Fixing a broken tuning peg on a guitar requires replacing it with a new unit. Guitars are fitted with two types of tuning pegs: in-line pegs, of three to six connected pegs, or single pegs. The tuning peg as a unit is called a "machine head," or "tuner," and replacements may be purchased at most music stores. Since there are many styles and sizes of tuners, it is important to bring the unit to the store for proper match and fit. Depending on your guitar model, you may not be able to find a perfect match, but you will likely find a replacement that will work just as well. If you are concerned with aesthetics, you may replace all the tuners with a matched set.
Loosen the string from the tuner by turning the peg and remove the string from the string post. String-peg winders are much faster than loosening the string by hand and are inexpensive. You may leave the string attached to the bridge for reinstallation and coil the string to keep it out of the way.
Determine the type of tuner (single or in-line) you have by viewing the back of the guitar headstock. In-line tuners will be connected and mounted with a single metal plate, while individual tuners will not have a plate connecting the tuners.
Determine the top bushing type of the tuner by looking at the string post of the tuner sticking out of the top of the headstock. If a hexagonal nut bushing is visible at the base of the string post (used primarily for some individual tuners), it is threaded. Remove it by turning it counter-clockwise with an adjustable wrench. If the bushing is round and smooth (no hex pattern), it is a friction-mount bushing and will be removed later.
Turn the guitar over to view the back of the headstock and remove the screws anchoring the tuner to the guitar with a screwdriver. The tuner will now pull out of the guitar easily, and the friction bushings can be removed by inserting a screwdriver into the hole to push the bushing out. You may take now take the tuner and bushing to the store to find a suitable replacement.
Place the new tuner into the hole to check for fit. If the hole in the headstock is too small for the tuner, it must be widened by a qualified repair person at the music store, or a different tuner must be procured. It is normal for the hole to appear too small for the string post, as the bushing will take up the slack.
Install the screws holding the tuner to the underside of the headstock with a screwdriver.
Replace the bushings by placing them over the string shaft sticking through the front of the headstock. Friction bushings are installed by pressing them into place with your fingers. Hex bushings are installed by inserting them over the string post and threading them into the headstock by turning clockwise. Follow by tightening the bushing with an adjustable wrench.
Reinstall the string on each tuner replaced, and your tuner repair is complete.
Whenever possible, take the tuners and guitar into the store for the best tuner fit. If replacing all the tuners, this is a good time to consider an upgrade. Better tuners will help your guitar stay in tune better, make tuning easier and increase its value. If the existing tuner mounting screw holes under the headstock are not a perfect match, fill the old holes by inserting a toothpick dipped in wood glue and breaking or cutting it off flush with the surface. Make a pilot hole for the new tuner screws by holding the tuner in place, and use an awl to make the new screw hole. You may touch-up visible toothpick repairs with a dab of nail polish or paint to match the headstock colour.
Never force a friction bushing from its place if it will not remove easily, as damage to the finish or wood of the headstock can occur. Some bushings may be kept in place if the new tuner is an exact fit. If not, entrust the removal to a qualified repair shop. Don't ream or drill tuner holes in the headstock to widen them to fit a new tuner. This procedure requires a drill press and precise drilling angle. Damage to the headstock wood and finish can occur, and over-drilling of the hole can result. Have a professional perform the work for you.