FAL Reviews - MiscFAL Review: Gas Block Drilling Fixture

FAL Review: Gas Block Drilling Fixture

(updated 04-12-2020)
Introduction
This project started turning in my head when I received some virgin gas blocks to mount to barrels. Identifying the exact location of the retaining pin hole and the handguard screw hole
is challenging, particularly because there are slight variations that make determining the “correct” location tough.

I used to do it manually – mount a used gasblock in the mill, find the hole center, swap for the virgin gas block, and drill. I came up with a short threaded rod for the front sight hole so I could establish level,
and a nut threaded for the gas cylinder so that the block would always teturn to the same position in my mill vise.

While this was adequate for the relatively rare virgin block, I also had plans to do a run of L1A1 barrels. Many Australian parts kits came in after Republican AG Gonzales “reinterpretted” the Republican’s
import ban to include barrels and recivers (non-sporting), so the barrels had to be chopped.

I had plans to do a run of US made L1A1 barrels and having 150 virgin L1A1 gas blocks to mount, I had to come up with a faster, repeatable method.

Primitive Method
finding hole center on a used FN gasblock. Rod for level and threaded collar to position the block so the web didn’t contact the vise.

remove used gasblock and replace with virgin block – hole location is now indicated.

L1A1 Gas Regulator Stop Pin
Virgin L1A1 gas blocks lack the regulator stop pin. This is a press fit. Since the front of the gas block is not flat,
I had to come up with something to hold the block perpendicular to use an arbor press for installation.

the original pin is grooved – the best commercial replacement pin I could find that would give a tight interference fit is a M2.5x8mm

This is the fixture for holding the block vertical in the arbor press. Not shown are heavy magnets I set around the fixture so I didn’t have to align it each time.
the end of the pin is beveled and it needs to be flat.That’s why I chose the 8mm pin – it’s a bit long, but I could then mount the block on the magnetic plate of a surface grinder
and reduce the pin to the exact height. Too long and the regulator won’t contact the gas block and the regulator retaining spring won’t fit.
Universal Fixture
FAL standard gas block, L1A1 gas block, and FAL Israel (Light and Heavy) gas block are all different. I wanted a universal fixture. I collaborated with my friend
John Hallis at North Freedom Engineering and we came up with this fixture. He’s a “real” machinist and could take my ideas and put them into practice with a much higher degree of accuracy than I could manage.

Here is what we came up with. The constant dimension on all the gas blocks is the gas chamber hole (except Indian, which is a little tight).

Using a spud for the cylinder, with different collars to position the different sized gas blocks to the same relative position,
and a threaded 1/2″ OD rod to insure the hole is perpendicular.
Eventually I’ll have the courage to drill the barrel and the block together, but as long as I’m finding annoying differences from one original block to the next,
I drill the block slightly undersize, mount and pin it to the barrel, and then finish drill it. This is particularly important on the L1A1 which correctly uses a tapered pin.
So I drill to the minor diameter and after mounting, use a #0 tapered drill and then finish with a reamer.
Mounting the gas block top dead center involves driving it on (some adjustment to journal diameter may be needed) and then tapping the block until it is top dead center.

A new L1A1 block will have parallel sight wings, but a used one this may not be so. If in doubt, I use a threaded rod in the front sight hole.

L1A1 barrel should have a horizontal flat at 12:00 on the chamber side, while FAL barrels you can use the vertical wrench flat – after verifying they are parallel.

The vertical pointed rod is the zero point to set the DRO. Theoretically, moving the DRO the same amount on the X and Y axis will put the hole in the exact same spot.
But the problem I found – testing with many original gas blocks – is the hole locations are close, but not exact. Many (not all) virgin L1A1 blocks have the pin hole location dimpled.

For the first dozen or so, as I refine the locations, it’s a bit annoying. Because I plot one thing (from hole back to zero point) on an existing L1A1 gas block and I get -15.25mm / – 65.53mm.
But then starting at the zero point, I go back to the pre-dimpled point on a virgin block and I get + 15.39 / + 64.78. Then the next one I get + 14.99 / 65.38. So far my max variation is +0.26mm / + 0.75 mm.
Not terrible, but I figure if I log each used and new unit, when I get a dozen data points, I can take the average.

Gas Port
The final step is to drill the gas port – .098″ – FAL is 90 degrees through the hole behind the front sight, while L1A1 is 32 degrees through the outside of the cylinder.

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