Bolt fitting question

[jonh172]

After reading over Peter Laidlers "fitting rifle bolts" (more than once) I have a question regarding bolt head fit.

I understand the least amount of bolt head over turn is desirable, but is there any advantage to a very light under turn?

Would this help give a tight fit to absorb recoil down the bolt body and evenly onto the locking lugs?

With this underturn the bolt locks tight with even contact on the lugs..

Thanks in advance,

Jon

Information

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[englishman_ca]

The brunt of the recoil forces are not carried on the threads, in fact, they have a slight clearance. The force is transmitted through the anuli of the end of the bolt body and on the back of the bolt head. These areas will appear as shiny rings when you inspect your bolt.

A sloppy fit with lots of bolt head over turn isnt good, but my belief is that the thread fit being tight isn't that critical. As long as the overturn is within limits, good to go.

[jonh172]

The brunt of the recoil forces are not carried on the threads, in fact, they have a slight clearance. The force is transmitted through the anuli of the end of the bolt body and on the back of the bolt head. These areas will appear as shiny rings when you inspect your bolt.

A sloppy fit with lots of bolt head over turn isnt good, but my belief is that the thread fit being tight isn't that critical. As long as the overturn is within limits, good to go.

Thank you Englishman, but if the bolt head overturns and there is no contact between it and the bolt body, as Peters article states, the recoil is taken up on the threads, which to me makes sense. Please correct me if i am wrong!
Less over turn helps accuracy as the recoil is taken up on a square bolt body, as opposed to angled threads giving a straighter axis of recoil.

I'm wondering if this tight bolt head will help get maximum accuracy, or at least eliminate one of the many variables to achieve max accuracy.

Jon

[jonh172]

Or am I just out to lunch on this and the extra tightness does nothing but make a stiffer bolt throw?

[englishman_ca]

You are not wrong with your concept, Jon.

The time when you wont get recoil transfer thought the bolt body anulus is when a new replacement bolt head is fitted on an old bolt and there is too much overturn.

An old sloppy bolt head and with its long time mated bolt will always bear on the body, not threads. I guess that the worn bolt threads allow more axial movement and the slack is taken up by the extractor pushing on the ramp on closing.
I've seen bolt heads so worn that they were wobbly on the bolt, 45 degree overturn. But the bolt head would bear on the body.
The rest of the rifle was also tired, I don't think a new new tight bolt head would have helped unless accompanied by a new bolt and receiver.

Tight is good, I agree.

If the bolt head sits tight with a slight under turn, one would have problems installing it into the receiver bolt way. Bolt head and lug have to be able to align.

[jonh172]

If the bolt head sits tight with a slight under turn, one would have problems installing it into the receiver bolt way. Bolt head and lug have to be able to align.

Great information thank you!

My current bolt does VERY slightly under turn but i am still able to install it in the body, It gives a tight lock up, is this an accuracy advantage over if i were to relieve it a bit to over turn by 1 degree or less?

[Peter Laidler]

Personally, and maybe it's just me, but why not just fit the bolt head into the bolt body as per the instructions passed down from my old and wise apprentice masters to me in the 60's and passed on to the flock so that the correct way isn't lost in the future

[jonh172]

Personally, and maybe it's just me, but why not just fit the bolt head into the bolt body as per the instructions passed down from my old and wise apprentice masters to me in the 60's and passed on to the flock so that the correct way isn't lost in the future

I was curious to see if this was something you had encountered or ever tried?

I just have a tight bolt head and was wondering what the general consensus is on the matter.

[muffett.2008]

All you are achieving with a tight bolthead, is excessive wear on the bolt guide channel, excess load on bolthead threads and a stiffer operation, requiring excess force to operate.

Go with Petes recommendation, he's correct.

[englishman_ca]

I think that you will find that once you start shooting the rifle with your super tight under turned (out of spec) bolt head, after a few rounds it will start to break in as things get hammered by recoil and no longer be super tight, it will likely then have a slight overturn.

In selecting a bolt head, it is always nice to have a bin full from which to choose as it is critical to get not only the thing to head space correctly, but also to have a correct overturn. The difference of fit between bolt heads is due to something called 'tolerance stacking'. There is an armourer's tool that fits over the bolt head to get leverage to allow one to crank on it to turn the head into the bolt with force when it bottoms out and it doesn't align with the bolt lug that last wee bit. Your bolt head would be one. The tool lets one run the head backwards and forwards and in effect starts the break in of the two parts to each other. It allows you to align an under turned bolt head with the bolt lug.

A different rifle, but I did hear something once about Lithgow replacement bolt heads for Sht.LE that were produced for fitting to used bolts, and the bolt head threads had the thread timing indexed a little to allow for thread wear. Sounds like a great idea, but I think it might be just another folk law. I have never seen such a bolt head and am not sure how I would recognise it if I did.

I'll shut up now and defer to the experts, I'm just a hobbiest who has worked on a few rebuilds in my garage behind the shade tree. But carry on Jon, you wont hurt anything other than feelings. I know that you are giving your rifle loving attention to detail, but just get it within specs and call it good.

As an apprentice in the machine shop it was explained to me that with a fit between parts, sometimes it was pointless to be measuring with a micrometer when I was marking with chalk and then cutting with an axe. ie, sometimes fine precision is overkill.
In your case within spec is good, at the tight end of the spec is also good but not necessarily gooder.

And don't be put off asking questions because you are curious. You will find the members here quite candid. They might seem blunt, but are to the point, they won't steer you wrong with bad advice. There is a lot of experience and a wealth of knowledge to tap into here. Its just that some here are crusty old farts, some crustier and fartier than others.