Shop-Made Mini-14 Gas Block Wrench

While cleaning my Ruger Mini-14 Ranch Rifle I noticed a little surface rust forming on the inside corner of the lower gas block. I had never removed the gas block before as this is not a recommended step in the normal cleaning process. Care must be taken when removing the four socket head cap screws as Ruger stakes the screw to the upper gas block at the factory. Simply removing the screws can damage the threads in the gas block as the staked portion of the screw rotates through.

By alternately loosening the screw a quarter turn and tightening it again, the staked portion of the screw will eventually wear down, allowing removal. The first screw I tried in this process turned very easily with a ball nosed 9/64" hex wrench. I shifted to the opposite side of the gas block and found one screw turned under a more reasonable breaking torque. The remaining two screws would not budge.

I tried snugging up the first two screws in hopes that this would relieve some of the pressure but the two screws remained locked in place. I did not want to tap the screw heads with a hammer for fear of damaging the scope and I did not see a good way to heat the screws without risking the gas block finish. Instead, I applied more torque with the hex wrench, and predictably, the screw head stripped out. Some people will advocate drifting a Torx driver into a stripped cap screw head.

This strikes me as a good way to deform or destroy the Torx driver and would require removing the scope from the rifle. Instead I decided to go with the sure fire method and drill the head off of the screw. Fortunately, this approach works very well with socket head cap screws as even a stripped screw head will center the drill bit easily. The bit diameter is chosen based on a size between the minor (root) and major (outside) thread diameter. Ruger uses #8 screws with 36 threads per inch.

This is the unified fine thread (UNF) pitch for the #8 screw size. Note that the 8-32 unified course thread (UNC) is more common and will probably be the only choice in most hardware stores, however, the 8-36 UNF is common to a gun parts supplier. A number 27 drill bit (0.144" diameter) is a good size for this job. It is best to fixture the gun in a drill press to avoid drilling at an angle but a hand drill will work if careful attention is paid to keeping the bit properly aligned.

After removing the head from the first stuck screw, I was relieved to find that the remaining stuck screw backed out normally. Once the gas block was apart, the remains of the stuck cap screw came out from the upper gas block with serrated needle nosed pliers. I chased the gas block threads using an 8-36 tap, which can also be purchased from any gun parts supplier. Since the gas block is threaded all of the way through, a standard tap can be used; there is no need to use a bottoming tap.

I ordered four new screws which are sold specifically for the Mini-14. At $2.00 each these area small investment. How to properly tighten the gas block on a Mini-14 is a subject that is well explored in online blogs. Some advocate experimenting with different values of bolt torque to optimize accuracy. Others believe that simply snugging the bolts by hand is sufficient. Additionally, there are a number of aftermarket gas blocks that offer improved barrel harmonics. I do not intend to explore all of these variables.

After all, my Mini- 14 was shooting just fine with one gas block screw practically loose. That said, I do believe in using the proper torque when reassembling guns. This turned out to be a bit of challenge with the gas block though, as the rear screws are located under the front sling swivel. Although it is possible to place a hex wrench straight into the rear screws with the swivel rotated toward the muzzle, the wrench will bump up against the swivel with each point on the hex.

I decided a ball nosed wrench would be the best option to avoid nicking the finish on the swivel. I do not own a set of socket drive hex wrenches with ball nosed ends. I did find a 1/4" socket drive 9/64" wrench, but without a ball nose. A brief search at my local hardware stores was not successful either— none of the socket-drive hex-wrench sets I found contained the 9/64" size. The most straightforward approach would be to cut down a ball nosed hex wrench from a full set and in-

stall it in the 1/4" socket drive, however, I didn’t want to ruin my hex wrench set and buying another set just to get one wrench seemed like a waste. Instead, I decided to make my own ball nose hex wrench. I found a regular 9/64" hex wrench in my collection of loose wrenches and used a cutting wheel to remove the short leg. I opted to use a hand held motor tool with a grinding wheel to modify the wrench, which would be held in the headstock chuck on my engine lathe.

I mounted the motor tool in a router base adapter and made up a simple fixture to attach it to the tool post on the lathe’s compound feed slide. While this makes for a convenient way to position and advance the cutting stone, it is not very sturdy. Light passes are definitely required to avoid breaking the grinding tools. However, this method allows the motor tool to be positioned anywhere from perpendicular to in line with the work and fed into the work using the carriage, cross slide or compound slide.

The hex wrench was held in a 3 jaw chuck in the headstock. It is not necessary to run the lathe for this project but the chuck provides a convenient way to securely hold and position the hex wrench. The plane of the grinding wheel axis is parallel to the lathe bed, so the chuck The author decided to make a ball-nosed socket drive hex wrench to avoid marring the finish. needs to be rotated such that one of the wrench’s hex edges is straight up, which presents a flat face on the front and back.

The motor tool was set to 45 degrees for the first pass. I opted to work on the face behind the chuck as this provided more clearance. The chuck was rotated through the six sides of the wrench. The compound slide was then adjusted to 25 degrees and the process repeated to create a shallow angle extending to a point just short of the socket cap depth. This completed shaping the wrench for the interior of the screw. Next, the relief necessary to clear the top of the screw must be formed.

The motor tool was positioned to 45 degrees and positioned with the outside corner of the grinding bit in line with the end of the socket cap screw. The cross slide feeds to grinding bit into the work. I took the slide in 0.015" after the initial contact. This depth provides enough clearance to let the hex wrench angle in the screw head without excessively weakening the wrench. The completed ball nose wrench was then pressed into the 1/4" socket drive with thread lock to secure it in place.

As I already mentioned, the proper amount of torque to be applied to the gas block screws is a matter of some debate. I believe there are two primary limiting factors in gas block torque: Stripping out the hex—either on the wrench or the screw head, as I already proved—or cracking the gas block. The gas block is designed to have a gap between the upper and lower halves. The upper half rests directly on the barrel but the lower half is seated against the gas port bushing.

This bushing ports the combustion gas from a small hole in the underside of the barrel to gas pipe piston where the gas pressure will act on

The screw shank was then easily removed. (tension from torque) higher than the force exerted by the gas pressure acting on the gas port bushing. A minimum torque value can be calculated using the cartridge pressure and actual part dimensions. The gas port bushing has a 0.16" diameter on the sealing face. Although the actual seal area is probably much less, using this diameter will provide a worst case scenario. Assuming the gas pressure reaches the SAAMI maximum of 55,000 psi the slide assembly to cycle the action.

The gas block screw torque resists the force from this pressure. The bolts need to have a preload during firing, the force on the gas port bushing will be (π × 0.16 2 ÷ 4 in2) × 55,000 lbf/in2, or about 1,100. To calculate the amount of torque required for a given bolt load, multiply the major thread diameter (0.161" in this case) by the load and then by the frictional coefficient, which is a dimensionless parameter typically ranging between 0.12-0.20.

Use a value of 0.20 for unplated screws; the lower values apply when coatings or lubricants are used. Also, we need to account for the fact that we have four screws, so our equations becomes Torque = (load × diameter × friction coefficient) ÷ 4. Here, this is (1,100 lb. × 0.161 in. × 0.2) ÷ 4, or 9 inch-pounds of torque. This value assumes that the gas block load is evenly distributed and does not take into account the vibration from shooting or transportation that might loosen to screws over time.

However, it does give usa general idea of the lower torque limit required to maintain a seal when the rifle is fired. Note the impact of the frictional coefficient in this equation. If we changed the frictional coefficient from 0.20 to 0.12 by using a thread lubricant, the force output would increase from 1,100 lbs. to over 1,800 lbs. For this reason, it is important to use only a thin film of oil to protect the screw threads from corrosion and to apply the film in a uniform fashion.

The Allen Manufacturing Company of Hartford, Connecticut (allenhex.com) recommends a maxi- Top A motor tool attached to the lathe cross slide held a grinding wheel to form the ball nose shape. Bottom

Make sure the gap is the same on both sides when reassembling the gas block. mum tightening torque for a high strength of 48 inch-pounds. This value needs to be lowered if the screws are threaded into a lower strength material such as aluminum. This is much too high for our application, as each bolt will produce a theoretical load of 1,490 lbs for a combined load of almost 6,000 lbs! This load would almost certainly break the gas block.

Most recommendations for gas block screw torque range from 22 to 36 in-lbs with the norm at 26 inlbs. Those of us who plan to check the gas block screw torque during normal cleaning can get by with a torque value on the lower end of this range, provided the screws do not show signs of backing off. For guns that are less likely to have the screw torque checked regularly, a torque value on the higher end of the range is advised.

A removable thread lock The gas port bushing sits in the counterbore. can also be used to assure the screws remain in place. Care must be taken to assure that the screw and lower gas block threads are clean and free of any oil because the thread lock requires direct contact with the metal to form a bond. However, a thin coat-

This allows the gas port bushing to engage the counterbore and hold the assembly together. ing of oil on the gas block is required to protect the finish. In my opinion, cleaning the screw threads while not compromising the protective film on the rest of the gas block is problematic at best. I suspect this may also be the reason Ruger decided to stake these screws in the first place. To reassemble the gas block, first insert the gas pipe piston with the counterbore facing up into the lower gas block.

Next, insert the gas port busing through the hole in the lower gas block and align it with the counterbore in the gas pipe piston. Next, place the lower gas block assembly into the barrel and assure that the top of the gas port bushing is fully seated into the counterbore in the underside of the barrel. The gas port bushing must fully seat into both the gas pipe piston and the barrel counterbores or you may damage the rifle. Align the upper gas block and thread in the four screws by hand.

Double check the gas port bushing alignment and adjust the screws to make the gaps on both sides the same. I used a set of feeler gauges placed between the screws to assure the gas block halves were straight. Torque the screws in a crossing pattern to no more than half of the final desired torque on the first pass. Make sure the gap is equal on both sides; if it is not, adjust the screws and re-torque. Increase the torque to three-quarters of the final torque, and then to the final value.

Repeat the final torque once more to make sure all the screws are tight. If you decide to periodically check that the screws are properly tightened, apply the desired torque using the same crossing pattern. Watch and feel for any rotation of the screw head. Noticeable movement indicates the screw has lost a significant amount of torque and a closer inspection of the screws and gas block is warranted.

To remove the gas block, first loosen each of the screws slightly (no less than half the original torque) in a crossing pattern and then back the screws out completely. This will help to insure that the gas blocks separate evenly. If the screws do continue to loosen over time, try reducing the amount of oil on the threads or increase the assembly torque within limits. Also, be sure to use bolts specifically made for this application. Weaker screws will elongate over time and reduce the bolt load.

Ruger does not recommend servicing the gas block if it is functioning normally, so careful consideration should be given before disassembly. There are a number of risks, including damaging the gas block threads with a staked screw and damage to the barrel or gas block if not properly reassembled. These risks can be minimized by tapping the gas block threads, carefully aligning the components and properly torquing high grade screws to secure the assembly.

Tightening the screws without damaging the rifle is made easier with a shop made socket-drive ball-nosed hex wrench. AG