Hopefully you were fortunate enough to go a high school that still had a shop class. While you wouldn’t walk out of one of these classes with enough knowledge to frame out a house or replace your truck’s engine, the curriculum usually provided a good working knowledge of most hand tools. But it’s pretty hard to capture the average teen’s attention, so something as mundane as torque specifications for different types of fasteners probably only got a cursory mention in even the most complete course.
That doesn’t mean it’s not important. Tightening fasteners enough ensures they stay where they’ve supposed to as the forces of whatever they’re holding together are acting against them. And not over tightening a fastener ensures that it won’t shear when the going gets tough, leaving you with half a bolt in a now unusable threaded hole.
Pretty much every nut, bolt, or screw has a proper torque specification that ensures it does its job as it was intended to.
When you think of a torque wrench, you probably see a ratchet type like this Tekton 1/2′ drive torque ratchet. This is a big tool with lots of leverage that’s more suited lug nuts on a car than bolts on a gun. Tekton also makes a 1/4′ version that would be useful on an armorer’s bench if you prefer this style. Amazon
There is anywhere from a few to tens of screws on a firearm, depending on the complexity of the action. Obviously a single shot has far fewer moving parts than a semiauto, and as the number of parts increases so does the number of screws needed to keep it all together. Barrel nuts, bolt carrier keys, action screws and scope rings and mounts all have a proper setting to ensure they stay tight but don’t snap off or adversely affect performance. Read on for a primer on how to get your guns’ fasteners as tight as they need to be—and no more.
If your shop teacher did mention torque values, they mentioned torque wrenches. They probably went over the variety that resembles a ratchet (like the one in the photo above) with a thicker arm that clicks when you’ve reached a certain threshold, or, if you’re hair is gray enough, the beam type (photo below) with a pointer indicating how hard you’ve cranked down on the nut or bolt.
Both are useful, but the edge probably goes to the click-type as they provide audible and tactile feedback when they reach the right torque spec, while the beam type requires you watch the indicator the entire time.
The units of measure were likely foot-pounds, as those are the most common when rebuilding an engine or tightening a lug nut (Over tightening lug nuts is the leading cause of warped brake rotors, which is what often gives your brake pedal that pulsing feeling).
But there’s significantly more finesse in firearms than there is in internal combustion engines. As such, the specified forces are measured in inch pounds, which are orders of magnitude less.
Below are some typical torque ranges for common gun parts, but be sure to check with the manufacturer of your items, as these are just guidelines.
|Type||Torque Value (inch-pounds)|
|Barrel Retention Screws||80|
|Scope Ring Screws||17-25|
|Scope Ring Crossbolts||65|
|M-LOK Accessories (polymer)||15|
|M-LOK Accessories (metal on metal)||35|
Some manufacturers may even specify the order in which the screws must be tightened, or require that they be torqued to set values until they are as tight as they need to be, i.e., the front action screw must be tightened to 25 inch-pounds before the rear can be tightened to the same. Once you have reached that value, they can both be cranked down to 65 inch-pounds, again following the front to back sequence.
Types of Wrenches
You can get traditionally styled torque wrenches with inch-pound settings, but there are also a number of screwdriver models and other styles that are well suited to these applications. I’ll go over each type and provide some instruction on how to use each and you can make your own mind up when it comes time to outfit your shop.
You don’t need the leverage that a ratchet wrench provides when dealing with the low torque values on common gun parts, but feel free to use one if your comfortable with them. To set these, you typically rotate the handle until you reach the prescribed value. Once you have tightened the fastener to that value the wrench will click and you will feel the head pop. Be certain to stop then; if you keep going the wrench will keep tightening and can cause damage.
Brownells has an excellent 1/4 inch drive ratchet torque wrench that can be set anywhere from 20 to 150 inch pounds, making this a good choice for tightening barrel nuts.
If you’re really old school, you might be interested in a beam style wrench, which uses a lever that deflects and an arrow to indicate the torque being applied, but these are few and far between these days. You can still find a number available in foot-pounds, but there are slim pickings in the inch-pound realm because it’s hard to mill a shaft that will deflect predictably at these torque values. If you simply have to have one, Home Depot carries a decent model. Pick up a square to hex ¼ inch adapter, like this one from Brownells, and use your existing gunsmith bits with either lever style wrench.
Screwdrivers that can accurately apply torque are typically easiest to work with when mounting scopes and the like. Their small form factor makes working in tight quarters easy, and only requires one hand. Perhaps the most popular on the market is the Wheeler F.A.T. Wrench, which has a torque range from 10 to 65 inch pounds that is set by simply pulling on the top cap and turning until the desire value is reached.
So far we’ve covered screwdriver and socket style wrenches, but there is another method for that’s becoming more common. Torque limiters act like fuses, blowing when they’re overloaded, except they just spin freely, not applying any more tension to the fastener. Because of this, they are probably the easiest to use, just put your bit in and turn until it stops tightening.
The downside to these is that they only come in predetermined settings, so they aren’t a one-size-fits-all solution like the adjustable wrenches. Instead, you’ll need one for each value. Fortunately, a company called Fix It Sticks put together a kit that contains a number of different torque limiters, a T-handled driver, and an assortment of bits that fits in a small carry case.
Before you mount that scope or other accessory, give some thought to how tight it actually must be. Most people tend to over tighten stuff, so use a torque wrench. It could save you from some catastrophic damage, like crushing a scope tube. Even if it saves you from damaging the screw head and making future work more difficult, the expense was worth it.