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View Full Version : What is history of FAL headspace technique?


shlomo
January 20, 2009, 16:33
In another thread, L Haney asked a very good question that I have wondered about myself: What is the deal with setting headspace with "two-thumbs pressure", closing on a H/S gauge?

I know that's the accepted procedure from before the time I started fooling with FALs, but with other gun designs like bolt guns, headspace checking is done by very gently lowering the bolt handle while feeling for drag, so as not to crush or damage the headspace gauge, which is a precision-ground instrument.

What I'd like to know is where this "two thumbs pressure" snapover got started. Military armorers procedure? And does it not damage the gauge?

Somebody please 'splain it to me.

ce
January 20, 2009, 16:47
You don't actually use two thumbs, you use one thumb, which is not enough to close the bolt on the guage, so you incorporate another thumb's worth of pressure to the original thumb, which gives the equivalent of two thumbs pressure to the one thumb, but avoids damage to the precision ground instrument by actually pushing on the bolt with two thumbs.

The practice was started by member number 19, also known as Pedro, who lost his thumb in a bricksaw mishap, and was given the name diez y nueve as a result by the Mexicans.

They can be so cruel.

I hope this helps, I really do.

tac-40
January 20, 2009, 16:48
You have been here longer than me but this is what I think. If you look at all of the US weapons where the headspace is checked, they all have some type of rotary bolt/lock-up that cams the bolt into battery - M1, M14, 03, M16. In each case, the proper headspace is achieved when the bolt is fully rotated into battery on a GO gauge. If the bolt won't close, then you have your less than max NO-GO.

On the FAL, the bolt is cammed down onto the locking shoulder and held in place by the B/C. The "two thumbs" pressure is most likely an approximation of the force to be used to push the B/C forward to ensure the bolt is cammed down into the LS recess. This "positive" pressure is required to make sure that false readings from burrs, grit, or dirt are not obtained. Just remember, bolt closed on an FAL means the B/C is all the way forward.

shlomo
January 20, 2009, 17:16
Originally posted by ce
The practice was started by member number 19, also known as Pedro, who lost his thumb in a bricksaw mishap, and was given the name diez y nueve as a result by the Mexicans.

They can be so cruel.

I hear you mang. His primo Eladio was a founder of Mara Salvadedo 13. They won't shoot or cut you, but they will give you the finger.

Hey, there's a guy in that thread I mentioned who needs some of your insight. Be a good lad, won't you, and give him a dose.

W.E.G.
January 20, 2009, 17:40
It has to do with the fact that even if your "all thumbs," you can probably still build a FAL.

"Crush" a headspace gage?
Man, if anybody put that much pressure on it (irrespective of whether its a FAL or an Obermeyer Über-Sniper) without knowing better, there's not much helping them.

shlomo
January 20, 2009, 18:07
Originally posted by W.E.G.


"Crush" a headspace gage?
Man, if anybody put that much pressure on it (irrespective of whether its a FAL or an Obermeyer Über-Sniper) without knowing better, there's not much helping them.

Well, I dunno. That's why I'm axin' the question. Hatcher, who (most folks will acknowledge) knew a bit more about this stuff than I, seemed concerned about it, and described crushing in a headspace gauge as roughly the same treatment of a precision instrument as, "tightening down the thimble of a micrometer with a Stillson wrench". The limited experience I have with machinery tells me that gauges are never crush fit, but I'm sure there may be exceptions.

My best understanding of the geometry involved in the locking step and shoulder is that the approximate 11-degree slope is there to match the profile of a short section of the arc swung by the rear of the bolt, and that it is intended to drop down as a tangent contact, and not a cam-over. That would seem to make it so a stripped bolt should smoothly drop down on a GO gauge at minimum headspace, without "snapping" shut. Or am I all wet?

I guess what I'm trying to get at here is whether the two thumbs crush fit has its origin in engineering, or whether it's just evolved as a part of do-it-yourselfer's folklore.

Ideas and comments are encouraged. We will wait and see about embraced.

L Haney
January 20, 2009, 18:25
Originally posted by tac-40
Snip This "positive" pressure is required to make sure that false readings from burrs, grit, or dirt are not obtained. Just remember, bolt closed on an FAL means the B/C is all the way forward.

Gotta' yell about this. If you are performing a "gauging" procedure, you better clean all that crap out. I know I've given at least one guy on here hell about obsessing over minuscule dimensions, but, if you want a good measurement, you better clean all those surfaces. That's why gauge blocks are wrung together rather than squeezin' 'em up inna' vise. Think about the torque you put on the spindle of a mic. Kinda firm, not cranked over, and as consistent time to time as you can. Steel is plastic and CAN be deformed with surprisingly little pressure, especially if the contact area is small.

Lowell

shlomo
January 20, 2009, 18:30
Lowell,

You are the right kinda guy to do machinin', or I'm mistaken.

PM me sometime, and I'll give you an email addy for when ya wanna talk about swarf and live centers and stuff.

vmtz
January 20, 2009, 18:38
Two thumbs if from the Aussies. See the Fal Bible for further information.

Fr. Vince

shlomo
January 20, 2009, 20:22
Originally posted by vmtz
Two thumbs if from the Aussies. See the Fal Bible for further information.

Fr. Vince

Okay, two thumbs if from the Aussies, and one thumb if from the Meskins.

What if from somebody else?

ByronF
January 20, 2009, 20:38
All your thumb belong to us

L Haney
January 20, 2009, 20:43
Originally posted by ByronF
All your thumb are belong to us

Fixed your syntax! :biggrin:

shlomo
January 20, 2009, 20:50
And while we're on the subject, can an Aussie even read the Bible with the gol-danged sun in his eyes?

W.E.G.
January 20, 2009, 22:18
Don't do any headspace checks on the FAL with just the bolt (without the carrier).

You'll get significantly different numbers if you do that.

Don't ask me to explain the mechanics of it. But it does seem to be a fact.

Always check headspace using the bolt AND the carrier together.

You're on your own with whatever other discoveries you find via other methods.

ce
January 21, 2009, 15:54
If you can keep your head when all about are swooning for Obama.
If you see Kay, tell her I'd love to.
If wishes were hormones, then beggars would get laid.
If only I knew then what I know now.
If you knew Sheila, like I knew Sheila, oy, oy, oy what a gal!

All these and more from The Chateau D'If.

It's in France, on the Cote D'Azure, just outside of Swarf.

shlomo
January 21, 2009, 16:24
On a clear day, you can hear a hormone.

If I have ta lissen to anymore drama about Obama,

I'm gonna barf in my swarf.

ce
January 21, 2009, 16:52
Hey, Shlomo, could I bend your ear, just for a moment?

No, no, your ear, like, would you mind listening to my problem?

You know I counseled that dear, dear boy about his locking shoulder, and I said .255, I'm in such a dither about it, I keep thinking, you know, .254 would be so much easier, I don't really subscribe to all that setback jive, I mean, who could afford enough ammo to set anything back further than your wallet, eh wot?

What if he somehow, what shall we say, deadeye's it all to hell, and then his bolt won't close on a cartridge?

Then he'll think I'm a sham, a charlatan, a flim flim man, a suppository salesman with a suitcase full of free samples.

Do I worry too much? Can we blame it on Bush somehow? Will it be all right?


Is it safe?

Pluribus
January 21, 2009, 21:39
I have never understood why all this thumb-schmumb stuff came about. Who ever the hell it was that dreamt it up. FALs, under normal operation, do not close with thumb pressure but, with the tension of the recoil spring camming the bolt down into the LS. Just use that pressure to hold the B/BC in place to measure for the locking shoulder. I learned this from many long distance phone calls to Laurence at Entreprise in 1995 when I was unable to find that last little bit of info before starting my then first build.

Build to the point of needing to measure for the LS including completely assembling the lower and, installing to the upper. Install the the bolt, minus the extractor but, with the firing pin so that the B/BC maintains alignment with each other. With a clean chamber,(and, gauge) place the go gauge in the chamber and close the B/BC slowly and, carefully on the go gauge. Now measure for the LS and, you're done except for installing said LS. No thumbs, no schmumbs, no wondering it was enough pressure to match the spring tension.

Bang-ity, bang-ity, bang-ity, bang. It's a FAL.


:shades:

shlomo
January 21, 2009, 22:12
Originally posted by ce
Then he'll think I'm a sham, a charlatan, a flim flim man, a suppository salesman with a suitcase full of free samples.

Face it, sooner or later he's gonna find out that yer a flip flop man with a case fulla urine samples, and ya make a livin' peddlin' em to probationers in the alley behind the courthouse, so they don't get violated, have to hire Vince, get slam dunked at the hearing, and go back to jail, which they woudn'ta been in in the first place if they hadn't hired a janitor to represent 'em right outta the gate.

But that don't mean you don't know yer onions when it comes to FALs. Plus, it's free advice. If ya called it wrong, he can take it to Barry Soetoro--he's gonna fix everything else, 'cept his birth records, which don't matter any more than for the other 25 million illegal aliens.

Hasta Luigi

kotengu
January 22, 2009, 11:02
Originally posted by W.E.G.
Don't do any headspace checks on the FAL with just the bolt (without the carrier).

You'll get significantly different numbers if you do that.

Don't ask me to explain the mechanics of it. But it does seem to be a fact.

Always check headspace using the bolt AND the carrier together.


This is another great example of an "angels dancing on a pin-head" mechanics/engineering discussion we need to have - why on earth would this be so? You would thing that if the bolt alone is fully seated on the locking shoulder the measurement should be the same. Maybe the carrier just ensures the full seating and alignment of the bolt, and it's subject to misalignment and not full seating?

I'll be the pin-head :tongue:

On the "two thumbs" question I would think (assuming perfectly clean surfaces) that only very light or no pressure on a fully seated carrier/bolt combination would give you an accurate match to that gauge. When you add one or two thumbs to that you're very slightly undersizing to the go-gauge (maybe 1.62999" instead of 1.63000") which is good for the tight headspace that some of us are looking for.

From an armorer's perspective I think it's just so close they don't want to change that stupid locking shoulder again, and if it closes with a little thumb pressure that's good enough.

erhauser
January 22, 2009, 11:25
What little I know about machine shop technique I learned from my German toolmaker Grandfather. The had been apprenticed at age 12, before 1900, so I have always assumed that he knew about such things.

His take was that you are trying to duplicate the technique of the guy who set up the specs. If the guy was a ham handed British armourer, you should use two thumbs pressure. If the guy was a German tool maker at Mauserwerke, use a just noticable drag.

Basically, the thing is that steel is an elastic material. Change the pressure, and the apparent size changes.

shlomo
January 22, 2009, 12:43
Well, the foregoing is probably gonna stir the pudding. :devil:

To further add to the mix, it has occurred to me that a possible explanation for the need for snap-over is bolt face tilt.

On the presumption that the bolt face is made to be square to the bore centerline when locked, then it follows that when the rear of the bolt is tilted up, the face of the bolt is canted downward; that is to say, that the top of the face is farther forward than the bottom.

This means that on the feed/locking stroke, as the cartridge seats, the top of the breech face counterbore contacts the top of the cartridge rim first, and there is a gap at the bottom of the breechface that must be closed as the rear of the bolt swings down to lock. I was interested to know how what the value of this gap was, so I took some measurements and did a few calculations. As it turns out, the gap is about ten times what I would have guessed by seat-of-the pants: .022" (!) (If anybody gives a damn, I'll post the calculations for checking later.)

There's more, but I've gotta go do something productive for a while.
Later.

kotengu
January 22, 2009, 13:54
I'll be curious to see the "more" - your theory makes sense. It also makes me want to go look at my parts and drawings some more :wink:

Does the top of the counterbored bolt face actually make contact with the round (gauge) before the bolt is seated, or does the protruding lip at the top make contact with the barrel face until the bolt seats? As the bolt slides forward into place, what's the actual relationship of the rear to front bolt height? The locking shoulder and locking face of the bolt are cut at an angle (13° from the mighty locking shoulder angle debate!), so the further the rear drops the further forward the bolt is pushed. Maybe that angle accounts for the tilting of the bolt as it seats?

Otherwise - you're saying that the top of the cartridge shoulder (or base) would be compressed and deformed 0.022" during the bolt seating process, then released as it fully seats. Either that or the actual closed bolt headspace dimension is 0.022" larger than the gauge in the chamber.

FAL Files access at work always kills my productivity too...:biggrin:

shlomo
January 22, 2009, 14:36
Okay. Got another couple minutes.

I had assumed earlier that the face angle of the locking step on the bolt was a tangent to the swing arc of the bolt body. Not so. It is steeper. In fact, if you take a dial caliper and place the tip of the standing jaw against the upper edge of the breechface counterbore (12 o'clock on the face) and place the moving jaw's tip against the upper corner of the locking step, you can sweep the moving jaw down to the bottom of the locking step with pressure on the caliper carriage, and you will see the dimension shrink by right at 20 to 24 thou. Pretty much the same as the gap dimension in the earlier post. :) Well, at least it is on mine.

It appears that the locking angle is engineered with enough slope to clear the .022" extra distance created by face tilt, and the angle is such that as the tilted boltface gains extra clearance (what would be extra headspace) during the tilt-down of locking, the slope of the locking surfaces moves the bolt forward at the same rate, to achieve zero-zero on lockdown without bent rims or cartridge drive-in. If that makes any frickin' sense the way I put it.

In hindsight, I guess we shoulda expected something like that. There's a reason they have schools for mechanical engineers. :wink:

One other thing. Later on, I'm gonna do some checking to see what clearances there are between the back face of the barrel and the end of the bolt, vis-a-vis Kotengu's question. On the one hand, if the bolt is intended to contact the barrel, that would pretty much make changing locking shoulders moot, if one wanted to close up headspace. On the other hand, I find myself wondering about how much headspace variation is or can be created by "drive-in" of the cartridge by a relatively heavy bolt/carrier combination moving at high speed without anything to arrest it other than shoulder crush. Gotta check for rear contact between carrier and bolt too...

I'll be back.

shlomo
January 22, 2009, 14:47
Originally posted by Raspeguy
No surprise about the .022 value as the bolt tilt is approximately 3 degrees.

I'm not sure about the meaning of "snap over" that you mentioned in the first post ...

The figure is 1/135 of a circle, or 2.666 degrees, as closely as I could measure, and that's close enough to 3.

The "snapover", is the effect you get when you run the bolt/carrier to contact with the gauge, and with the application two thumbs, the cam surfaces load and release, and drop the bolt on the locking shoulder. The whole question is whether that sensation is an indicator of a too-tight condition, and whether the bolt should drop snugly and cleanly in place without snapover.

shlomo
January 22, 2009, 14:58
Originally posted by Raspeguy


Generally you will find the clearance or gap between the barrel breech face and end of bolt to be nominally .015 when set to a headspace value of 1.632.

Check this link:

http://www.falfiles.com/forums/showthread.php?postid=2485460#post2485460

Fine post and link. That is precisely what I'm wondering about, but better said.

I see we're running parallel to some other discussion.

Thanks much.

kotengu
January 22, 2009, 15:46
All good questions though - never assume anything, and always measure for yourself. Especially don't take for granted that an engineer had it right the first time and thought of everything!

..this engineer sure has screwed a few things up the first time and forgotten more than he should...

garandguy10
January 22, 2009, 16:09
It is of British origin,if you use both of your thumbs to check headspace that will confirm that at the time of said inspection one thumb was not were it was not supposed to be while inpecting the Queens rifles.

shlomo
January 22, 2009, 16:11
:rofl:

ce
January 23, 2009, 09:57
Thank you, Raspeguy, for your inclusion, it always adds validity to the discussion.

And that Shlomo, he's such a mensch, always with the angles and the sizes, such precision, he shouldda been a moyl.

Didja hear about the constipated mathemetician?

He worked it out with a pencil.

shlomo
January 23, 2009, 10:10
Originally posted by ce
Thank you, Raspeguy, for your inclusion, it always adds validity to the discussion.

And that Shlomo, he's such a mensch, always with the angles and the sizes, such precision, he shouldda been a moyl.

Didja hear about the constipated mathemetician?

He worked it out with a pencil.

How about the constipated engineer? Talk about a mensch, HE worked it out with a slide rule.

I got a wallet made by a moyl years ago. Whenever I travel, I just rub it, and it turns into a two-suiter.