FAL Headspacing
The following is mostly quotes from posts on falfiles, by the various board members, most notably Radio.
Headspacing is a fun and challenging procedure on the FAL; and unlike many other rifle designs, easily within the grasp of the average shooter. That said, you do want to be cautious; setting headspace is perhaps the most critical step in assembling your FAL... if it's off, either your boomstick is just a stick (too tight, won't chamber) or is a major danger to you and nearby spectators when it explodes (waaay too loose, case rupture or catastrophic failure).
The cost of the proper tools for this job is INSIGNIFICANT compared to the price you might have to pay if something bad happens. These tools will allow you to measure EXACTLY what headspace THAT PARTICULAR rifle demands. You will also have the means to insure that rifle REMAINS safe over time as the headspace opens up due to parts wear. Here's what you will need:
- ZZ pin gages (from MSC Industrial Supply, for example). Sizes .254" to .265" will cost $25 at $2.25 each, even less if you skip every other size (even or odd). A more expensive way to go, but faster, is to buy sizing rods. Board member Casey Elliot, the man who brings you that fabulous receiver wrench, also features those.
- Some method of stripping the extractor from the bolt. I built my own tool; dedicated tools are available from DSA, for example (about $20). You will need this tool anyway for standard maintenance of your rifle. While it's possible to simply use a vise and a nail, the tool makes the job MUCH easier.
- Calipers to measure the existing locking shoulder you got with your kit. (Keep in mind that chances are very great you will need a different size regardless.) You might already have this in your toolbox.
- A set of GO and NO-GO gauges. Will set you back roughly $35 or cheaper for Forster, more expensive using Clymer.
Go/No-go sizes and specifications:
- 1.630" Forster GO
- 1.630" Winchester .308 minimum
- 1.6315" NATO 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" NATO 7.62 maximum
Things to pay attention to when headspacing:
- Barrel is properly indexed (gas block straight up) in the receiver. (You've already gotten this far in your build if you're now ready to headspace.)
- Extractor is removed from the bolt.
- Bolt and carrier are together, NEVER just the bolt alone.
- Be clean: CLEAN chamber, CLEAN headspace gauge, CLEAN boltface, CLEAN gage pin or sizing rod, CLEAN locking shoulder hole in the receiver. Dirt will affect your readings.
- Repeatability, your readings should be about the same on different measuring attempts and even different days. If not, something is wrong with your mechanicals and/or your technique.
With the above ready, select a gage pin or sizing rod (I like to start with .260") and try to close the bolt. If it closes easily, you need a larger pin/rod; not at all, smaller. What you are trying to do is to seat the bolt/carrier using "two-thumbs pressure", or a good firm push with a little resistance. Not too easy, not too hard; it's sort of subjective, I admit, but you'll probably know it when you feel it. (Don't be worried about the pin/rod being smaller than the locking shoulder hole in the receiver; the act of seating the bolt will move the pin/rod rearwards in the hole and give you a true reading.)
Let's assume you've closed your bolt on a .262" pin/rod. That's your starting point for figuring the correct locking shoulder. Next, you need to compensate for something called "setback." Factoring in SETBACK will make your initial pin gage/sizing rod thickness GROW by .001" because the locking shoulder will move AWAY from the chamber, "set back," and thus require a THICKER gage/rod (or locking shoulder) to return to the same headspace dimension you started with.
(People who are smarter than me seem to think that introducing firing pressure into a just-assembled rifle, in addition to affecting the locking shoulder, also seats the threaded barrel into the receiver. It usually all adds up to about .001" anyway. Also, to be fair, some people don't believe "setback" occurs. I always plan for it myself.)
- Your bolt closes with two-thumbs pressure on a .262" pin/rod
- To account for "setback" ADD .001"
- Correct pin/rod will now be .263"
- This is true regardless of which brand of headspace gauges you are using.
Many knowledgeable gunsmiths such as Board member gunplumber of ARS, also the fine folks at Fulton Armory, recommend setting headspace on these rifles at 1.632". This allows for reliability using any 7.62 NATO milsurp ammo, plus the ability to safely fire commercial .308 with its generally thinner brass.
FORSTER GAUGES EXAMPLE
- Your bolt closes with two-thumbs pressure on a .262" pin/rod
- ADD .001" for setback, equals .263"
- You are using a Forster GO gauge, 1.630"
- SUBTRACT .002" from .263", equals .261" locking shoulder.
CLYMER GAUGES EXAMPLE
- Your bolt closes with two-thumbs pressure on a .262" pin/rod
- ADD .001" for setback, equals .263"
- You are using a Clymer GO gauge, 1.632"
- You're done, you need a .263" locking shoulder.
If for some reason you want to resist conventional wisdom and set your rifle's headspace larger than 1.632", take the difference between your target figure and whatever gauge you're using and SUBTRACT from your nominal pin gage/sizing rod/locking shoulder reading, again after first ADDING .001" for setback. EXAMPLE:
- Your bolt closes on two-thumbs pressure on a .262" pin/rod
- ADD .001" for setback, equals .263"
- You are using a Forster GO gauge, 1.630"
- You'd like a 1.634" headspace
- SUBTRACT .004" from .263", equals .259" locking shoulder.
Probably more people use the Forster set than the Clymer. What perhaps contributes to confusion is that +.001 -.002 = -.001, so builders are constantly trying to remember if that's PLUS .001 or MINUS .001 to get where they're trying to go. If you do it the long way, you'll always get it right. SETBACK GROWS THE LOCKING SHOULDER READING BY .001".
Now that you know the exact size of locking shoulder your rifle needs, there's one last question to solve. "To file or not to file" on a larger locking shoulder you might already have, down to the size you need. Very true, indeed, some guys hack away with a file or stone to thin out one that's too thick, but I think such a practice is taking the "backyard" aspect a little too far. Tell me how confident you are to be able to maintain the 11-degree surface against which the bolt seats. Tell me how even the new thickness will be (not .261" on the right side, .259" on the left). Tell me how pounding the locking shoulder in and out as you trial-fit again and again won't open up the hole in the receiver. All this on a critical part that has to hold against circa 50,000 psi, to save $20?? My personal opinion: BUY the correct-size locking shoulder or at least swap out your old one for the proper one at vendors who allow that.
You can do this. You will feel great pride and accomplishment. Be safe and have fun.
How to Order Pin Gauges
For Vermont (US made) ZZ: 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 $2.78 Each (2007 price)
MSC Big Book, Page 1352 (2007 Edition) (These are for MINUS. Use 889 prefix for PLUS.)| size | MSC part # |
|---|---|
| .252 | 8902520 |
| .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 |
| 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 |