View Full Version : Headspace: technique & dimensions

February 19, 2001, 17:29
I got the barreling timing down now. How about headspace. Whose gauges, pins , etc should I buy and how do you tell the right shoulder to use.

Give a shooter a hand, My barreled upper looks lonely!

February 19, 2001, 21:11
You will need Go/No-Go headspace gauges, pin gauges to determine which thickness locking shoulder to use, calipers to measure the locking shoulders you already have, and an extractor tool.

You may find them for less, but I got the headspace gauges from Bushmaster, $16.95 each, 1-800-998-SWAT. Go is 1.630 inches, your minimum headspace, No-Go is 1.634 inches, your normal maximum. Since the FAL is a mass-produced military rifle for use with NATO-spec ammo, it's possible to have headspace as large as Field 1.638 inches. I've never felt the need to get a Field gauge since if any of my rifles headspace beyond No-Go I'm taking remedial action; after all, despite opinions by certain liberals to the contrary, I'm not an armory. Your Mileage May Vary.

I decided to go with "cheap" and picked up a set of pin gauges from MSC, 1-800-645-7270; the most common sizes needed run from about .254 to .265, $2 each. You could instead go with "fast" and get gauge rods from Casey Elliot, the guy who also makes the best receiver wrench out there, 972-790-2636.

Buy a dedicated extractor tool from DSA, TAPCO, etc, use a cotter pin puller or even a nail, or build your own like my “$2.58 Extractor Tool” as posted earlier this month. If you use a nail etc you’ll a) need good health insurance, b) lots of bandages after you gouge yourself, c) someplace clean and well-lighted to aim your extractor spring at so you can find it later “if” (make that “when”) it gets away from you and launches itself across the room/garage/shop. Part of the charms of building a FAL, that's why I made my own so I can CONTROL the spring.

With extractor removed from the bolt, place the bolt in the carrier and slide into your receiver. Chamber the Go gauge, then select a pin (try starting with .260), insert the pin where the locking shoulder goes, and attempt to close the bolt/carrier. Keep going up/down in size until you find one that allows the bolt to fully close. You should be able to seat the bolt with minor pressure. Double-check, one size down should close easily, one size up should close hard or not at all. That’s your nominal locking shoulder thickness; smithing expert GunPlumber recommends shooting for halfway between Go 1.630” and No-Go 1.634” by subtracting one size from your nominal reading, plus allowing for “setback” of about .001 after the first few times the rifle is shot. Example: your bolt closes “just right” on a .258 gauge pin, with setback would be equivalent to a .259 pin, so use a .257 locking shoulder to obtain 1.632” median headspace. Double check again, replace the Go gauge with the No-Go. You should not be able to close the bolt at all.

Measure your existing locking shoulder(s) with a set of calipers or micrometers. Remember that the locking shoulder tab points to the rear, and you are trying to measure the front/rear dimension (the round part where the tab is, and the flat part on the side away from the tab). Hopefully you’ve got several locking shoulders to choose from and can select the correct size from stock; otherwise buy one from Entreprise or Larry/FN, 913-592-2168. In my opinion grabbing a file and hacking on an oversize locking shoulder down to fit is not the way to go, but again, YMMV and some guys do this, and the world hasn’t ended yet.

Sorry if this was a whole lot more than you wanted to know and I’ve told you a bunch of basics you were already aware of.


February 19, 2001, 21:16
Actually, that's so damn good somebody ought to move it to the FAQ's page. Nice job Radio!

February 19, 2001, 22:31

That information is as complete and easy to understand as it can be.

Man you outta write a book on the FAL.

Thank you very much


February 20, 2001, 02:36
If you plan on building only a couple and don’t want to purchase the pin gages, try this. Get 3 inches of 19/64 drill rod; turn down 2 inches in a drill chuck until it just slips through the locking shoulder hole. Very little material needs to be removed. Now file the flats required for the bolt clearance and bolt lug using the locking shoulder for a guide. Sneak up on the locking lug flat dimension until the bolt closes on a go gage. This is the maximum locking shoulder that can be used and minimum headspace, ignoring setback. Some builders like a little larger headspace. I tried this when I found a Hesse that required a larger pin gage than I had and did not want to order one. It took about 30 minutes. It actually works better than the pin gages because the bolt clearance flat supports the rear of the bolt as does the locking shoulder and does not allow the bolt to drop down giving a false “reading “ as the pin gages can.

February 20, 2001, 12:52
Gone to FAQ

1*.....Train Like You Fight: Second Place is NOT an Option.

E.M. (Ted) Dannemiller II

February 21, 2001, 08:39
Thanks for that creative idea. I might try it if I can get the proper size rod.

So any caliper would do then? Any recomendation for someone building just a couple of fals.

February 24, 2001, 18:28
1.630 (.308 Win. minimum headspace)
1.632 (7.62 NATO minumum headspace)
1.634 (Forster NO-Go)
1.636 (Clymer NO-Go)
1.638 (Forster Field)
1.640 (FN Field)

See also http://www.cruffler.com/accessory-review-october-00.html#ClymerGauge

Sorry you asked?

February 27, 2001, 07:41

half trac
August 06, 2003, 22:10
good idea were would i find drill stock that size?

August 08, 2003, 11:53
Trying to use drill stock for setting headspace is not a good idea.
* Mainly, you cannot find ALL the sizes you will need to make adequate readings.
* Second, pin gages are designed as a standard of measurement, and are EXACT in size.
* Third, pin gages are cheap!
You can get pin gages for as little as $1.56 each, at MSC Direct. Here is a link to the pin gage page.

MSC 2003 Catalog, Pin Gages, Page 1362 (http://www.mscdirect.com/PDF.process?pdf=1362&x=5&y=6)

For (American) Vermont ZZ: "To order gage pins, the ordering number is determined as follows: Class ZZ, Minus Tolerance, 890; next four digits are Decimal Diameter Of Pin. Be sure to use four digits even if ordering full thousandths: .260" would be 2600 for example. Give this seven-digit number to sales; they will tell you the eighth digit." Price $2.29 Each

(These are for MINUS. Use 889 prefix for PLUS.)

size / MSC part #

.253 8902530
.254 8902540
.255 8902550
.256 8902560
.257 8902570
.258 8902580
.259 8902590
.260 8902600
.261 8902610
.262 8902620
.263 8902630
.264 8902640
.265 8902650
.266 8902660
.267 8902670
.268 8902680
.269 8902690
.270 8902700
.271 8902710
.272 8902720
.273 8902730
.274 8902740

For (Import) SPI ZZ: "Call Nationwide Sales with exact size and ordering number range. 78382462 - 78386851 (Size Range .061" to .500") National Sales will locate the order number." Price $1.56 Each

size / mfgr. part # / MSC part #

.253 CMI11098-243 78384385
.254 CMI11098-244 78384393
.255 CMI11098-245 78384401
.256 CMI11098-246 78384419
.257 CMI11098-247 78384427
.258 CMI11098-248 78384435
.259 CMI11098-249 78384443
.260 CMI11098-250 78384450
.261 CMI11098-251 78384468
.262 CMI11098-252 78384476
.263 CMI11098-253 78384484
.264 CMI11098-254 78384492
.265 CMI11098-255 78384500
.266 CMI11098-256 78384518
.267 CMI11098-257 78384526
.268 CMI11098-258 78384534
.269 CMI11098-259 78384542
.270 CMI11098-260 78384559
.271 CMI11098-261 78384567
.272 CMI11098-262 78384575
.273 CMI11102-23 78384583
.274 CMI11102-24 78384591

Call 1-800-645-7270 To Order.

Feed Your Addiction.



"Very sad life. Probably have very sad death, but at least there is symmetry.” --Zathras, Babylon 5

half trac
August 09, 2003, 20:34
thanks for settin me straight radio. do i want to get plus tolerance, or minus?

August 10, 2003, 03:51
Matter of preference.

As somebody once told me, you might put a minus .250" gage pin in a .250" hole, but you'll have a hard time getting a plus .250" gage pin in a .250" hole. However, either will work for headspacing. Tolerance of .0002" ain't gonna kill you, either way.


August 10, 2003, 09:43
Hey Radio since you are the "MAN" :bow: :bow: :bow: For real let me ask you this....since I head spaced for 308 Win I guess I need to buy the Nato go no-go gauges??? and re head space or is there a quick math for me to use?? My gauges are Forester from Guns&Things And I can't get the Aussie Ammo to close.

August 11, 2003, 01:25
Since you asked me...

Here are the numbers commonly seen on the Files. Recently, it has been brought into question whether Clymer GO is 1.632" as popularly thought, or actually 1.630"... but anyway:

1.630" Forster GO
1.630" Winchester .308 minimum
1.6315" FN FAL 7.62 minimum
1.632" Clymer GO
1.634" Forster NO-GO
1.636" Clymer NO-GO
1.638" Forster FIELD
1.638" Winchester .308 maximum
1.640" Clymer FIELD
1.640" FN FAL 7.62 maximum

For the purposes of headspacing your FAL, there is NO difference between .308 Winchester commercial and 7.62 NATO milsurp. Unless you have a bad Century or a poorly-assembled home-built, you are probably NOT going to be in "large" headspace territory, where the true differences between commercial and milsurp become a problem.

Milsurp ammo traditionally features thicker brass than commercial, thus it's able to tolerate headspace even beyond FIELD "reject" without causing undue safety problems. Commercial ammo, at that range, is going to be at or near its limits where case failure occurs. Thus what could be considered "acceptable" headspace in your 7.62mm machine gun (though perhaps not "desireable") could rupture brass right and left if you shot .308 commercial in it.

That said, milsurp also quite often measures SHORTER than standard SAAMI .308 commercial, near 1.627" to 1.629". Thus a rifle headspaced to accept .308 commercial will virtually ALWAYS accept a NATO round.

Forster is a great gauge, perhaps the "unofficial preferred" gauge here on the Files (compared to the fewer number of builders using Clymer). The question should not be about your gauges, it should be: to what headspace did you set that rifle? The AVERAGE "target" headspace for the FAL appears to be 1.632" which theoretically should be plenty room to chamber a NATO round. If you can't chamber it:
* You have a funky run/lot of Aussie that's excessively long
* Your headspace is too tight, probably UNDER 1.630"
* Combination of A and B above
* Something is mechanically wrong, debris in the chamber or on the bolt face
* Failure or binding of the extrator (I'm grasping at straws now)
* Your firing pin is broken/protruding (and God loves you since you haven't had a slam fire yet)

Check your rifle: remove the extractor and try your gauges. Your Forster GO must chamber; if it does not, YOUR HEADSPACE IS TOO TIGHT. (And if it does chamber, you're okay and simply have some funky Aussie ammo.) Your Forster NO-GO should not chamber; if it does, YOUR HEADSPACE IS SOMEWHAT LOOSE. Personally, my rifles get a new locking shoulder if I can chamber my 1.636" Clymer; I won't wait for 1.638" FIELD.



"Very damaged. Zathras can never have anything nice.” --Zathras, Babylon 5

August 11, 2003, 06:27
Thanks Man, I was so worried about the this, but I found the problem. After I got home from buying the Aussie and all excited the first thing I did was try to chamber 1 of the rounds, and when it didn't go....freek out Oh OOO, :confused: Then I remembered after I had shot 60 rounds of Win 308 the weekend before I didn't clean the gun :o there was brass build up in the chamber and on the end of the bolt & ejector. After a good clening and light oiling, bingo :) back in Biz. I do think that my head space is 1.632 because after the video I watched from AGI and what I've read here My Go Gauge 1.630 seats just right and my No-Go will not for about a half inch Thats eye ball guess. And also If I remember right when I had Mickey Mouse here in Ohio help with the last bit of barrel timing he checked it for back up and said it was perfect.
Well I'm going this morning to try it out I'll let you know how it goes :beer: And thanks again. :bow: :bow: :bow:
Just one of these cases of playing it safe and being safe than sorry deals. As always I'll be reading your post with eagerness :D

half trac
August 13, 2003, 10:05
hey radio, got my gauge pins from MSC today. when checking head space this is what i found. forster go gauge closes on .257 with slight thumb pressure, closes with hard thumb pressure on .258. no-go will not close on either of these.what do you recomend? i know this has been answered a thousand times but i just want to be curtain:)

August 15, 2003, 11:26
Yes, this question has been asked and answered at least that many times. However, the fact that you put your question HERE in this forum will hopefully benefit many others as they attempt to headspace their FAL for the first time, and come here to research the job. By the way, I'm no particular headspace expert, I've just done it a good number of times. Anybody else who feels to, chime in, this isn't my thread.

Use .257" as your baseline reading. If you were measuring something with a micrometer, after you get your true reading, you could probably force the instrument down another thousandth or so, but it's now become a C-clamp instead of a precision tool. Your ACTUAL reading might in fact be .2575", but locking shoulders don't come in half-thousandths originally. Don't FORCE it.

You now know a .257" locking shoulder will give you 1.630" headspace, which is rather tight for an autoloading battle rifle although some guys run that. Now, choose your desired headspace: 1.632", subtract .002" from .257"; 1.634" (which is Forster NO-GO, by the way) subtract .004" from .257", etc. This is your semi-final figure.

(Keep in mind that the looser your headspace, the more danger of case failure with commercial .308 or questionable reloads. This becomes increasingly likely as you get above 1.636" or so. Commercial .308 FIELD is 1.638"; milsurp NATO brass will tolerate to 1.640" or even beyond, as some NATO machine guns have a FIELD figure as high as 1.6455". However, the more headspace, the more bolt slap from the expanding brass and thus the more pounding on your rifle; also, the more need for resizing if you save your brass. The standard recommendation here is 1.632", halfway between Forster GO and NO-GO.)

If you desire to correct for expected setback, ADD .001" to your semi-final figure above. Many builders recommend this, some don't believe it's necessary. Factoring in setback will make your headspace slightly tighter initially, until your kit parts "set in". Not factoring in setback will either a) mean nothing if "there ain't any such thing" or b) make your headspace slightly looser after your kit parts "set in".

Additional "heads-up": If you use a factory, FN locking shoulder, you're done. If you use a US-made Vanden Berg locking shoulder, subtract at least .002" from your semi-final figure. (I often recommend subtracting .003", but in your case you are NOT crunching down on your gage pin at .257", AND you are not using a particularly thick locking shoulder size.)

* Get your initial reading WITHOUT forcing hard the bolt. If you have what feels like an in-between reading and you are uncertain, go with the smaller figure.
* SUBTRACT .00x" from your Forster GO figure to reach your desired headspace; .000" if 1.630", .001" if 1.631", .002" if 1.632", etc.
* Decide if you will factor in setback. If so, ADD .001". If not, do nothing.
* Decide if you are going to use a Vanden Berg locking shoulder. If so, SUBTRACT either .002" if standard headspace and smaller sizes, or SUBTRACT .003" if tight headspace and/or larger sizes (upper .260's and above). If not, do nothing.
* Order the size locking shoulder you've determined from the above steps, or select that size from your stock. Install. Shoot. Enjoy.


BMG Mike
September 09, 2003, 07:40
I recently had the opportunity to compare a Clymer GO guage with a set of Forster guages. The Clymer GO guage - at least mine - runs about 1.632,
determined by comparison of bolt closure effort against a set of Forster guages in 0.001" increments.

Admittedly imprecise, but confirming of my earlier problem with chambering the Clymer guage in a tight chamber.


September 09, 2003, 08:34
I too, was told by Clymer a few years ago that their guage was 1.632". I find it very confusing that they give us conflicting info. I will just stick with Forsters...they are cheaper anyway!

Pa. Patriot
October 14, 2003, 17:20
You can also order a 1.632 Forster "Match" GO gauge from Brownells.

Bruce Allen
October 14, 2003, 22:00
Brownells offers a complete line of Forster gauges in series of .001 increments.

You can get any size you wish.

Also I recently finished a kind of drawn out experiment with the headspace on two of my FAL's.

After initially headspacing the rifles I read and followed the idea that a looser than minimum headspace might be a good thing as far as reliability goes.
That is the rifle might be a little more tolerant of crud and grit if not real tight.

What I found was the accuracy became affected, that is the groups became larger, to the tune of 2 - 3 inches per rifle at 100 yards.

I reheadspaced the rifles (I use the NATO min spec) and the accuracy was restored.

There have been others that done this with no change of accuracy so I cannot promise it will happen to any other FAL.
I do believe a minimum headspace is desirable for a host of reasons.

October 15, 2003, 13:05
Yeah... I like 'em tight!:biggrin:

October 17, 2003, 20:48
Originally posted by Radio That’s your nominal locking shoulder thickness; smithing expert GunPlumber recommends shooting for halfway between Go 1.630” and No-Go 1.634” by subtracting one size from your nominal reading, plus allowing for “setback” of about .001 after the first few times the rifle is shot. Example: your bolt closes “just right” on a .258 gauge pin, with setback would be equivalent to a .259 pin, so use a .257 locking shoulder to obtain 1.632” median headspace.

I think you're mistaken. Setback means headspace is increased. With a setback of .001" a .258 locking shoulder would be equivalent to a .257" locking shoulder. Obviously, a smaller locking shoulder would increase heaspace. Assuming that with a locking shoulder measuring .258" headspace would be halfway between Go and NoGo (1.632) you would want a .259" locking shoulder which would give you a headpspace measurement of 1.631" before setback.
I hope this message made some kind of sense.

Radio, either you made a typo, got it backwards, or I'm not reading it right.

October 17, 2003, 22:49
Jambo - Radio is allowing for the .002 difference in setting HS with a 1.630 GO gauge VS a desired 1.632 final HS (with allowance for setback).

I recently added a Forrester Match 1.632 gauge to the Forester GO and Clymer GO I already had. Trust me, the Clymer GO is 1.630. There is a hint of difference in the "feel" between the Forrester GO and Clymer GO, (but it'is a very blond one).

The Match 1.632 will take firm, but not forced pressure to close on the pin that offers very lite to no resistance with the Clymer GO or Forrester GO.

I used the 1.632 gauge to HS the R1 I just finished with firm (not forced) pressure on the pin and ended up about perfect 1.632 HS after the first range trip (120 rounds). SWAG is I had about .0005" setback on the DSA receiver. The 1.632 will now close with moderate “2 thumbs” pressure.

October 18, 2003, 00:33
Originally posted by shootist87122
Jambo - Radio is allowing for the .002 difference in setting HS with a 1.630 GO gauge VS a desired 1.632 final HS (with allowance for setback).

I understand that, but that doesn't do anything for explaining the mix-up that I pointed out.
.268" minus .001" (setback) is not equivelent to .269". Either installing a larger locking shoulder increases headspace or he got it flipped around. It's O.K. We all make errors sometimes, it just wasn't me this time. Am I making any sense, now?

October 18, 2003, 08:01
Originally posted by Jambo

.268" minus .001" (setback) is not equivelent to .269"....

A .268 pin measurement plus a setback allowance of .001 = .269. Radio's math is correct. Setback increases headspace so the "setback allowance" is in the opposite direction to compensate.

October 20, 2003, 23:36
As others have pointed out, you did not catch me in an "error", Jambo... not to say it hasn't been done before! But YOUR math is BACKWARDS. Oops!

Setback GROWS your headspace... SET plus BACK. Thus you must move the bolt FORWARD in order to RETURN to a calculated 1.630" baseline by ADDING .001" to your initial reading to compensate. You are subtracting, sir.

Now you're mathematically again at 1.630". From here you can dial in your desired headspace; most often recommended is 1.632", so a SUBTRACTION factor of .002" is required, since you are now PURPOSELY trying to "grow" your headspace to your desired figure.

.258" initial reading at 1.630"
PLUS .001" to "compensate" for anticipated .001" setback
SUBTOTAL .259" your "corrected baseline" back to 1.630"
MINUS .002" since you want 1.632" headspace (minus .003" for 1.633" etc.)
EQUALS .257" locking shoulder. Voila.

Please note that for the last several years I have always told builders to go through ALL the math steps to keep it straight. Some builders have gotten the .001" shortcut into their heads, but then can't remember which direction to take the shortcut: plus or minus. Do ALL the steps and you'll avoid confusion.

Also, might I point out that the quote you've pulled from me was posted exactly 32 months ago today?? Since that time I have altered my terminology somewhat to avoid any confusion such as yours, though the example I gave within that very quote took you through all the math steps. Of course if you already have math problems you are likely to not understand fully the process. Don't worry, this is correctable, as headspacing is dirt simple once you understand it. If you're smart enough to learn how to type and run a complicated computer, you'll soon be the master of headspacing an FAL. That covers about everybody here, right?

Frankly, if somebody were going to "catch" me at something, I've been holding my breath concerning the following statement from just a couple months ago.Your ACTUAL reading might in fact be .2575", but locking shoulders don't come in half-thousandths originally.As they said in the movie, "That's not entirely correct." The statement served for the purposes of my discussion at that time, but ORIGINALLY locking shoulders came as measured in MILLIMETERS even though we commonly refer to them in inches. Seems nobody's caught that one yet (or they're being kind) but here's an interesting little chart for your enjoyment and further education.gunplumber
posted March 04, 2001 19:58
V 6.63 mm .2610"
W 6.65 mm .2618"
X 6.67 mm .2625"
Y 6.69 mm .2633"
Z 6.71 mm .2641"
A 6.73 mm .2649"
B 6.75 mm .2657"
C 6.77 mm .2665"
D 6.79 mm .2673"--Radio