thrower would be more fun getting them while on the tree.........


After welding the torched barrel socket I needed to massage it a little to get it to accept a Bren barrel. Socket internal diameter in .903"

The copper slug I put in the cut section did its job and I had a very slight amount of material to remove. Rather than buying a flex hone I made a simple lapping tool from round stock.

I used some valve lapping compound turned by hand with a socket to lap it out.

I will hand stone the face and other welded areas , blend them & blast it and more forward to doing the barrel nut section when I can get a block of free time. Work sucks at the end of the year for me but at least the yard work is ending.

Fixing the barrel socket is a fair amount of work and what keeps a lot of cut up Bren's from ever firing again.
The rest of my build should be pretty conventional.

Next up was drilling the detent holes for the dust cover and barrel nut. Dust cover hole is parallel with the front barrel socket face. Barrel nut detent in on an angle. Everything machined nice without any surprises.

Moving on to the next section : Trashed barrel but area.

Torch guy wiped out the front barrel nut section as in its gone. I have a section from another parts set to use as a donor.
I will turn a copper slug the length of the barrel nut + an undecided amount to account for weld shrink.

Need to trim some pieces and fixture it up.


Blasted in the white~

Thank you torch guy~

 

About time to start cutting the weld I added to the missing sections. Copper slug in barrel socket did its job and had good fill out. My receiver has a socket insert where the barrel is inserted. They torched about 1/3 of it away so I welded it up on the face to hold it then build up the other missing areas. I have a cheap IR thermometer and weld until the temps hit 450'ish then give it a break then weld some more.

Left part of the original dust cover detent hole visible and the new one I drill should line up and clean it up.


Thank god for TIG welding!

Complete control of heat, filler and weld area.

Barrel socket was about 3 sticks of filler rod and good practice for laying beads.

Next up -

 

Cleaned up the rest of my L4 , stripped all the parts and degreased everything including the wood. Discovered my bolt , slide , lower frame and what's left of my receiver all have the same serial numbers which was a big surprise. My other Brens are typically mixmasters. Internal parts are in really nice shape and not fired much from the appearance of parts, bolt face and gas piston. Wood has a good amount or wear around the sling attachment point. It was put away dirty and have some surface staining and has likely been that way for decades.

Tacked the barrel nut section together in six places , removed the oversize ( +.012" length ) copper spacer and clamp assy. I fabricated and will weld it up. Hopefully my fixture helps keep the barrel nut surfaces parallel.

Guessing the shrink factor is always fun.

 

I am happy to be off this week and cant remember the last Christmas week I had off from work in decades.

Welded the barrel nut section together and the weld shrink was about .012" more than I guesstimated. I was more concerned with keeping the barrel nut surfaces parallel for even contact with the nut. I decided to make a basic tool to cut the front barrel nut surface clean then fit the nut OAL for a nice fit.

The tool I made pilots off the barrel socket driven from a 1/4" socket extension from the rear. The cutting face is .010" larger than the barrel nut diameter. I used the adhesive backed "slip tape" people put on walkways , because I had a roll and everything was closed yesterday. It actually worked out pretty well.
After welding everything stayed pretty straight so I used a Sharpie and colored up the front barrel nut contact area and buzzed it with my Ryobi drill.

After it cleaned up I measured it then put the barrel nut in a 3-jaw chuck with and faced off the front of the nut a couple thousands , test fit then back and forth until it just started to slide in. I switched to valve lapping compound and worked it until the nut throws smoothly and goes in and out correctly.

Its a simple task but I try to get every section right before moving on, its time well spent and avoided compound problems.



Tool~

It slides in from the front , put large diameter in first , slide pilot thru into barrel socket then push in 3/16" dowel pin that drives it.
I stuck a piece of the non-slip tape on the face of it and trimmed it with a razor knife.

Nice and clean~

Perfect fit~

Now that the barrel nut area is finished I can measure the front section , cut and get ready to weld.

 

Sarco....... and I spent a half day getting it fitted and its been interesting.

On mine the extractor relief clearance is clocked about 10 degrees off center when I got it to lock up.



I ended up lapping the hell out of the barrel threads ( the lead or extractor relieve is off ) to get it centered and locked up.

To do so I will require a .022" shim on the barrel face to lock up correctly with contact.



As a sanity check I have a bucked of several factory 303 barrels and all of them install and lock up correctly so its the Sarco barrel.



After getting it fitted I welded the front gas cylinder section on , without surprises.

Strangely , these all fit correctly~

Extractor relief centered , need shim for the gap~

Locked up~

Front section is reunited ~

 

Pinned the gas block , pressed a dowel pin in place and milled the front of the barrel for the front sight assy.



Need to pin it and drill the gas hole in the barrel.

 

Bummed that I have to go back to work tomorrow.

Drilled the barrel gas port and drilled retaining pin hole for front sight assy. in barrel without issue or surprise.
Spent a good hour with hand files and grinder cleaning up the torch cut middle section. Most of the slag is gone but the torch melted metal into the grooves and other areas that makes it a PITA to clean up / out.

After clean up I see my cocking slot closed up in width after being torched. I will straighten it before setting headspace and welding sections together.
I have some big gaps to fill in. They are thin sections and fill in quickly with little filler rod.

The one surprise is a repair or revision section on the front cocking handle stop / retainer. This L4A3 came with a folding cocking handle Vs the unique MKII.
No idea if its a revision to use the standard cocking handle or a repair. Either way its been brazed / soldered in place and if you were unaware and tried welded on it solder or braze really makes a nice show when you light up on it with a welder.


Need to add bolt denial pins in both sides of the barrel socket section before welding the mag section to it to keep BATFE happy.


The factory revision / repair is very well done.

Braze visible on top~

Bottom dovetail visible~

End with braze line visible~

Too bad torch guy cant cut a straight line~

Rear of slot closed up and separated with a parallel & shim.
This will be straightened out before welding sections together.

Mag adapter fits and so does an L1A1 mag~

 

Before joining the mag section together ( and set the headspace ) denial pins are required to keep an unmodified bolt from dropping in.

Bolt denial pin locations:

1.250" from the receiver barrel socket face
1.660" from the base of the receiver rail up.

I turned two denial pins from a grade 8 bolt .250" diameter x .230" OAL.
Tapped in flush with receiver wall/s then welded in place.

Tested with bolt in carrier for function then weld in place.

Milled off the full auto sear catch from the carrier and piston post~

Rear section must have been torched by the same guy that cant cut a straight line on the other end of my L4.
Spacer in charging handle slot sets the correct space in the cocking slot. When mine was cut it spung in about .050". If its not set straight you will have issues with the striker , carrier assy and cocking handle fit. All have to travel freely and should slide back and forth by tilting.

I have a fixture that pins off the rear sight holes and take down hole to set the correct length.

Tacked and now ready to finish weld rear section together.
Might do some more welding tomorrow.

 

Managed to get some build time on the L4.
Welding & porosity , a fun fight.

The rear section was cut thru the rear retaining pin hole ( that's not horrible to fix ) but ran into a few other issues that was a fight to get taken care of.
Torch guy wiped out the internal stepped area that has multiple intersecting cuts and filling them in typically is not a hassle. The rear edge of my receiver was cut thin like a sliver with the piece being pretty thin. When you weld things you get shrink. When you have a thin section welded to a larger section it pulls the smaller piece more than the larger piece.

in my case the thin rear section pulled in like a dish where the back edge of the receiver looked like a smiley face.
This area needs to be welded up and re-cut. No biggie.

The nagging issue I had was weld porosity welding up the sides. This usually comes from dirty metal , bad filler rod , gas issues , etc.
My barrel socket was torched hard and I ground the slag clean and welded it without issue. This back section was more melted by the torch than a cut and I think some crap was melted into the base metal. When you weld or have a puddle and run into dirt you get porosity and the dirt or slag makes a bigger crater or porosity in the weld. Sometimes you can add a dab of stainless steel filler rod and get it to flow out but this is going to be parked so it needs to be filled with similar carbon steel filler rod ( ER70 ).

I welded until I hit a spot that would not puddle with porosity then grind it out like a dentist does to fill a cavity then repeat. I did this WAY more times than I wanted to but its the only way to do it right. In hindsight if I was doing it again I would have ground away more metal in the melted area.

I have enough filler added to clean up and hit again if required.
Charging handle dropped in so my shim / spacer did its job.

Before making it pretty~

Torch missed all the stepped area~

Torch ate the other side~

Filling the gap pulled over the thin edge like a smiley face ~

Had to fill in the cut gap and edge and re-cut receiver face~

 

Cleaned up welds and dimensionally everything is correct. Cocking handle fits and slides back and forth under its own weight. Rear take down pin hole welded up and re-machined so the pin fits like it use to. Rear section slides on the frame and the bolt and carrier slide easily. I am ready to weld the front and aft section together. I will be using a dimension from the barrel socket face to the mag latch pin hole. This is a set dimension and I will be adding my guess for shrink amount to it. My barrel is "short" chambered with about .100" sticking out. You can see the same amount between the bolt face and the bolt stops in the receiver. You need to hit your headspace in this area because if you need more that the case stick out you will be hitting the bolt stops and cant go deeper. Using the Bren receiver dimension is safe because the magazine should fit and have room to ream the 7.62 chamber deeper to correct HS.


LOTS of unsupported case and way off the bolt stops~

Better~

Now to find time to stick them together~

 

Setting center section spacing~

I used a standard parallel to measure over.
I had a drawing (unknown source) with dimension from the barrel socket face to the ejector / mag catch retaining pin center line. I did the math to the edge of the hole from the barrel socket face then you plug in your guess for shrink factor.

Zero calipers -

Hold parallel flat against socket face and measure -

Chicken scratch -

Ready to weld..... after checking again.

 

Closing the gap.



Torch guy must hate bren guns but I think I am gaining on the damage done. I want to even the gap side to side for even finish welding. I will double check my spacing (I added .020" for shrink) then bridge the gap in alternate spacing LH bottom then RH, top of LH & RH then middle. After the receiver is tacked together in multiple places I can finish the welds.



The RH side by the mag well was bent / kicked out from the torch cutting or handling. I have it clamped with machinist clamps until it's attached to the receiver.



This section is welding WAY better than the rear section. No porosity or strange surprises.

I did grind the gap back more on both sides to guarantee clean metal. Grinding more material away is opposite what you want to do but you need clean metal or you will have more work to do.

 

Nothing more irritating than going to use your shop equipment and something does not work.

Went to use my tig welder ( I have a water cooled torch ) and noticed coolant on the floor....... return coolant hose blew and leaking.
Could not find a stand alone replacement hose so I removed the part from the welder and tapped it to a standard 1/8 NPT pipe thread and made a line.
Ended up flushing the old coolant , cleaning the pump filter and adding new coolant.

Blue hose is the original / leaker.

Back in action.

Its nice having the receiver back into one piece.
Checking the target print dimension after welding I am .010" short which should not be an issue. I added around .020" which got me close but had I added .030 " I would have been right on the money. If the magazine fits and my bolt stops do not hit the frame I should be able to set the headspace.

I underestimated how much filler metal was required in the larger sections by the frame rails and got a little more weld shrink.
Need to look at it with fresh eyes this week and fill anything I missed , then give it an over ride @400 F and remove it from the fixture.

 

Exterior weld cleanup.

Using aluminum oxide abrasive 2" disc's and my air die grinder on exterior welds. It can knock it down quickly but if you are not careful it can cause you extra work. I will get it cosmetically OK but will not really finish it until I have successful test fire and function. I used 40 grit sanding drums in the milling machine for side radius blend.



Now I move to the inside of the receiver to recut all the slots that are welded shut.

I plan to parkerize & paint the receiver.

 

Internal weld cleanup.

Machining out the lower frame and bolt carrier guide rails.
Keycutter did it's task and lower frame slides on and the bolt carrier has full travel without any tight spots.

Cleaning up the dust cover slot and area to accept the L4 magazine catch. Lots of little things to clean up after a successful test fire and function.

 

Cutting back mag well weld material and L4 magazine adapter now fits again. Tested a few L1A1 20's and all 12 of my L4 mags and they all fit VERY nice.
I think I am close to being too tight / too much weld shrink but mags literally fit perfect. Easy in , out and no wobble.

Inserting bolt its short of fully locking. I am tighter than the nominal print dimension and it moved the locking shoulder closer to the frame bolt stops.
I am going to do a cast of the locking shoulder space with it removed and measure it to the L4 locking shoulder overall length.

I need it to lock and have some space between the frame stops and bolt. After I get this dimension set I can headspace the barrel.


When it fits it makes me smile~

When the magazine fits and locks in I really smile~

 

Fathers day , a day for working on gun stuff !

Put my bolt together with the internal transfer bar.
This L4 / 7.62 bolt thru me a little curve from the dimensions i used on my MKI & MKII bolts. Machined the thru hole from both sides with a carbide ball endmill. One side cut very nice , no issues or surprises. Flipped it over and went to put the hole thru the other side ( They have to line up for the roll pin ) and it was more exciting than I expected. New carbide end mill and it ate it in short order. Typically when you get thru the surface hardness the base material is not as hard. Well the other half of my bolt is REALLY hard all they way thru. Ended up going thru 4 or 5 other carbide end mills getting thru it.

After dropping the transfer bar in along with the firing pin and spring the firing pin was sticking out of the bolt face slightly and I was short on travel , my transfer bar was contacting the internal bolt clearance. My 7.62 bolt is slightly thicker in the front section by about .028".

I used a 5/16" carbide end mill and cut some clearance from the front of the pocket and faced off the rear of the firing pin so its flush with the bolt face.
Roll pin installed and its ready to go.

Rounded up my stainless steel tubing to make spring guides and striker tube along with some O-1 drill rod for the spring guide. Put lower frame together.
Need to cut down the piston post to remove the FA face, add clearance for the transfer bar and weld in an extension on the piston post for the disconnector.

 

Finished winding a couple new striker and recoil springs , making separator tubes and recoil spring guide. Also slotted the bottom of the striker to clear the disconnector and not drag moving forward or back.

My other MKI & MKII builds both had recoil spring tubes that were slightly out of round and I had issues with the striker spring tube dragging with the retainer "lip" on the tube. I changed the design by making the striker pusher tube a straight diameter ( no retainer lip ) and opened up the front of the Bren recoil spring tube a straight diameter , no reduced diameter to stop the striker tube. Now there is nothing to drag.

Before you had to remove the plug in the buttstock to insert the spring separator , now it goes in from the front , just drop everything in and set the striker on the sear.

Test fired a few primed cases and everything went bang. Test fire planned on the 4th.

 

Well I can now claim victory / I had a successful test fire and wrapped up a couple minor issues that I missed and created.



My initial test fire I thought I had it but I was getting singles , doubles then no firing. I was stumped at first but after looking closely at my parts I noticed my ignition issue.

My L4 bolt was slightly different in dimensions than my MKI & MKII builds. My firing pin was actually proud of the bolt using the same dimensions for my transfer bar.



That was an easy fix by taking a slight amount off the rear face ( impact area ) of the firing pin. I set it to flush with the bolt face thinking it would help prevent any prime flow back. I never had any flow back issues with my .303 guns and used stock bolts so I thought it was an "enhanced" thing I was doing.



This snapped primed cases and never missed a beat. Test fire day 1 I was getting one or two shots then nothing would fire.

Looking at my bolt I noticed my firing pin was now about .025" recessed back in the bolt. Turns out the harbor freight 1/8" roll pin was dead soft and bent like a "U".



Bent pin~

My fix was using a standard steel solid dowel pin and I set the firing pin recess the same as a stock Bren gun. Once again test fired several primed cases and it was 100&.

Go to the range and thinking I "got it" I was still getting some light strikes. Looking at my parts my tube that pushes the striker and rides over the other tube in the buttstock was rubbing internally. Simple spring tweak to the closed end loop and ignition is 100%



What I missed was chamfer on my barrels face to allow rounds to enter the barrel without damage or impact. The magazine is a double feed from L&R sides and the bullet nose was hitting the barrel face , not enough chamfer. The wild part was it was always on the 2nd round. One round in the mag fed and fired , 2nd round would slam the barrel face and not fully chamber. I had one round the bullet slid back into the case. It was obvious when I looked at it. The L4 barrel fitting instructions shows a barrel with a heavy chamfer all around the end of the chamber. They did this for an easy machining operation. The chamfer is only needed opposite the feed ramp area.



I got the dremel tool out and put a small chamfer that meets the feed ramps in the frame. Now it feeds smoothly by hand or under power and no bullet point damage.

Clearance is opposite the extractor clearance cut.

I need to wind a new striker spring with closed ends and pull it apart to pretty it up and park & paint it.

Proof of life and some random pics along the way~

 

I’m astounded by the skill !!!! Thank you for sharing

 

Hope something helps another builder.