Tune-o-Matic bridges are actually pretty easy to adjust, since the entire bridge sits on two thumbwheels at either side. The radius should be adjusted at the factory, but a good setup tech will check this to make sure that it is correct.

Checking the fretboard radius

You can check the fretboard radius using an undersaddle radius gauge. Place the gauge under the strings to make sure that the saddles are equal in radius to the fretboard. In the event that they don’t match (sometimes overseas guitars and even some USA Gibson production will be a little off), you can use gauged nut files to deepen the string slots on the saddles. 

If the radius on the bridge is too flat, you can file the string slots on the low and high E strings to lower these strings down to the gauge. Leave the center strings as they are, then adjust the A and B strings. 

If the radius is too round, you can lower the center strings so they match the gauge. Just be careful not to bury the strings too deep in the saddles. A string slot that is too deep can choke the string and impede clarity.

Also, you should not file straight across, but rather tilt the file back to the stoptail so the string jumps off of the front edge of the saddle. Then round over this angle so the saddle also does not cut into the string. If you notice that the saddles are beyond saving, it isn’t a bad idea to recommend replacing the saddles to the customer. This would obviously be a repair and therefore an additional cost beyond the setup.

Most of the time, you will find that the saddles are matching the fretboard radius and only need to be adjusted for action height. To do this, you will tune the guitar, and then suppress the first fret and read the string action height at the 12th fret. You can then either raise or lower the thumbwheels until you reach the desired string action height. You should start at 5/64” on the low E and 4/64” on the high E. The rest of the center strings should evenly flow with this radius if it matches the radius gauge.

Attaching the stoptail

After the string action is set, you then adjust the stoptail. You want the strings to have a clear shot to the bridge saddles after coming out of the stoptail holes. They also shouldn’t be hitting the back edge of the bridge on the way. This is referred to as “break angle” of the string and it is a function of the overall neck angle of the guitar. You might find that the strings do in fact clear the back edge of the bridge even with the stoptail posts slammed all the way down. If this is the case, then you are good to go. 

If the strings do not clear the back edge of the bridge, then you will want to raise the stoptail posts up until they do. Just make sure that you adjust the posts equally so the stoptail sits flush with the top.

Fixing an overset neck

An overset neck is a set-neck guitar with too much neck angle. In the event that the neck is overset, you might have to raise up the posts excessively to make the strings clear the bridge. Consider installing the strings through the front of the stoptail in order to wrap them over the top so the break angle will be higher.