Key Rebushing 101

Updated: Apr 30, 2020

By Hannah Beckett, RPT, Reston Virginia

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, dental tool, rebushing cloth, size 147 cauls, xacto knife}

A few notes on the tools:

- Dental tool? Glad you asked. Read about them on the Tips & Tricks page before continuing.

- 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. You may find a few sizes available, but 147 is the standard size for pianos from the 50s on. Pianos with different sizes of pins are old enough that you shouldn't be working on them for money. To be on the safe side, measure the pin using digital calipers. The measurement should read 146mm.

- The best xacto knife tip is the small square one in your Mulwark 16-PC Precision Kit.

- Renner, Pianotek, and Schaff all sell high-quality rebushing cloth. Order at least three sizes, small, medium, and large so you can have some options.

The first step is to 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 rebuilder or working on a low-quality piano, it could get complicated.

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 ten seconds. The cloth should now peel out easily with the use of the needle nose pliers. If it does not come out easily, stop, and reapply the iron. 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.

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 briefly before applying the felt. For example, you could steam the bushings out in the morning and then start phase two in the afternoon. If this is a complicated job, use the wood sizing cauls and let them dry overnight before beginning phase two.

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").

Start with a small 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 cloth until it is snug in the key. By pulling the caul slightly in and out of the hole, you should feel some resistance, but not excess resistance. If the caul slides very easily in and out, try a larger size of felt. If the caul is difficult to pull out of the key, try a smaller size of cloth. When in doubt, do a test key bushing on key 1 or 88 and test the fitment of the key on the frame. You can also gauge the fitment initially if you can lift a dry fitted bushing by the bushing caul and shake the key three times before the key drops. Obviously, don't drop the key on the floor.

Once you have selected the cloth, apply a small amount of glue to the sides of the mortise using your dental tool. Only apply glue to the desired depth of the bushing, and do not use excessive amounts of glue. It should not drip into the bottom of the key. Apply a bit of glue to the edges of the mortise as well for the "wings" of the bushing. Insert the caul, and cut the felt from around the caul with the xacto knife. Leave to dry overnight and reinstall the keys the next day. Be prepared to do some key easing when you reinstall. Your results should yield a close-fitting, frictionless key travel that is indistinguishable from the original manufacturer's bushings.

A few notes:

- This is the same process for rebushing balance pin holes except that you should note if the original bushings are flush cut or winged. Match the original manufacturer's bushings when possible.

- The clamp I am using in the video is an amazing and affordable third hand, but is not necessary for the task.

- Learn how to make your own depth gauge to check the bushing length here.

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