Pianoshop :: Info :: Figures :: Key Levelling
The keys have holes in the middle and at the front. The middle hole is passed over the balance rail pins: (see below)
The height of the keys is adjusted by adding or removing paper washers under the red felt balance rail washers. The keys are weighted at the back so that they at at their highest point, the gold things visible in the picture below are the clips that hold the 3¾ oz lead weights onto the backchecks. See picture below:
In the bottom left of the picture are visible green washers (0.1 mm) and yellow washers (0.3mm). Also used are white washers (0.075mm). When a key is placed back on the balance pin it is tapped down firmly with the butt of a screwdriver to take up any air gaps between washers.
Note the large gap between the third and fourth white keys from the left, the keys have not been "spaced" properly yet. When the keys are spaced, all the gaps between the white keys should be the same.
Sometimes a key is not exactly horizontal to the spirit level, "squaring" the keys is making sure that they are all horizontal. This is done by tapping the top of the balance pin with a large screwdriver to gently rock the key over in the desired direction.