Step U4: Gas System Installation
Objective: In Step U4 of upper receiver assembly, we will install the gas system (gas tube and gas block).
NOTE: This step does not apply if you are building an AR-9 on the Aero Precision EPC platform, which operates on direct blowback of the bolt, rather than direct impingement.
Tools Required
1: We recommend securing the upper receiver directly in a vice for installation of the gas block. You will not be able to perform visual alignment with an upper action rod installed. Be sure to use non-marring (plastic/nylon or rubber) vice jaws to avoid damaging the finish of the upper. Note that if you are NOT using a gas system illuminator to perform visual alignment, you can use an upper action rod to secure your upper during installation of the gas system.
2: We have tried several retail gas block jigs and have yet to find any that fit a wide range of gas blocks without modification of the block. We use a 3D printed product from Etsy. The Midwest Industries lower receiver vice blocks that we recommend include a universal gas block jig for roll pin installation.
3: If not using a borescope and gas system illuminator to align the gas port and gas block.
Components Required
| Quantity | Component |
|---|---|
| 1 | Upper Receiver |
| 1 | Gas Block |
| 1 | Gas Tube |
| 1 | Gas Tube Roll Pin1 |
1: Gas tube roll pin is often included with the gas block and gas tube (you may get one with each). All of the products that we recommend in our kits include the required gas tube roll pin. However, if you select another gas tube and gas block which both do not include the roll pin, you will need to purchase a gas tube roll pin$ separately.
Instructions
Secure the Upper
If performing visual alignment of the gas block, you will not be able to do so with the receiver on an upper action rod. Add non-marring vice jaws to the vice and gently clamp the upper receiver in the vice.
Step U4.0: Remove Existing Gas System (replacement/upgrade only)
If you are replacing your gas block or gas tube (or need to remove the gas system for any other reason), first ensure the firearm is unloaded.
If you need to remove the gas block from the barrel, you will need to remove the handguard first and you will most likely need to remove the muzzle device (unless you use a low profile muzzle device like the VG6 Epsilon SL muzzle brake$).
Back the gas block set screws out (or loosen the clamp screws, if a clamp on gas block) and carefully slide the gas block and gas tube off of the barrel.
If you are reusing either the gas tube or the gas block (but not both together), you will need to punch the roll pin out of the gas block using a 5/64″ roll pin finish punch.
NOTE:Â Do not reuse roll pins.
Step U4.1: Dimple the Barrel
If you are installing the gas system on a new barrel (or one that has not been dimpled), we strongly recommend that you dimple the barrel to prevent shifting of the gas block over time (misalignment with the gas port on the barrel will cause cycling issues).
Identify the set screw spacing for your gas block. Below are the measurements for the products that we regularly recommend and use.
| Gas Block | Set Screw Span |
|---|---|
| Luth-AR Low Profile | 0.350″ * |
| Aero Precision Low Profile | 0.485″ * |
| Odin Works Adjustable | 0.450″ |
| Odin Works Tunable | 0.450″ |
| Superlative Arms DI Adjustable | 0.500″ |
* Note that there is no dimpling jig that supports this span. We recommend at least dimpling the barrel for the screw that is closest to the shoulder (closest to the chamber; directly under the gas port).
Secure the barrel into a vice with the gas port facing up. Use a barrel vice block, padded/non-marring vice jaws, or wrap your barrel to avoiding marring the barrel finish.
Install the dimpling jig onto the barrel according to the manufacturer’s instructions. The instructions below are for use of the recommended SLR Rifleworks Barrel Dimpling Jig$ or the KAK Dimple Jig$.
Locate the set screw hole marked with the applicable spacing for your gas block. Slide the jig onto the barrel with the set screw hole toward the rear of the barrel (away from the muzzle). Roughly align the screw hole on the jig with the gas port on the barrel. Thread the alignment set screw into the jig and drive until it contacts the barrel. Wiggle the jig as you gently tighten the set screw. You should feel the point of the set screw engage the gas port. DO NOT OVER-TIGHTEN. You only need to tighten to the point that the jig is securely indexed to the gas port. If you over-tighten, you will damage the gas port.
Once the jig is installed, loosen the vice and rotate the barrel 180 degrees so that the gas port is facing down. Install the appropriate drill bit into a hand drill or drill press. Insert the tip of the bit into the rear hole in the jig (under the gas port) and drill a slight depression into the barrel. DO NOT DRILL TOO DEEP (this is barrel dimpling; this is not barrel perforation). You do not need to dimple the barrel twice if installing a standard gas system. If you are going to install a piston system, we recommend dimpling the barrel twice for added security. If installing a piston system of you want a second dimple for any other reason, drill the second dimple.
Remove the jig by loosening the set screw and sliding off the barrel. Your barrel should now have 1 or 2 shiny silver depressions on the side opposite the gas port.
Step U4.2: Assemble the Gas Tube into the Gas Block
If you are using an adjustable gas block, adjust the orifice to the full open position, per the gas block manufacturer’s instructions.
Remove the set screws, if applicable.
Identify the front end of the gas tube (plugged end). The gas tube must be oriented such that the larger hole faces the gas port on the barrel.
Identify the rear of the gas block. If it has a closed end, that is the front. If it is open on both ends, the gas port hole will be drilled closer to the rear of the gas block.
Insert the gas tube into the gas gas block (from the rear) with the larger hole facing down (toward the gas port).
Align the gas tube roll pin hole in the gas tube with the corresponding hole in the gas block. It may help to use your roll pin finish punch to verify alignment. Once aligned, verify that you can see the gas port opening on the bottom of the gas tube through the hole in the gas block, as viewed through the rear set screw hole (if possible).
Place the gas block onto your gas block roll pin installation jig.
Insert a new gas tube roll pin into the roll pin starter punch and begin to drive the roll pin into the gas block. As the roll pin is driven into the gas block, you may need to adjust the gas tube slightly to ensure alignment.
Once the starter punch bottoms out against the gas block, switch to a 1/4″ steel punch. Drive the roll pin until almost flush with the gas block.
Switch to the finish punch and drive the gas tube roll pin until it is flush with the gas block or is slightly recessed.
Step U4.3: Mark the Barrel and Gas Block (if not performing visual alignment with a borescope)
The gas port on the AR barrel is positioned to allow for use of the A2 gas block sight and such that the A2 gas block sits against the shoulder behind the gas block journal on the barrel. This gas block is slightly longer than most aftermarket gas blocks. A standard gas block should not be indexed to the shoulder on the barrel. There should be approximately 1/16″ between the gas block and the shoulder. Even if you dimple your barrel, we recommend using a gas block alignment tool to verify proper alignment of the gas block and gas port. Failure to properly align may result in failure to cycle or unreliable cycling.
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Insert the gas block alignment tool into the gas block according to the manufacturer’s instructions, ensuring that the gas port on the gas block aligns with the applicable feature of the tool. Using a pencil, mark the front edge (gas port is closer to the rear) of the gas block on the tool on either side of the center slot. Mark the front face of the gas block at the center slot of the tool. Remove the tool from the gas block.
Snap the gas block alignment tool onto the gas block journal on the barrel with the hole toward the rear of the barrel. Align the feature on the tool with the gas port on the barrel. Using the mark applied to the tool, transfer the mark to the barrel to mark the appropriate depth and center line.
Step U4.4: Install the Gas System onto the Barrel
Slide the gas block onto the barrel, with the gas tube pointing to the rear.
Feed the gas tube through the applicable hole in the upper receiver (Aero Precision Enhanced upper) or barrel nut (threaded uppers).
Thread the thumb screw into the rear set screw hole. Do not tighten.
Step U4.5: Align Gas Port and Gas Block
If using a borescope and gas system illuminator, feed the fiber optic cable down the gas tube from within the upper receiver and turn on. Insert the borescope into the barrel through the muzzle until the gas port is visible. Ensure the gas block is aligned with the gas port.
If using the gas block genie, seat and index the gas block such that the mark on the gas block aligns with the mark on the barrel.
Tighten the thumb screw when the gas block is properly aligned.
If tightening the set screw into the rear dimple causes the gas block to be misaligned with the gas port, switch the thumb screw to the front gas block hole and retry. If your barrel is only dimpled in one place, you should be able to secure the gas block in any position to ensure alignment. If the rear dimple is not perfectly aligned, we recommend putting a drop of ultra high temp thread locker in the rear screw hole over the misaligned dimple before threading the set screw in. This will fill any void in the dimple and ensure that the gas block does not move.
Step U4.6: Install First Set Screw
Apply a drop of ultra high temp thread locker onto the threads of one set screw.
Begin to thread the set screw into the front set screw hole in the gas block (or the rear if you needed to move the thumb screw to the front hole).
Using an appropriate hex wrench, hand tighten the front gas block set screw.
Step U4.7: Torque the Front Gas Block Set Screw
Set your torque driver to 25 in. lbs. and insert the appropriate driver bit (usually 3/32″) for your gas block’s set screws. Torque the set screws until the torque limit is reached.
Step U4.8: Install and Torque Rear Set Screw
Remove the thumb screw from the gas block.
Repeat steps U4.6 and U4.7 for the second set screw.
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