Advice needed ... bolt opens on firing!

[5thBatt]

Yes i believe that is the case, i have experimented with a few of my rifles & just by inserting a piece of paper under the bolt handle when closing the bolt & then removing the paper leaves a small gap between the handle & body which seems to stop the bolt lift.
Think what happens to a steel washer sitting on one end of a steel workbench & what happens when you strike the other end of the bench with a hammer.

[Bruce_in_Oz]

Try this:

1. Cock the rifle on an EMPTY chamber..
2. Take piece of paper and create a small gap between the bolt handle and the "socket".
3. Fire the action.

Does it still "kick up"?

Repeat the process with a DUMMY round in the chamber.

How about now?

Remember that the locking surfaces on the bolt and body are cut on a helix (1: 0.10" pitch); they are NOT dead square. This is to provide "camming" for the final closure of the bolt during chambering. The "unlocking cam" in the little lug-hole on the left side has a pitch of 1: 1.0". P-14 and M-17 rifles also have helical locking surfaces.

If the bolt can "float" around in the body, (wear/mismatch), there is a greater likelihood that it may "bounce".

Note also that the right side locking shoulder on the body has TWO helical surfaces: the top section being at the 1: 1" "twist and the bottom at 1: 0.1". This is to allow primary and secondary "ramming" of the cartridge and clearance in the unlocking / primary extraction phase. The hard "skin" on these surfaces is VERY thin and can be made even thinner by people attempting to "smooth" them with dodgy hand tools. (How accurately does anyone think they can form a precise helical cut with a file or hand-held stone?). These surfaces were brought to final spec in the factory using a hand-driven shaving tool, ( bit like a boring bar but with a suitable helical drive "guide"). That is just one of the many reasons that the Lee Enfield Rifle was not exactly cheap (or quick) to make.

It was once suggested to me that a CNC machining centre could "approximate" the relevant surfaces using small high-speed rotating tool bits. When I asked about surface grade, "matching" the "approximated" surfaces to the bolt lugs and material compression during proof, not to forget accelerated wear of these "approximated" surfaces affecting headspace, there was a bit of a pause.

Corrosion and/or crud inside the bolt assembly may also cause the ends of the striker spring to bind on the inner rear of the bolt body and/or the rear of the collar on the striker. Ditto a "replacement" spring that does not have its ends PROPERLY closed and ground.

[303Guy]

So this particular rifle has a groove worn into the socket from that kicking impulse. Perhaps the long lug is now contacting the bold head guide. Would that make it worse? Would a thin soft buffer placed under the bold handle in the worn groove stop that jumping?

[gewehrsniper]

I have a no4 mk1 rifle that does the same thing. When id fire it the bolt hangke would kick up about a centimeter. Also the take down gap on the bolt head track had the corners worn off...and I never could find out what the screw on the left center of the receiver was for but I replaced it and tightened it (not too tight) to where the action feels brand new. The bolt body/handle no longer kicks up and the bolt head hasnt once came off track since I fixed it. I'm guessing the screw on the side of the receivee is just to adjust the tightness of the action...

[5thBatt]

Give me 5 Min guys, just got to nip out & get some popcorn & a drink.

[Parashooter]

. . . the bolt kick-up that some rifles have, is that caused by the bolt handle resting on the action body (which I accept must contact - there's nowhere else to stop the bolt rotation)? . . .

No, it's not caused by the bolt handle - and closing rotation would be quite nicely arrested by that sturdy rib on the right side of the bolt body even if the handle were absent.

What actually happens is fairly complex. ('Son' already touched on it in post #4 of this topic.) If you can bear with the extended narrative, I'll try to explain in more detail.

1. With the bolt closed and the striker cocked, the mainspring tries to pull the firing pin and attached cocking piece forward - but the sear prevents this. The rear end of the mainspring, bearing on the bolt body interior, consequently wants to pull the bolt to the rear - but this is stopped by the locking lugs bearing on their corresponding surfaces on the receiver. Friction there provides a modicum of resistance to bolt rotation, usually just about enough to keep the bolt closed unless intentionally turned or unintentionally jarred.

2. Also, during the closing rotation of the bolt, the mainspring (compressed between firing pin and rear of bolt interior) experiences some degree of torque. If its bearing surfaces at both ends are not smooth and/or well-lubricated, some or all of this torque remains in the mainspring, tending to turn the bolt to the open position. This torque is normally, however, too weak to overcome the friction between locking lugs and receiver.

3. When the sear releases the cocking piece, the firing pin moves forward and the pressure of locking lugs on receiver is relieved, significantly reducing friction.

4. When there is no primed cartridge (or "snap cap") present in the chamber, the flange on the firing pin strikes a blow to the rear of the bolt head, momentarily jarring the locking lugs completely out of contact with the receiver and eliminating the friction between lugs and receiver.

5. After the firing pin flange comes to rest on the bolt head, mainspring pressure puts tension on the bolt head threads, again restoring some degree of friction that resists bolt rotation. (If a primed cartridge or effective "snap cap" is present, the pin flange seldom contacts the bolt head. Instead the tip of the firing pin comes to rest on the primer and mainspring pressure is again transmitted to the locking lugs, albeit at reduced force compared to the cocked position.)

6. With the pin forward, the small "safety" lug on the cocking piece interacts with the cam surface at the rear of the bolt body to prevent bolt rotation unless sufficient torque is applied to overcome mainspring pressure. Due to normal clearances, this lug does not contact the cam surface until the bolt is rotated some 5 to 15 degrees.

7. During the brief period while the pin is falling, and especially after it has just whacked the bolt head during dry firing, any residual torque on the mainspring can (and often does) overpower the reduced frictional forces that resist bolt rotation. Consequently, the bolt can "jump" toward the open position those 5-15 degrees until the safety lug on the cocking piece comes into play.

8. This completely normal mechanical action seems to scare the hell out of droves of Lee-Enfield novices fixated on watching what the bolt handle does during dry firing and paying no attention to its movement (or lack thereof) during live fire. The same action occurs on many Mausers and their derivatives, but since the one-piece bolt lacks the Lee's bolt head friction element (See item 5.) the bolt handle usually right drops back down after its little jump and nobody panics.

All of this may have little bearing on the problem described in post #1 of this thread, which seems to be about a bolt opening much more than 5-15 degrees - and during live fire. If the description is accurate, the condition of the cocking piece safety lug and/or the mainspring would be reasonable starting points for detailed inspection - unless the bolt or receiver has been altered/damaged beyond reason.

[Son]

Thank you Parashooter, for going into the mechanics of the of the reaction. I only hope it isn't lost on those not technically trained or technically minded.
To anyone who cannot get your head around it, try stripping out a bolt and go back through it with the parts in your hands creating all the scenarios that together make up the cycle of action. Understanding what is going on is vital to working out what is going wrong. Plenty of great help here...

[Peter Laidler]

We're wayyyyyyyyy off the subject now, but I believe that it'a all down to the fact that you can compress steel (and air) and it WILL react as opposed to a liquid that you cannot compress and it won't react. Get a poly bag or baloon of water, empty it of air and tie it up. Drop it onto a surface plate or something similar and see what happens. It'll just deform and lay there like a wet fart.

Get a ball, fill it with air and do the same thing. It'll bounce to a set drop to bounce ratio

Get a ball of steel, like a ball bearing (or even a hammer head, ring spanner, the harder the steel the better......) and do the same. It will bounce. Don't believe me, take a look! Once again, to a set drop to bounce ratio. This is in effect just what you are doing to the bolt when you hit the bolt head, which is attached to the bolt. If I had a 40 minute physics lesson I'd explain it better but that's about it. There is no secret or black magic there.

Forgot to say. You can easily and simply eliminate the striker spring torque theory by taking it out and firing the cocked cocking piece/striker forwards with a catapault (or something similar) contraption. Yep....... the bolt still rotates slightly.

There is a word for this physical property but I forget what it is. JM...., help!

I know Muffer thinks that I'm getting cranky but I don't suffer fools any better or worse that the rest of you. I'm wise enough to answer the questions - just so long as you don't keep changing the questions or even worse.......... I won't go on!

[5thBatt]

There is a word for this physical property but I forget what it is. JM...., help!

Harmonics maybe??
The two parts being of different masses will "ring" at different frequencies & repel each other, not trying to be technical as thats not my field (I'm a spray painter) but thats the best way i can explain it.

[303Guy]

Thanks all. I've noticed this bolt jump on dry firing, now that I'm reminded. I don't like to dry fire - not sure if it wears the working bits or not and besides, pulling the trigger is serious business! I expect it to go bang. I don't recall bolt lift on firing maybe because it doesn't happen. Would y'all say 5thBatt has nailed it? Makes sense to me.