A Closer Look At Screw Torque In Gunsmithing Applications

Screw application can have an untoward effect on the mechanical function of a gun including accuracy and annoying bullet impact changes. Damaged screw heads detract significantly from the appearance and value of a firearm as well. If you are like most devoted riflemen and hunters you may be into at least some moderate gun work involving removing and tightening screws and mounting scopes.

Scopes are often changed from one rifle to another in search of that perfect optical combination or a rifle action may be taken from its stock and replaced for one reason or another. Frequent use will leave screws vulnerable to damage. Most gunsmiths, as well as the experienced handyman, have at least a working knowledge of proper gunscrew application but for many there is much to understood about proper screw torque.

Webster’s Collegiate Dictionary defines torque as, "A turning or twisting force." Of course this basic definition is somewhat over simplified when it comes to real applications. For example, a calibrated torque wrench is used to measure the force used to tighten a threaded nut, screw or bolt.

Each size and type of bolt or screw is designed to withstand a workable torque depending on the type and hardness of the screw, the pitch or number of threads, the size of the screw and the hardness of the threaded mate From rial the screw enters. Here we will discuss screw torque as it applies to firearms in general.

Some of the issues associated with proper screw thread is attributable to their many different sizes and shapes, the hardness and quality of the steel used in screw construction, the hardness of the metal where the attachment screws are threaded, and the quality of the tools being used. In scope mounting alone there are at least three different screw head configurations. Those most

Wrench comes with the necessary bits and adapter for most screw applications. This wrench will likely show that you have been using incorrect torque on many gun and scope screws. From left, Torx head, common slotted screw (Fillister) head, Socket (Allen) head screw. Proper screwdriver bits area must in working with these screws along with properly applied torque. commonly used include the slotted Fillister head, Allen or socket head and the more recent addition of the Torx head.

The socket and Torx head screws are also available in metric sizes. Common screw heads vary widely in slot shape and size to further compound the problem of proper torque as the screw is tightened or removed. Not all gunsmith shops, to say nothing of average gun owners, are fully equipped with the many screwdriver sizes needed to precisely fit the screws used in gunsmithing or gun care either. This adds a problematic dimension to it all.

We often see damage done as a result of screw torque applied by those who possess extraordinary strength and sometimes little touch. Hand strength varies among individuals and some don’t seem to know their own. Excessive torque will really mess up screws by muti lating the heads, stripping threads, or possibly breaking the screw. Too much torque on scope ring screws can actually compress and permanently distort scope tubes or even bend the top half of a scope ring.

The same goes for the over tightening of guard screws, particularly on wood stocks, where action support varies from one gunstock to another. In carefully controlled tests involving variance of guard screw torque on rifle actions I learned that accuracy was clearly affected. This was particularly true where a third center action screw was present. The quality of action bedding has a significant bearing on this.

For example, a well supported, pillar-bedded stock is less subject to action springing under heavy or uneven guard screw torque than a poorly bedded stock. But few, if any, are totally immune to accuracy changes as guard screws are over worked in the center of a poorly supported action.

In controlled tests with benchrest and varmint rifles capable of extremely fine accuracy, even those rifles pillar bedded in the stocks would show some change in grouping as well as point of impact changes as the guard screws were unevenly or excessively torqued or manipulated while firing a group. It was not uncommon for bullet point of impact to undergo a gradual change for a period of time after an action was removed from the stock and replaced.

This came to be a real problem in long range shooting where a rifle was sighted-in one day, only to be off the next time it was used. Early on in my career I acquired a few sets of screw drivers then available to the handyman and gunsmith. Both the earlier Chapman and Bonanza brands come to mind here. The Bonanza set consisted of seven screw drivers with square-ground ends and grooved, round handles for better grasp.

The Bonanza line has been discontinued while the Chapman line is carried by Brownells in a much more expanded selection. My original set consisted of quarter-inch hex driven shafts with a good selection of bits for Fillister and American Phillips type of screws. There were no metric Allen wrenches in this set. In 2004, Wheeler Engineering introduced an 89-piece Professional

Gunsmith Screwdriver Set, a product made by Battenfeld Technologies, Inc. (www.battenfeldtechnolo gies.com, Columbia MO 65203, This Deluxe set consists of fifty-four hollow ground flat bits, eight Allen bits, four Phillips and the Torx, along with fifteen specialty bits. Two major driving handles come with the set with the necessary fittings to handle all the bits and specialty tools. Screw Torque Wrenches I recognized the need for accurate measurement in my work and made a torque wrench back in the late 1960s.

It was fashioned after a standard automotive torque wrench but in miniature size. I calibrated it in inch-pounds and used it primarily to apply uniform torque to the guard screws on some of my rifles. My wrench proved useful with scope mount screws once I learned the amount of torque needed on the base screws. As it turned out, I found myself using the torque wrench on a regular basis, particularly after discovering a difference in each of the four scope base screws used on a rifle.

It had been very difficult to get each screw tightened with the same torque. Thankfully, gunsmiths today are no longer forced to cobble together their own solution because the much welcomed and needed Wheeler FAT wrench has entered the scene. FAT is an acronym, Firearms Accurizing Torque, and is perhaps the closest assist to correctly tightened screws the average handyman or gunsmith There are 15 specialty bits and accessory tools for many different guns like Colt, Ruger, S&W, Glock, Remington, Mauser and more.

This is one of the more complete screwdriver sets you will find geared to the gunsmith.. Just select the proper torque and turn the handle until it releases at correct setting. A torque of fifty-inch pounds or more may require the use of both hands. will ever hold in his hands. The FAT wrench comes equipped with a square drive adapter bit, T10, T15, and T20 Torx bits, numbers 10, 11, and 32 flat blade bits, numbers 3/32" and 5/32" Allen bits, and a Leupold/ Buehler windage bit.

This is a perfect compliment to any professional gunsmithing screwdriver set. The square drive directly off the wrench will handle those small 1/4" drive socket wrenches most shops and ho meowners have. This torque wrench removes the guesswork in tightening scope ring screws, scope base screws, rifle guard screws, even the windage screws on Redfield-style

scope mounts. Wheeler Engineering communicated with manufacturers of rifles and scope mounting accessories to get their input and then tested these data on their rifles and others. The results are very informative and interesting. One must continue to use good common sense in working with gun screws. A good technician develops feel for what is being done.

At the least this should alert the gunsmith to the degree of torque being applied and help provide a correlation between hand torque and the calibrated torque wrench. For example, you might tighten a scope base screw by hand and follow up with a torque wrench to verify your findings. In some cases I have used screw bonding cement, such as Loctite, but I don’t use this as a routine, especially where scope swapping from gun-to-gun may be the case. Such screw bonding cements can really make screw removal difficult.

Where screws thread into aluminum scope rings, or even aluminum receivers, care must be taken to not over-torque such screws. The same applies where thread depth is shallow. A rule of thumb for any bolt or screw is to have a thread depth at least equal to the diameter. As an example, a 1/4-28 screw should have a minimum of seven threads holding. Cheap screwdrivers or those with soft bits should be avoided. Most name-brand sets can be relied on for dependability.

The slotted screw in general is the weakest screw head and the easiest to ruin. The Phillipstype socket head screw is good but must have a well-fitted wrench and not be mistaken for a metric size. Always be sure of the differences between Unified Thread Standard (American) socket head screws or ISO metric sizes. A mistake here may cost you a mutilated or stripped screwhead. The Torx head is ideally strong for scope bases and rings and resists cam-out.

In fact, it holds so well to the wrench that it’s easy to apply too much torque. The possibility of breaking the screw or stripping a thread is more likely with the Torx head. The Fillister head (square slot) screw provided a sort of safety gap here as screwheads would give way before too much torque could be applied. Conclusions As with all elements of gunsmithing, understand your problem and use the best tool for the job.

It may pay off to learn more about screw threads and the necessary torque for various screw threads in a variety of applications. The chart provided below will get you started. The use of a torque wrench can be an education in itself. AG Screw Application and Torque Recommendations Screw Type or Screw size/thread Comments Position on Gun Recommended Torque Fillister or socket 1/4-28 40 inch lbs. No pillar bedding, wood stock head guard screws at receiver ring. 1/4-28 40-45 inch lbs.

With pillar bedding or synthetic stocks reduce rear tang screw torque by 10 inch pounds on wood stocks. 1/4-28 50-60 inch lbs. Fiberglass stock with bedding block 10-32 20-25 inch lbs. Unless center guard screw is well supported. Only light torque is recommended. Scope ring screws 6-size 10-12 inch lbs. Reduce by 10% for aluminum rings Scope base screws 6-48 18-24 inch lbs. Screw size and threaded metal (all head shapes) 8-40 25-30 inch lbs. varies widely here.