There are many things to consider when purchasing a piano. The action is certainly important. The complexity of all of the parts working together in precision is awe inspiring. Interestingly, the action is the one part of a piano that can be restored to like new condition to the vast majority of pianos. Of course the level of performance is relative to the quality of the original design and manufacture. Many companies produce first rate actions.
It is rare to replace an action. When you hear of people utilizing Renner action in a rebuilt piano, they are not putting in a new action, they are replacing all the worn parts. This accomplishes exactly the same results, but rarely do keys, key frame and the basic structure deteriorate at all. (However, it could be possible with water or fire damage.)
The action is comprised of different elements that wear at different rates.
Bushings: The keys pivot on rails in the front and back of each key. Around the rods are felt bushings. In time the felt becomes compacted and there is play in the keys. The front rails can be turned to achieve tighter tolerance since the rails are not round. Eventually the key bushings need to be replaced when there is too much play.
Hammers become worn from hitting the strings. The felt becomes compacted at the point of impact with the strings. They also become somewhat flattened out in time. They can be reshaped with filing and voiced by needling to get uniform tone. Eventually after many filings there is no more felt to work with and the hammers must be replaced.
Hammer shanks and knuckles: Sometimes the joint that the hammer pivots on (the knuckle) becomes worn to the point that the hammer doesn’t hit the strings in exactly the same point each blow. That is when it becomes necessary to replace the knuckles and shanks.
Dampers: Above the strings are the dampers. When the felt becomes hard, noises and inconsistencies in the termination of notes results. Often these problems can be regulated, but there comes a time when optimum results aren’t possible without replacing the felt.
Back check: There is a leather piece that engages the hammer after the key is pressed. If the felt becomes hard, it can be roughed up to avoid excessive noise. If the leather is too hard it must be replaced too avoid action noise.
Felt action rail and key punchings can harden and replacing is desirable.
Whippen: Now here is where it gets interesting! There are around 100 parts to the action and all the rest of them come pre-assembled in a piece called the whippen. So when rebuilding an action to new condition, there is nothing more to do than what I have listed.
First rate parts are available from a variety of companies. The important thing is using the right parts that match the original specifications. When this is done, the performance of the action is indistinguishable from a new piano. When you hear of a “New Renner Action”, they don’t actually put in a whole new action, they are using Renner parts to rebuild the action.
Are the newer composite materials utilized in many Asian pianos superior to the traditional wood parts? Here is one clue to the answer to that question. Both Yamaha and Kawai use synthetic materials in their actions. However, their best pianos, Yamaha S series and Shigeru Kawai have all wood actions. In fact, other than Mason & Hamlin which is utilizing the questionable practice of sourcing pre-assembled composite actions and plates from China, no hand made piano uses synthetic materials in their actions!
The fact is, their are aspects to a piano that are fundamental to the quality of the instrument. Even the best action will wear out and need attention over time. But the sound board, scale design, rim materials, bridge composition, string tension, and plate manufacturing process are intrinsic to the quality of the instrument. These things cannot be improved. So, when looking for a piano as a long term investment, this is what is most important. That is why a hand built piano cost more and are more valuable over time. It is also why it is not advisable to rebuild Asian production pianos. There is limited potential for what results are possible. Over time the basic structure of these instruments deteriorates because of the design and materials utilized in manufacturing allowing for the right price for mass marketing.