A combination of bad design, soft key-wood, 40 years of heavy concert playing and rough handling by technicians has led to the elongation of the balance holes on this Steinway concert grand. What is needed is to provide a durable replacement for the original unsatisfactory design, and this is done by inserting plugs of hornbeam—the very hard wood used in making piano action parts—to replace the softwood round the balance hole.
These plugs are designed as wrest-plate bushings and have a pilot hole drilled centrally about half-way through the middle. A special tool is used to mill out a socket 3 millimetres deep centred on the original balance hole.
The central guide of milling tool is a press fit in the balance hole and aligns itself with the flanks of the hole. Since the elongation of the hole is towards the front, slight pressure from the back of the key will prevent any inclination of the tool to move to the crushed side while the milling ring makes contact and begins to cut the socket.
The outer stop ring is set to prevent the mill cutting deeper than 3 millimetres, which is the depth of the original balance hole.
The wrest-plate bushings in this case are very slightly larger in diameter than the socket cut by the tool. To reduce them to a good tight fit in the socket and also to provide a rough circumference as a key for the glue, they are spun in a fast drill against a sheet of abrasive paper.
The newly milled socket is blown clean and the wide drilling above made clean.
The plug is stabbed and picked up with a fine scalpel so that glue can be applied to the top surface and the circumference.
The plug is then pressed into the socket and hammered home and the key is then set aside for the glue to dry.
The ring of glue is removed and a fine saw is used to cut the plug almost flush with the underside of the key.
The hornbeam is trimmed flush with a very sharp paring chisel and the key lightly cleaned up with abrasive paper. The pilot hole is now visible and serves to centre the drill used to open up the balance hole to the required diameter.
The balance pins on this piano are 3.7 millimetres in diameter. A drill of 3.6 millimetres is fitted in a small hand drill and the drill run backwards in the hole until it finds a true centre. The drill is then run forward to complete the boring and the hole is compressed and burnished using an awl smeared with a little deer tallow. The key now slides down the pin with slight pressure and we check that the key balances freely and that there is no play.
