Steinway rails need to be replaced when they become visibly cracked (as samples shown to the right) and when the piano is no longer delivering the dynamic range you would expect from a Steinway Piano.
Why? I hear you ask.
I will explain.
Every time the hammer head hits the string the recoil force makes the rail the hammer is attached to, twist. This over many years of playing causes metal fatigue which is basically very minute cracks that weaken the rail's structure, some of these cracks will develop into large visible fissures, as you can see (upper right), but before this happens the dynamic range of the piano will have diminished. The reason being, as the rail becomes weaker more of the energy that is applied to the key during playing, is absorbed by the weak rail and is not available for the hammer when it hits the string.
To bring back the dynamic range of the piano, new rails must be fitted.
The new rails are made of heaver gauge brass and are therefore more able to withstand the rigours of heavy and prolonged playing. See cross section diagram (lower right).
The rails can be replaced in two ways.
The first is to replace the whole action frame with a new one from Steinway & Sons. This will of course match the scale of pianos built today, but not necessarily match the scale of the piano the action has come from.
The second is to replace the rails in the original action frame. Having the rails drilled to the exact scale of the original ensures that the rails will match the string spacing of the piano when it was built, and therefore avoiding any problems when fitting the action back into the piano.
The spacing of the rails has also varied over the years to accommodate action design changes. When the rails are replaced the spacing can be adjusted to match new parts if fitted, or left as original if the the old parts are to be refurbished.
I hope this has been of some help, if you wish to discuss this further or for more information, please contact Phil who will be only to pleased to help.