Last updated: September 11, 2018
Complete instructions for performing a neck reset are out of the scope of this page, but I did want to include a few general considerations for those attempting to do this operation. If you want to get an idea of what the neck reset process looks like, take a look at Benjamin Strange's nice pictorial here.
A neck reset is a non-trivial repair even on instruments that have easily removable necks. And since material is removed in the process, a neck reset is not a readily reversible process. That being the case, it behooves the repair person to be sure that all other contributions to high action have been taken care of before the neck reset is attempted. If the neck is bowed it should first be straightened. If the bridge is coming unglued from the top that should be fixed first. It is always a good idea to make sure there aren't any cracked or missing top braces contributing to the high action and to fix these if they are present. Finally, since it is common for previous attempts at lowering the action to include lowering the bridge saddle, the saddle should be raised up to a reasonable level or replaced entirely prior to a neck reset.
The geometry involved in the neck reset can be seen in the following figure:
As can be seen, the reset involves cutting a wedge from the “cheeks” of the heel, which angles the neck back a bit, thus lowering the action. Before beginning the operation it is necessary to figure out how big a wedge to cut out to effect the desired action.
The formula for calculating the size of the wedge to cut out of the heel (in spreadsheet format) is:
D = ((Ai – Ar) * 2 * Lh) / Lb
Refer to the figure for more information on the parameters. Note that this formula takes as parameters string action measured at the 12th fret, as this is the conventional way of measuring action. All measurements should be taken while the instrument is tuned up, and the resulting depth of the cut for the heel should be carefully scribed on the heel as small differences here can result in rather large differences in action.
Want to know what typical action values are for guitars and bass guitars? Tables of that information can be found here.
Folks that can follow the geometry will notice that the formula is quite simplified, considering some arcs as simply straight lines. Please note that, given the small displacements involved here, this simplified formula will yield results well within the tolerances of the most precise woodworking.