A good question Steve.....!

[Peter Laidler]

GOOD QUESTION STEVE..........

A good question cropped up a week or so ago and it’s this. Why was a replacement piston post spring carried in the spare parts tin within the wallet’? I suppose the simple answer would be in case the one on the gun breaks......... It’s correct of course but now look at one of the springs and ask yourself this. Is it REALLY likely that a spring;

Post, piston, of 21 ¾ coils with a .087” wire diameter, an inside dia of .484” and free length of between 3.130” - 3.122”

Of these sturdy dimensions and compressed over a plunger AND within a tube could break. The answer is that for reasons mainly to do with the action of this spring it did and does break with monotonous frequency! Put simply, this spring is the only thing that buffers the piston post as it cams down and unlocks the breech block. And if this unlocking were a humanitarian event it’d be described as absolutely barbaric! As I usually try to do, I was trying to think of an everyday event/example of where a short stubby spring would be subject to the bound and rebound forces that this piston post spring is subjected to. Valve springs on your car or the strut springs just don’t even compare......

This fault, with the piston post spring breaking was with the ZB’s and Bren since the earliest days and trials and was never truly but only ever partially rectified.

During the initial sustained fire endurance trials the fact that the springs broke caused other problems too. Due to the ferocity of the a) unlocking of the breech block and b) the imparting of the rearward impetus to the breech block, a broken spring would directly lead to irreparable damage to the unlocking bent of the breech block and forward face of the unlocking cam of the piston post. Because now, the broken ends of the coil would simply wind themselves into each other, effectively making a SHORTER but COIL-BOUND spring Due simply by the now un-buffered action of the piston post on the breech block during unlocking and the rearwards action. But the Enfield* engineers/designers came up with was a novel answer. They’d call it thinking outside the box nowadays........ And this is what they did.

Aware that the actual breaking spring problem was a fact of life, they set about designing INTO the spring a feature that ensured that even if the spring did break, even in several places, it could not damage the breech block, piston post or prevent the gun from firing. And it’s this. Just look at that spec again, especially the number of coils over the length and the diameter of the wire. Now, even if the spring did break, it was impossible for the coil ends to wind themselves in. It is impossible..... The only thing that can happen is that the damaged broken ends of the fractured spring could and did score the exterior surface of the plunger and internal surface of the piston post. Both simple to clean up and make good.

It was a simple matter for the usually well trained Bren crew to strip, clean and inspect the piston assembly and replace the spring if necessary. The arms storeman usually had a pack of them in any case ready to replace a broken item in the spare parts tin. But there was something else that had to be considered now. Because the gun crew were now able to strip the piston assembly, assembly of the horizontal cotter that holds the piston post securely into the piston extension HAD to be made foolproof. As you’ll see if you ever need to replace one, it will only fit one way – the CORRECT way. And it’s been like that ever since

I’ll jump ahead a bit now to 1962. The metallurgy problem of the spring (it’s not a true metallurgy problem per se of course) was subject to further consideration during the L4 trials programme but deemed to be inherent in the design configuration. It then glossed over the fact that the gun training syllabus catered for the fact that the springs can and do fracture. Also that a spare spring is carried as part of the gun CES.

*Nobody really knows whether this was an Enfield or joint engineering design but it appears that the patent for this change was taken out by Enfield. Though quite how you can patent the laws of physics or what now seems to have been the bleedin obvious is quite another matter!

There.......... Not a lot of people know that. Now take your gun apart and marvel at the thought that went into a simple part of the design of this magnificent gun

Click PICS to Enlarge

The breech block, piston post, spring, plunger and cotter
Attachment 64141

The block and post with the unlocking cam of the post ready to make contact. This phase would in fact be part way through the mechanical safety before commencement to unlock the breech block. The forward face of the unlocking can is upright at the base of the angled unlocking cam on the piston post.
Attachment 64142

The breech block, piston post and extension, unlocked and on its way rearwards.
Attachment 64143

Pictorially once again. The horizontal cotter shown in the piston extension is a one-way-fit. You cannot insert incorrectly!
Attachment 64144



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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.

[tankhunter]

I have also encountered Piston post's fractured in service. During Routine Inspections on L4A4's.
They usually break at the joint between the round section, that guides the spring & it's internal Guide rod.
And the thicker machined area where it is more squared off.......

[Peter Laidler]

Here's another Q that I thought that you'd mention Tankie...... Do you recall the very late A class L4 modification, to machine/grind a second sear bent into the piston post about 3/4" or so to the rear of the original sear bent? There was a reason for this but the first mod wasn't well thought out as it was causing P-Posts to fracture and it was quickly re-designed. The second mod meant grinding a clearance space across the piston extension too to clear the sear.

A classic case of if it ain't broke, don't fix it!

You can tell the distinct hollow sound when you 'ring' a cracked one whereas a good solid one 'sings'..... Don't know the right words to use, not being a musician - but you get my drift.........

[tankhunter]

Yes Pete, I believe it was listed in E.M.E.Rs. I can only assume that who designed the mod. was either looking ahead for another potential problem. Or,
Defect reports had been submitted to enact this mod being implemented?.....I assume that the second bent was like the GPMG L7A2. A 'Safety' sear when a weak round was fired? Not nessacary on an LMG though. I have never encountered a 'Run away' LMG in Service. The only reason I can think of if it did ever happen.
Would be a collapsed Sear Spring.

As to your reference to using a piston post as a musical 'Triangle', so to speak! Yep, we all did 'The Ring' Test. Also extensively used the same way, with the L2A3 Sterling SMG. You REALLY were in no doubt about a cracked Breech Block on one of those!...

[Vincent]

How common were L2A3 Sterling SMG cracked Breech Blocks?

The L2A3 Breech Blocks seem very sturdy. What caused them to crack?

[Peter Laidler]

It was the case hardening/heat treatment/quenching that caused them to crack. The case hardening was very deep. I think it was to about .014" or so but feel free to correct me there. They usually cracked up the back end towards the very substantial centre section. I'm not sure whether it'd fail eventually but we tested them and changed any that were cracked

[Vincent]

Thank you, Peter. I will check the Breech Block next time I clean my Mk4.

[tankhunter]

A light 'Ding' with the cocking handle, struck near the bottom 'Skirt' area of the Breech Block. Is all that is needed.
It SHOULD 'Ring' like a tuning fork! (Hold the block near the top with your forefinger & thumb)
If there is a crack in it, you will hear a dull 'thud', that you cannot mistake!...You might not even see the crack at first. But You will hear if it is damaged/ Defective.

[Vincent]

Thank you, tankhunter. That's very helpful. I will try it with a Mk6 semi too.