Pianos have soft parts that wear out over time. These are usually felt: hammers, key bushings, and many other areas where the natural lanolin in wool felt reduces friction and noise between dissimilar materials.
On this grand piano, with the keys pushed down, it was easy to wiggle them side-to-side. This is how you test for wear. Often it’s visible just by looking at the keys, and noticing the irregular spacing between adjacent white keys.
This set of photos documents the removal of the old bushings, and the replacement with new felt. A precise fit within 0.005″ is needed, so that there’s almost no side play (with the key pushed down), but free movement up and down. It takes careful selection of the right thickness of new felt (from half a dozen sizes), and fastidious installation with the correct glue, so when replacement is needed again, no wood is removed with the old felt.
A nice, tight fit gives even control over the keys, and prevents energy loss due to side play. It makes the keys feel new again.
The photos are here: