Key Rebushing 101

By Hannah Beckett

As a pianist, one of my pet peeves is playing a piano with wobbly keys. Compressed key bushings make keys distinctly hard to control whether the player realizes it or not. Key rebushing is a job that can be done without a shop and is a wonderful skill to have as a technician. If you have never done it before, practice on a few junk keys before offering it to a client. It's a straight-forward job, but there are a few steps that should be refined before you take on a full set for a client.

Required tools:

Iron with a steam function, washcloth, yellow wood glue, small needle-nose pliers, glue applicators, rebushing cloth, bushing cauls, Xacto knife

A few notes on the tools:

Cauls

There are different types of cauls, brass, plastic, or aluminum. My personal preference is brass because I like the weight and they have given me consistent results. Unfortunately, there is no standardization of front or balance rail pin sizes, so be sure to measure the pins and order the correct cauls before you start the job. A full set of cauls can be expensive, so factor that in to your price as you build up your collection.

Spurlock cauls (plastic) are available at Erwin's Piano Forte Supply, brass cauls are available from PianoTek.

Use digital calipers to measure the pins. There's a bit of technique involved in getting an accurate measurement. Hold the fixed end of the calipers level against the pin, and close the opposing end on to the pin. I always recommend doing this three times for accuracy.

Once you have measured the pins, select a caul that corresponds closest in size to the pin size. Bushing is about correcting the relationship between the key mortise, and the balance/front rail pins. The caul serves as a portable pin - a size that does not change. Later in the article we will talk about selecting the correct cloth that will allow the pin-to-mortise relationship to be close, but friction-free.

Knives

It is really important to use a knife that cuts cleanly and does not pull, tear, or rip the bushing cloth. After experimenting with many types of knives, my recommendation by far is to use an Xacto knife with a chisel blade. The chisel blades allow you to cut straight down on the cloth instead of pulling or slicing, which can cause damage to the cloth, the mortise, and your hand. It's also super efficient and will make the job go by faster. Be liberal in your use of blades - start with a fresh knife and don't hesitate to switch it out as soon as it starts to become dull. It's worth the investment.

Bushing Cloth

Renner, Pianotek, and Schaff all sell high-quality bushing cloth. Order at least three sizes, including thin, medium, and thick so you can have some options.

Glue Applicators

Glue control is an important part of attaining good results from your bushing job. "Wax carvers" are great for applying glue in a controlled way, and - if you clean them off before the glue dries - they're reusable.

Step 1: Steam out the old bushings.

Depending on the condition of the previous cloth, this could be quite simple, or could eat up lots of time. If you're working on a Yamaha or Kawai, it should be easy. If you are following a previous rebushing job or working on a low-quality piano, you may want to use a solution of wallpaper remover and water.

Start with one key. Apply your damp washcloth (not too wet, but damp enough that there can be a good amount of steam going into the bushing hole) to the bushing, and lay a hot iron on top for about fifteen seconds. The cloth should now peel out easily with the use of the needle nose pliers. If it does not come out easily, don't try to force the cloth. Wrestling the cloth out can cause damage to the wood, or the bushing could tear leaving hard-to-remove pieces of felt in the key. Be more liberal with your steaming so that the cloth pulls out easily and does not tear.

The piano shown here is a relatively new Kawai grand, and I'm likely the first to rebush the keys. This is a best-case scenario rebushing job. If you're following a messy technician, you may want to buy Spurlock's wood sizing cauls. After you've steamed out the bushings, apply the caul to the bushing hole to resize the wood while it dries. This will create a neat, consistent job.

Once you have a feel for the amount of required steaming time, you can start steaming sets at a time. If this is a best-case scenario job, let the wood dry out before applying the felt. For example, you could steam the bushings out in the morning and then start phase two in the afternoon. If the mortises are in bad shape, use the wood sizing cauls and let them dry overnight before beginning phase two.

Step 2: Select the bushing cloth.

Now that your keys are ready for new cloth, you'll need to select the cloth size most appropriate for the keys. Bushing cloth should only be 4.5-5mm deep in the key. Cloth too deep in the mortise will result in excessive friction, which could cause the key to stick at the bottom of the stroke or yield a slow return. Too little length and you'll have a wobbly key. The width of the key mortise is around 4.9mm (3/16").

To get the proper depth, use the sides of the mortise to measure. Start with a thin size of cloth, and line one end of cloth across the mortise to the opposing side and hold with your thumb. Do the same for the opposite end, resulting in an overlap of cloth across the mortise.

Push the caul through the cloth until it is snug in the key. By slowly pulling the caul slightly in and out of the hole, you should feel some friction, but not excess resistance. There should be a bit of a "grab" just as the caul is released from the mortise. If the caul slides smoothly in and out with no "grab," try a larger size cloth. If the caul is difficult to pull out of the key, try a smaller size of cloth. When in doubt and if time allows, do a test key bushing on key 1 or 88 and test the fitment of the key on the frame.

Step 3: Glue the cloth.

Once you have selected the cloth, apply a small amount of glue to the sides of the mortise using the glue applicators. Only apply glue to the desired depth of the bushing. Do not apply so much glue that it pools slightly on the sides of the mortise. Excess glue tends to accumulate at the edge of the bushing cloth and creates a lip that can be felt as friction once the key is on the keyframe. Apply a bit of glue to the edges of the mortise as well for the "wings" of the bushing and insert the caul.

Line the cloth up with the sides of the key and hold tightly while you insert the caul. Be careful that the caul does not drag the cloth deeper into the mortise than desired.

After your caul is inserted, cut the cloth using the Xacto knife:

Leave to dry overnight and reinstall the keys the next day. Be prepared to do some key easing when you reinstall, and be sure to check for any excess glue that may have seeped out from the bushing. Your results should yield a close-fitting, frictionless key travel that is indistinguishable from the original manufacturer's bushings.

A few notes: