Bolt head jumping track, wot, again? yup, fraid so

[RJW NZ]

I know this topic is a bit worn out, but I looked over the previous posts and the consensus has been either accumulated wear or damaged receiver, and yet today I've been going over a 1943 Lithgow that appears neither, yet jumps the track when retracted about one inch.
The rifle has a green band, and in all other regards appears to have had very little use, because of this low use condition, ie near new rifling, blueing undamaged, all matching serials (5) etc, its being considered as a competing rifle, yet with several shots in the mag it tightens up on the second or 3rd one and ultimately the bolt head jumps the track and the bolt won't draw back any further, its tight.
The bolt head can be forced (gently) over the track by hand.
The track blueing shows very little wear, and no sign of dents or hits on the receiver anywhere.

I just thought the whole thing really odd considering the unused condition of the rifle.

Doesn't the green band indicate a cadet rifle that would have had regular but light use? It seems in keeping with the appearance of little aging, but if the rifle was jamming etc wouldn't they have flagged the rifle? There's no DP or other marks.

If it appears to be undamaged, is it safe to assume thats its cause by accumulated loose tolerances and therefore safe to build up the groove in the bolt head so that it won't come free and continue using it?

Information

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[newcastle]

Mine (as mentioned in another thread) is similar but instorage right now so I can't get at it to check anything since that thread was open. Mine LOOKS pretty much unused but the bolt does have looser tolerances than other No1s I have and when the rear of the bolt is pushed to the left the bolt would, if it were upside down fall off the track.

[RJW NZ]

Here's a couple of pics. Regarding the red arrows, if I lay a straight edge on that plane , it is dished 2-4 thou and I can see daylight under the straight edge in the middle. The white arrow is pointing at a scratch mark which is the place where it comes free, forward and rearward of that area its OK.

Because the rifle is otherwise so good, with matching serial on the bolt, I'm inclined that the owner should try a few bolt heads and/or try building up the groove in the bolt head. Would there be any reason not to do this?
thanks.

[Thunderbox]

I find that an exchange of bolt head nearly always "cures" this problem.

I think that possibly the bolt track and bolt head groove might have been part of the tolerance relaxations during WW1 to ensure rifle functioning, and that this has led to a proportion of rifles/bolt heads not working together well due to normal manufacturing variations.

An elderly and highly experienced armourer/gunsmith told me that, on rifles that genuinely seemed to have an indented bolt track - they simply put a big cold chisel into the magazine well, and gave it a whack with a hammer! Obviously it wasn't seen as a problem that needed to condemn the rifle...

[Peter Laidler]

This is a load of old clap trap, written before engaging brain. Read below for explanation
There is only one thing that will cause a spring loaded No4 bolt head to jump out of the track and that is if the firing pin spring is twisting the bolt head in a anticlockwise direction while it is running along its path. When it does start to lift at the track opening, that's when it chips away at the edge and the real problems start

The same directional force theory applies to SMLE bodies except that it is exaggerated by wear on the underside of the bolt head or theoutside of the track. EY rifles suffer from bowing of the right hand bodyside. I'd start by making sure that there are no sharp edges at the start and end coil of the striker spring

[Peter Laidler]

OOOOOOOOooooooooooooooooops............. I just hope nobody's read the above totally absurd answer wot I wrote! I was thinking of something totally different and engaged mouth before putting brain into gear......... Bolt heads have nothing to do with spring torque or loading.

I was reminded within about 10 minutes by one of the Armourers over the road who'd been browsing the site who phoned me and said '.......you're thinbking about the SA80 ejection cover we were talking about..........' Only he didn't say it as politely as that

[Norton1]

My smile for the day - Thank you Peter -

[bouletbill]

An old Sussex armourer told me it wasn't an uncommon problem ... and suggested I (gently) hammer down the groove lip on the bolt head, to make it a tighter fit. This I never did because I just didn't like the idea (of hammering my gun) and anyway, it has only jumped track a couple of times in about 20 odd years. However, it is certainly not caused by any WWI tolerance issues ... because my N°1 is a '39er.

[RJW NZ]

Thank you gents, all very interesting.

[Son]

I had a rifle here a few weeks ago with the same problem. It was in good condition, all except the bolt head- and it also failed headspace, by several thou! I surmised the bolthead might have been swapped to fix another rifle, fitted a new bolthead that headspaced good. The new bolthead was still able to be lifted off the track, but only in one spot. I started measuring the diameter of the two boltheads and found the new one to be perfectly round, but the old one had a definite wear pattern that lined up with it's contact on the receiver as if it were being held up as it cycled. I compared the bolt body with a newer one and found the same thing. Putting the newer bolt body and new bolt head together in the rifle and it no longer was able to disengage, no matter how hard I lifted it! It appears to me that with a little bit of wear on the bolt head and bolt body, it is allowing it to move (firers view) up a little and it goes to the right, guided by the inside shape of the left receiver wall. The wear on the bolt parts are mainly on the right because the contact area on the receiver is smaller on the right. This free play up and to the right lifts the bolthead out of the track.

Seems simple enough, but a question to people with the metalurgical knowledge... is the inside surface of the receiver harder than the bolt body?