This is one of those projects that, on the face of it, looks simple but it’s one I’ve seen botched more than a few times.
If you’re replacing your tuners with ones that are exactly a match then it’s a simple matter of just taking the existing tuning machines off and dropping the new ones in. This is rarely the case. Usually upgrades are what you’re after and that can be challenging.
First lets get into what’s available. There are two basic kinds (with all sorts of variations) open backed and sealed. Open backs are most often seen on acoustic flat tops but also on older cheaper electrics. Open back does not mean cheap. Waverly tuners sold by Stewart MacDonald are extremely high quality and the cost starts at $140 and up past $300. The posts are bronze and the gears are nylon bushed stainless steel. They are copies of pre WWII Grovers and far surpass the originals in quality and materials. They also are the same foot print as Grovers so it makes upgrading your pre-war Martin easy to take. No extra screw holes and you can replace them without hurting the value of a vintage instrument. Grover Sta-Tite is the modern version of their vintage tuner. They are about 25% of the cost of Waverlys and an excellent choice for replacing worn machines on a budget. Several other companies that produce this kind of machine are Ping, Schaller, Gotoh, and Schertler.
I include the older style Klusons and their copies in the open back category because they are basically open backs with a cover. These are not to be confused with the next group: sealed machines.
Sealed machines were an invention of Grover. I’m guessing in the late ’50’s although I couldn’t find much information on their history. These machines called Rotomatics had permanently lubricated gears with sealed backs. This meant no more messy oil to contaminate the head stock. Almost overnight this replaced the open back. Martin, Gibson and later Fender all put sealed tuning machines on their guitars. Grover had some quality control issues in the 70’s and Schaller, a German company, quickly became the tuning machine du jour. Gotoh, a Japanese company, makes what I consider the finest of the sealed tuners, the 510. It has an 18 to 1 gear ratio and almost no backlash. It is well finished and in my opinion a better value than Schaller. Other companies that make sealed tuners are Ping, Spertzel and Planet Waves.
A sub genre in both open back and sealed tuners are “locking” machines. These are machines with an internal post that pins the string to the main post through the threading hole. These are either adjusted by a knob on the bottom of the machine or on the top of the post itself. While these work very well on guitars with tremelos, I don’t think they’re necessary on most guitars. Most designs add excess weight and if the string is put on correctly with an ordinary turner you won’t get much slippage anyway.
While I have preferences in what I like to put on my guitars, with the exception of economy machines you can’t go wrong in your choice. All will preform well and get the job done.
Now after that long winded intro, on to installation!
To start with, most of the open back use a 1/4″ post with a bushing that is between .328 and .393 inches in diameter. While you can just drill the hole at the bushing diameter, it leaves the post only supported at the bushing. The correct way would be to drill an initial hole at 1/4″ then use a bushing reamer or pilot bit to enlarge the hole or the bushing. This reamer is a cutter of proper diameter with a solid 1/4″ “pilot” to guide the cutter. These are sometimes called step drills. Of course, if you are replacing tuners you will already have the holes so you won’t have to do anything but swap parts.
If you’re going from the open back to sealed machines you have two choices. Most sealed machines have a base that is .390″ or 10mm. DO NOT under any circumstances try to drill the hole with a standard or brad point drill bit. You have no way of ensuring that the hole drilled will follow the proper course. If you have no other choice, you can plug the hole with the proper sized dowel and drill with a brad point bit but his can still be tricky as you will be drilling into the end grain of the dowel and the bit can still wander off center. I’ve seen more than one head stock cracked because someone tried to use a standard drill bit and the torque from the drill got the better of them.
The easiest is to use a reamer like that used for the bushing on a open back. Most of the threaded bushings on sealed tuners are .310″ so it’s the opposite of open back as the top of the peghead should be smaller than the back. Pilot bits are also made for this but sometimes it can be very difficult. I also feel that the tuner is much better supported as the bushing is threaded into the base so this is not as critical as the step in an open back tuner.
Going from a sealed tuner to a open back is a little easier as you can buy bushings that will fit the .390″ hole. These are press fit and will look pretty close to original open back bushings. The shaft will only be supported at the bushing but in most cases that will not be a problem.
If you’re using open back tuners you should fit the bushing now. They should be tight but be careful not to make the fit too tight, so as to not crack the peg head. If you’re using the pilot bit the fit should be perfect. I pad the back of the peg head so it won’t be marred and use a clamp to press the bushing in. DO NOT use a hammer as the trauma of brute force can also crack the peg head, plus a misdirected blow can occur. Neither of these is good for business! If your machines are sealed use the appropriate wrench to tighten the threaded bushing. Again go easy and don’t over tighten. Some builders will countersink the washer so that when torque is applied to the bushing it doesn’t pucker the lacquer.
The next common problem is that hole position of the screws that hold the tuner in place are often in different positions than the replacement. You can try to fill the holes but I find that this often looks worse than just leaving them. If you’re worried about moisture getting in the hole just drop a bit of lacquer in to seal it. Just be careful not to get any on the surrounding peg head. The problem comes when the screws are put in without first pre-drilling. At the least you can break the screw and worst, crack the peg head. I’m repairing a guitar now with this issue. You want to pick the bit size that is equal to the barrel of the screw (the solid part not including the threads). Temporarily put the machines on the peg head and line them up where you want them. Get them even on the peg head. If they are off line your eye will pick that up and it will look like a very unprofessional job. Take a piece of tape and after measuring the depth of the hole needed, put the tape around the drill bit. Don’t ruin your day by drilling all the way through to the face of the peg head! The next important step is to lubricate the screw with wax or a small amount of soap. Don’t use auto wax as this can contain silicone which will be hard to remove if you want to refinish someday. Make sure you use the proper sized screwdriver and carefully install the screw. These screws are very soft and easily stripped, so go easy.
Some machines have a small post on the underside of the surface (Spertzel). This post is used to stop the tuner from turning. To install these, put the machine in the proper position and lightly tighten the threaded bushing just enough to imprint the peg head, then drill the hole to the proper dimension.
I hope this helps dispel any misconceptions and happy lutherie!