Why do we lap bolts? Is it for safety, reliability or accuracy? - M14 Forum

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Why do we lap bolts? Is it for safety, reliability or accuracy?

This is a discussion on Why do we lap bolts? Is it for safety, reliability or accuracy? within the Gunsmithing forums, part of the M14 M1A Forum category; Something that I've wondered about is why we lap bolts. I know that you're supposed to have about 80% or more (I pulled that number ...


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Old April 25th, 2016, 10:33 AM   #1
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Lightbulb Why do we lap bolts? Is it for safety, reliability or accuracy?

Something that I've wondered about is why we lap bolts. I know that you're supposed to have about 80% or more (I pulled that number out of my head, not a tech manual) on each side; but why?

Is it a matter of safety and distribution of pressure when the round is fired? SAI bolt lugs most likely won't have that kind of contact unless you do it yourself, yet there are 300,000+ rifles out there that work and shoot just fine.

And how many "gunsmiths" just drop in a bolt and check for headspace and function, then call it good? Is there definitive proof or research testing that insufficient bolt lug contact leads to blown up rifles regardless of case-hardening?

I glean(ed) what I could from the experts and pretty much accept it as gospel, but I never asked the question.

OR....Do we lap bolts for reliability and accuracy?

Is the real purpose of lapping bolts to square up the bolt face with the bore and to provide a smoother, uniform surface for the bolt to unlock on, thus speeding the rate at which the bolt unlocks?

If the bolt face is not perfectly perpendicular to the bore, then are we inducing an angle at which the case sits inside the chamber?

I don't ponder much, so maybe I should lay off the hash pipe and get back to work.

Thoughts...?

Tony.

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Old April 25th, 2016, 10:53 AM   #2
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cuz we be bored. always gotta be messin with these sticks of fire.

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Old April 25th, 2016, 10:56 AM   #3
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Quote:
Originally Posted by tonyben View Post
...Is the real purpose of lapping bolts to square up the bolt face with the bore...
This is the reason I always heard, and to ensure it stays square.

I have also seen/heard of people doing it to get a chrome plated barrel to minimum headspace.

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Old April 25th, 2016, 11:16 AM   #4
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Main reason most commercial receivers are not machined to USGI blueprint specs on locking lugs.
The locking lugs are part of a perindicular thread with a lead of .200".
Most if not all are just simply machining a flat surface thats cut close to lead angle ,resulting in NOT mirror image of bolt.

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Old April 25th, 2016, 11:52 AM   #5
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Making a round peg fit into a square hole.

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Old April 25th, 2016, 12:14 PM   #6
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I always thought it was to spread the load [bolt thrust] over the largest area possible, kind of like a tire in a sand dunes, if you have the tire pressure high the contact patch is smaller making the PSI higher, let the pressure down and the contact patch is bigger and the PSI drops.

Casey

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Old April 25th, 2016, 12:32 PM   #7
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It's to correct improper geometry of commercial receiver lugs so colapsed pressure seating doesn't occur.
If a USGI bolt is used which were thread ground after heat treat and receiver is machined correct a mirror surface requiring no lapping will be achieved.

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Old April 25th, 2016, 12:58 PM   #8
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Tony, I honestly do not believe that in virtually all cases, there is any sort of safety issue that requires that the bolt lugs be lapped. Even if there is uneven and light contact when unloaded, once chamber pressure nears maximum levels I would bet that at that point in time there is pretty heavy and uniform contact. We are dealing with deformable bodies in this application so things do move under load. Like Lysander, I was under the impression that a precision rifle often had the lugs lapped to square up the bolt face with the centerline of the chamber. For most applications, that would make little if any difference since the largest error sources predominate and this phenomenon is way down the list. Unless other larger error sources are addressed, it makes no sense to mess around with something that is a minor influence. Rick

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Old April 25th, 2016, 03:15 PM   #9
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For reloading, it also helps keep the base square with the body. So the resize process is a little more even.

Jay Kosta
Endwell NY USA

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Old April 25th, 2016, 04:55 PM   #10
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ALL OF THE ABOVE.
But especially because I am a quality control freak,
and I like to get it RIGHT!

And,
up here in the frozen North,
we get wayyyy to many Chinee/Canuckistan imported M14 clones which have HORRIBLY DEFORMED left locking lugs on the bolts as they come from the factory. Some of these seem to be [ QUITE LITERALLY/NO EXAGGERATION ] ground by hand, by a drunken one eyed palsy victim. Bearing on that crucial left lug may range from excellent to non-existant.

AND,
along with those very rough left bolt lugs,
for some reason, the majority of our Chinese/Canuck import 14s also come with MASSIVE head space issues. I think at one time, there were some barrels that had the shoulders cut too far forward, so the Chinese started in on grinding the bolts to fit the over spec barrel intrusion into otherwise excellent dimensioned forged receivers. And they just kept on grinding those bolts, even when the barrels to receiver connection was proper and in spec for intrusion and head space.

The Chinese receivers are, in my experience, mostly very good for receiver internal lug cuts, as proven by how easy and quick it is to lap in a US GI bolt, used or new. It is just the Chinese left BOLT lugs that make it almost necessary for a civilised, quality oriented, fussy, retentive, perfectionist, M14 owner to drop in a GI bolt, and
" lap till happiness is achieved".

PS: I have personally shot some SUB-MOA groups with a Chinese 14 that had a horribly fitted, wayyy too long for headspace bolt. I have also seen a 3 MOA Chinese rifle drop to MOA with nothing more than a GI bolt swap. But IMHO, proper ammunition selection, a good trigger job, and a properly fitted stock, will have more impact on accuracy than a perfectly fitted bolt.

That being said,
BOTH my personal Chinese M14 rilfes have US GI bolts lapped in.

Like I said,
I like it dun right!

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Old April 25th, 2016, 05:08 PM   #11
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PS: At one of my M14 seminars, a student showed up with a near new M1A, that had just returned from SAI repairs. THEY REPLACED THE BOLT WITH A US GI BOLT, and when we gaged head space , we confirmed it was exactly as written on the tag, .002" over .308 GO.
WHOO HOO!

BUT,
when we put a bit of red sharpie pen ink on the lugs, and looked at the wear pattern on the left lug, it was obvious the bolt had NOT been lapped in for best lug engagement. Lug bearing was "adequate", but not exceptional.

So we can confirm that SAI, the biggest producer of M14 type rifles, feels that a drop in bolt with no lapping is ...
good to go.

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Old April 25th, 2016, 05:13 PM   #12
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Quote:
Originally Posted by IRON WORKER View Post
Main reason most commercial receivers are not machined to USGI blueprint specs on locking lugs.
The locking lugs are part of a perindicular thread with a lead of .200".
Most if not all are just simply machining a flat surface thats cut close to lead angle ,resulting in NOT mirror image of bolt.
I would have though cutting the lugs on a helix would be relatively easy if you're doing the machining with a CNC machine....

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Old April 25th, 2016, 05:14 PM   #13
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I thought it help'd accuracy

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Old April 25th, 2016, 05:49 PM   #14
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Quote:
Originally Posted by lysander View Post
I would have though cutting the lugs on a helix would be relatively easy if you're doing the machining with a CNC machine....
Originally the receiver lugs were cut with a double sided cutter, tool steel with the cutting edges perpindicular to receiver centerline and tool edges ground to cut both lugs similtaneously. The cutter or receiver rotated while traversing the correct lead ,but this was just a partial movement as the right shelf and left lug raceway limits travel.

Either a tracer or cam operated machine would have performed this operation. Remember to cut a true helix the cutter tool edge has to be on centerline while either the tool or workpiece rotates around the axis and at the same time traversing the lead of said helix.

If you make a straight line mill cut across the lug it will mill a flat.Were talking small deviation here .001'' - .0015'' .assuming lead angle is correct.

If an endmill is used a very small diameter would be needed to clear the right shelf radius relief . An indexing axis would be needed to rotate the receiver while traversing the helix lead .Would be like milling a square thread but not having ability to make a continous rotation to the cut.

I actualy successfully duplicated the receiver lugs on my receiver and done it with a single point perpindicular ground tool all set up on a lathe set to 5TPI (.200''lead right hand) Cut both lugs in same pass pass continous and worked exactly like USGI operation and provided surfaces that required no lapping .

You can read and view my findings in the locking lug helix thread.

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Old April 25th, 2016, 05:58 PM   #15
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I'm about as far away from an expert on the M14 as is possible , but
I always thought running an unlapped bolt is a lot like only tightening one bolt on a connecting rod when you bolt it to a crank
until you lap the bolt only one side is carrying the load ,
not to mention the torsional twist that having only one side in contact imparts
to top it off an unsquared bolt is not conductive to accuracy.

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Last edited by ray55classic; April 25th, 2016 at 06:13 PM.
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