Truss Rods by Bob Smakula

I’ve recently been reading many newspaper and magazine articles on the greatest inventions of the last 98 years. You tend to see the same things over and over: air travel, nylon, transistors, and even Little Debbie Nutty Bars. My personal favorite is one that few people ever see, though most guitar, banjo, and mandolin players rely on it to keep their instruments in shape. This hidden treasure-the truss rod-has saved me countless hours of neck adjustments.

The earliest manufactured guitars were intended to be played with gut strings and neck reinforcement was a minor. As players demanded louder instruments, steel strung instruments became more popular. To prevent constant warranty repairs, fretted instrument manufacturers had to stiffen the necks on their instruments. Fully strung and at-pitch steel strung guitars have about 150 pounds of pressure trying to pull them apart 24 hours a day. Guitar tops have bracing glued to them not only to modify the instrument’s tone, but to give strength and make the instrument long lasting. Necks have their own reinforcement to keep them straight and playable.

In the early part of the 20th century makers devised various reinforcement techniques. Gibson used a large V-shaped piece of maple glued into a similarly shaped slot in the neck. The C.F. Martin company’s first reinforcement was a 1 / 4″by 3/ 8″ piece of ebony glued in a slot the entire length of the neck, though by the mid-’30s they were gluing in a steel T-bar. Washburn was one of the first to use a rectangular steel bar for reinforcement.

As helpful as these additions were, they didn’t always keep a neck straight. An instrument left in a hot place under 150 pounds of string pressure could easily pull up. Changes in humidity could also cause necks to curve.

The adjustable truss rod was invented by a Gibson employee, Thaddeus McHugh, and patented in 1921. This invention made it possible to straighten a minor neck curve with the twist of a nut. This first truss rod was a 1 /8″ steel rod that had one threaded end with a nut installed. The other end was anchored in the neck’s heel. The rod had a curve to it so that the nut end was close to the fingerboard and the anchored end was about 1 / 2″ deeper in the neck. The premise was that when you tightened the nut the two ends would be drawn closer to each other and straighten the neck. This simple mechanical invention was a marketing coup for Gibson. It let them slim down their guitar necks considerably and was heavily advertised as a Gibson exclusive. When Gibson’s truss rod patent expired after World War II, most instrument manufacturers rushed to make truss rods a feature of their necks too.

The 1970s and 1980s had inventors improving the design of truss rods. Though Gibson’s early design worked, it was limited as to how warped a neck it could straighten. The truss rod renaissance gave us rods in aluminum U-channels, double rod systems, and truss rods that could take out forward and backward bow. These improvements allowed luthiers to adjust instruments with greater precision for the increasingly demanding musicians.

Many people ask me how to tell if a neck is warped. A quick check is to sight down the neck and compare the neck’s curvature with the straightness of a string. The method I prefer is, with the instrument strung to pitch, fret a first or a last string at the first fret and the fret at the body of the instrument. Using the string as a straight edge, look at the distance between the string and fret in the middle of the neck. No clearance means the neck is too straight or back bowed. More than .010″ clearance would mean too much forward bow. Either extreme means a truss rod adjustment is in order.

Truss rod adjustments are simple, but like any other repair they take a little care and knowledge. After you have determined the degree of forward or back bow, locate the truss rod adjustment nut. It is either at the peghead under the truss rod cover, or in the sound hole. Tightening the truss rod will make the neck bend backwards, straightening a forward warped neck. Loosening will make the neck pull forward giving you more neck relief.

I recommend turning the truss rod (with the appropriate wrench), a little bit at time. 1 /8 of a turn at a time with the instrument tuned up to pitch is good for a novice truss rod adjuster. After each turn check the neck as previously recommended until you have achieved the .010″ clearance. Why a little clearance? This gives the string a slight hollow in which to vibrate. With a perfectly straight neck string rattle can occur.

The worst thing that can happen in truss rod adjustment is breaking off the nut. This can occur when the truss is maxed out and you try one more turn to make the neck perfect. The nut should have some resistance, but if it doesn’t move, don’t force it. Take the instrument to a pro for guidance. They will have plenty of tricks up their sleeves to make it work.

If your instrument is truss rod-free, necks with too much curvature can be straightened with a heat press. Though successful most of the time, heat pressing a neck is not the exact science as is my favorite invention of the 20th century.

Bob Smakula is n musician and resident of Randolph County, WV. He is the proprietor of Smackla Fretted Instruments specializing in the restoration and sales of vintage stringed instruments.

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