Bren and L4 spring and thread sizes

[Peter Laidler]

There won't be many who need this info but if you do, here's a few important bits of info for Bren and L4 spring sizes

SPRING AND SCREW THREAD DETAILS FOR THOSE IN NEED

I can't get this to tabulate with my dire computer skills but the first measurement is the number of coils, second the external diameter and the third size, the free length

SPRING No coils approx External dia. Free length
Bipod 13.5 .484 2.2
Buffer piston 8 .89" 2.75
Catch, cover eject opening 10 .153 .51
Catch, carrying handle 15 .346 1.61
Catch. leg bipod 7 .165 .336
Catch, magazine 16 .169 .95"
Catch nut barrel 8 .272 .67
Detent, change lever 8 .13 .354
Pin firing 15.5 .228 .906
Plunger, butt bracket 11.5 .13 .52
Plunger, handle carryng 10 .232 .787
Plunger handle cockng 8 .232 .748
Plunger backsight 27 .224 1.85
Post, piston 22 .484 3.126
Retainer barrel nut 14 .189 .79
Retainer, pin locking body 18 .110 .67
REtainer, pin mag catch 9 .110 .335
Return, inner 92 .358 13.2
Return, outer 70 .477 13.2
Sear 11 .264 1.26
Stay, extractor 13 .264 .98
Trigger 13 .17 .79


SCREWS
Same again. Due to my dire computer skills, First is the TPI. Second measurement is the +/- length of the shank and 3rd measurement, the diameter
DESIGNATION TPI LENGTH DIAMETER
Screw; OF SHANK
Bracket, bipod 32 .865” to .875” .311”
Handle, carrying 48 .566” to .570” .234”
Foresight protector, Mk2 2BA .185”
Retaining mounting pin 72 .255” to .215” 1585”
Locking shoulder 32 .354” to .342” .316”
Sleeve, bipod 22 1.53 to 1.53” .315”
Spring, backsight leaf 2BA

OTHER THREADS
Return spring tube Butt slide thread 26 TPI r/h
Nut thread 32 TPI, r/h
Pistol grip, Mk1 32 TPI r/h

Information

Warning: This is a relatively older thread This discussion is older than 360 days. Some information contained in it may no longer be current.

[Lee Enfield]

So does that mean that the recoil and buffer springs are the same for both the .303 and the 7.62 Brens?

[Peter Laidler]

Yep! But the L4 doesn't have a recoil spring, only a gun buffer spring (the one behind the buffer plate at the back of the butt slide)

[Brit plumber]

Peter, Did you send a message into BBC radio 2 today? If it wasn't you then there's another PL who served in Malaya!

[Peter Laidler]

Yep, that was me!

Back to the main thread and the interesting Q by Lee Enfield. So far as I am aware and recall, no springs changed during the conversion from ,303" to 7.62mm. As for the 'buffer', the Mk1 Bren had a GUN buffer and a RECOIL buffer.

The gun buffer buffers the recoiling gun body and the large silver plate you see at the rear of the butt slide absorbs the energy of the piston extension as it strikes it. Behind that is a spring and a friction brake. No change in any components there
The Mk1 Bren also incorporates a recoil buffer within the butt plate. This is a simple spring loaded butt plate. But it really is/was redundant from day one! This is because during the trials, the designers had modified the buffer plate (see above) from the early design so that it incorporated little 'wings' left and right that would also buffer the gun body during recoil. So why have TWO means of buffering the gun body during recoil? And don't forget that RECOIL isn't piston extension or gas buffering.

Common sense prevailed and on the Mk2 onwards all buffering was taken by the gun buffer. Any Mk1 guns we came across in service, mostly DP guns by the 60's, had the flimsy Mk1 butt assembly replaced by Mk4 butts (Don't forget, the Mk4 butt is the butt fitted to the Mk3 gun). I know what you're going to ask now.............. What is a Mk3 butt then? Ah, yes. the Mk3 butt is a Mk2 butt patched-up and lightened to Mk3 spec for the proposed Mk4 gun. We would still occasionally see these in service

Maybe that demon photographer KG could put up a photo of the internals/skeletonised section of the gun buffer. It's well thought out to buffer the recoil and the reciprocating heavy piston extension. Mind you, it needs to be checked regularly because if it gets dry and jammed up it really makes firing the gun uncomfortable.

[Lee Enfield]

By "recoil spring" I meant "main" or "return spring", as rather obviously the "gun buffer" doesn't provide the spring pressure to operate the forward movement of the bolt group and piston assy.

I wasn't thinking of the spring loaded butt as being a "recoil buffer", though I suppose that it was.

If they were useless, why did the Australians continue to use them on their 1950's FTR's?

[Peter Laidler]

Ah, yes. But someone else asked me a similar Q as a PM so I thought I'd elaborate, aimed at both of you!

Why did the Australians continue the practice? Simple...... They didn't have the benefit of Mk2's although they did manufacture some Mk2 parts to simplify/speed up manufacture. Mk2 barrels spring to mind!