This grey spring has weakened with time. The one on the left is the new one.
With the spring removed, glue is applied to fit the new spring in tightly
Each key has it’s own spring below the jack, this enables the jack to push back under the hammer but after escapement.
New spring being placed in a small slot on the flange
New spring has much better tension, and is able to allow the pianist to strike play the key in rapid succession.
Screwing the flange back onto the action
While doing so, the jack must be underneath the hammer butt, in its correct position to avoid any breakages.