Diagram courtesy of singlecoil.com

Several months ago we examined potentiometers to understand how they’re constructed. (You can brush up on this essential info here.) Now it’s time to discuss some typical pitfalls you might encounter when replacing pots or knobs on your guitar.

The first scenario is a hot topic in guitar repair shops worldwide: installing U.S. pots (measured in inches) in a guitar that sports metric electronics. For example, Epiphone guitars have a very good build quality, but they often suffer from poor electronics, so replacing the stock metric pots with high-quality U.S. CTS pots will be a real upgrade. The problem is the holes in the guitar are too small and the U.S. pots will not fit. The solution is to enlarge the holes from 8 mm to 9.5 mm (3/8"). This sounds easy, but it isn’t!

Never try to enlarge a hole in a guitar or pickguard with your handheld drill and standard drill bit. Instead, the quick-and-dirty solution is to use a sharp, fine-tooth round or half-round wood file to carefully enlarge the hole. Don’t worry if the hole isn’t perfectly circular—the washer will mask your dirty little secret. The professional solution? Use a sharp handheld reamer. It will only take about 20 seconds to enlarge each hole.

When modding guitars, you may face the opposite scenario of wanting to install metric pots in a guitar with stock U.S. electronics. Most military-spec pots and switches are made in Europe or Israel and are excellent for guitars. They’re available in the typical 250k and 500k values and several bushing lengths, but they all have metric bushing diameters.

Sometimes mil-spec pots have a bushing diameter of 10 mm, which makes them a bit too big for the 3/8" holes in the pickguard or body. If that’s the case, go fetch the reamer. However, most mil-spec pots have an 8 mm bushing diameter, so your 3/8" holes are too large by 1.5 mm. The solution is simple: First slip a lock washer on the shaft and then install the pot. After the shaft pokes through the 3/8" hole, add a plain washer that’s big enough to cover the hole, and then thread on the hex nut and gently tighten it down.

You can customize a guitar by replacing its stock knobs with new ones, but, again, this can be perplexing. One problem occurs when you try to install knobs designed for solid-shaft pots onto a split-shaft pot. Let’s say you want to put Telecaster-style dome knobs on a Stratocaster, but you want to keep the Strat’s stock split-shaft pots. Typically knobs for solid-shaft pots use a setscrew to fasten them to the shaft. If you put this type of knob on split-shaft pots and tighten the screw improperly, you can break one or both halves of the shaft (Photo 1).

Our final piece of the knob-pot puzzle happens when you attempt to install a push-on knob on a solid-shaft pot, like putting plastic Strat knobs on a Tele.

There are two ways to prevent this. The quick solution is to position the setscrew in the gap between the shaft’s two halves and tighten it very carefully. The pro solution is to fabricate a piece of wood or plastic that slips precisely into the gap between the two halves. In the shop we use a piece of nylon that’s already the correct thickness, so it’s simply a matter of cutting out a piece of the right height and width to fill the gap. We use super glue to hold this gap-filling piece in place and trim off the excess with a sharp knife and file. Voilà—the split shaft is converted to solid-shaft specs and the knob will fit like a charm.

Here’s another problem: You want to install a knob designed for metric solid-shaft pots onto a U.S. solid-shaft pot. We run into this quite often when we put metal Schaller knobs on a U.S.-made Tele. Schaller hardware is beautifully machined, but all their knobs are metric. A metric knob usually has a 6 mm hole, but the solid shaft on a U.S. pot measures .250" (6.35 mm), so the hole needs to be enlarged.

This requires a drill press. Don’t attempt to do this with a handheld drill because the hole won’t be straight. Put a sharp .250" countersink bit intended for metal into your drill press, don protective gloves and goggles, and slowly enlarge the hole, being careful not to drill through the top of the knob. But here’s the trick: You also have to secure the knob to keep it perfectly vertical, and you can’t do this by simply holding it. In the shop we use a vise grip covered with a thick layer of foam rubber to protect the knob from getting scratched. We also use wooden blocks with holes sized to hold our most popular knobs while they’re being drilled.

Consider the opposite dilemma: You want to install a U.S. solid-shaft knob on a metric solid-shaft pot—Tele dome knobs on an Epiphone Les Paul, for example. In this scenario, the hole in the knob is too big for the shaft. No worries: You can buy a brass sleeve developed for this specific purpose. Slide it onto the metric shaft and the U.S. knob will fit perfectly.

Ever tried to install a metric push-on knob on a U.S. split-shaft pot or vice versa? Perhaps you want to put an Ibanez Strat-style push-on knob on your beloved Fender U.S. Strat, but it won’t go on. The problem is that U.S. push-on knobs have 24 “fine knurled” teeth, while metric knobs have 18 “coarse knurled” teeth. The only thing you can do is fine tune the fit by squeezing the shafts together slightly or spreading them further apart. This works, but requires finesse. Don’t overdo it; otherwise one or both halves can break and you’ll have to replace the pot.

Our final piece of the knob-pot puzzle happens when you attempt to install a push-on knob on a solid-shaft pot, like putting plastic Strat knobs on a Tele. First, use a Dremel tool or a small round file to bore out the insert of the push-on knob until it fits on the straight shaft. Then get a setscrew designed for dome knobs (they’re sold as spare parts), measure it with your digital caliper or micrometer, choose a drill bit that’s slightly smaller, and carefully drill through one side of the knob. Put the knob on the pot and gently tighten the setscrew against the solid shaft. Done!

Next month we’ll explore the last of our Les Paul “master wirings.” Until then ... keep on modding!