The photo above left shows a hammer that is crooked as a result of the hammershank twisting over time.  This can eventually become so bad that the hammer will rub an adjacent hammer, causing the note to perform poorly or not at all.  The photo above right shows the grooves that develop as a natural result of the piano being played.  The grooves on these hammers are not very bad, I've seen them more than 1/4" deep, but this still needs to be addressed as part of the reconditioning of this piano.

In the four photos above, we see the crooked hammer and string grooves being corrected.  The top left photo shows application of heat to soften the glue holding the hammer to the hammershank.  The top right photo shows the hammer being "coaxed" back into position.  The bottom left photo shows the final part of the process of filing or shaping the hammers and the bottom right photo shows the hammer, now straight, with the string grooves removed.  After cleaning the action with compressed air, and checking for any other parts that need attention, the keys and action can be reinstalled in the piano and the adjustment process started.

It is very important to perform the various adjustments (called regulation) in proper order, since each builds on the previous step.  We start with measuring the distance from the tip of the hammer to the string - known as hammerblow (left photo).  This distance changes as the hammer rest rail felt blocks compress and when the hammers are filed to remove string grooves.  Correction, if needed, is accomplished by adding a thin amount of felt to the felt block supporting the hammer rest rail (right photo).

Lost motion is most easily defined as the distance the key travels before the hammer begins to move.  This distance increases as the capstan felt and hammerbutt felt wears slightly from use.  This is most easily checked by watching the backcheck (green felt in the photo above) approaching the catcher (tan leather immediately in front of the backcheck) and noting the amount of travel seen before the hammer assembly starts to move.  It should move just the tiniest amount before hammer movement is detected.  If it moves excessively, or if the hammer moves at the exact time the key is depressed, adjustment is necessary.  This is one of the most critical adjustments on the piano - too much and the hammer will "double strike" (strike the string twice or more when the key is only pressed once) or repetition (the speed at which a note can be played repeatedly) will be diminished.  Using a special tool which varies depending on the maker of the piano, the capstan is turned to adjust the lost motion.  This is usually done with the key in the piano (lower left photo) but for the purpose of showing what the capstan looks like we took one photo with the key out of position, showing the tool engaged on the capstan (lower right photo).

It is also important to check the distance at which a hammer is stopped or "checked" from the string.  Too close and there is a risk of the hammer "blocking" or not allowing the string to ring until the key is released (or the sound fades with time).  Too far, and the speed at which the note can be played (repetition) is reduced.  Usually, with use, the distance increases and the backchecks are bent slightly forward (left photo) to compensate for this type of wear.  This is the case in the top left photo, the corrected distance is shown in the top right.