I would like to add just a bit to this discussion since I have
performed the replacement of a Steinway XR system.  Most of what has
been said is correct.  What has been said about the scarcity of finding
the correct replacement parts is a bit misleading.  Given enough time
and patience, the exact replacement should be no problem.

Keep in mind that the stack must be specifically for a Steinway and
it must be from the correct era of production.  The good news is that
many XR systems were built.  Additionally, the Steinway OR stack is
identical to the XR stack, so two sizes are interchangeable.

Two factors not mentioned are the spool box mounting style and the
control levers.  Obtaining the correct levers may be a bigger problem
than getting the correct stack.  Making the correct levers from scratch
can be done, but it is a huge task.  The two general sizes of spool
boxes are not interchangeable in some cases.  This is due to
interference with the compartment lid.

To start the project, identify the style of stack that is missing.
The older style, generally found in pianos made before 1925, have the
larger stacks that play all 88 notes and the stack pneumatics are aft
of the poppet rod assembly.  The stack extends well aft of the front
legs.  1925 pianos and later usually have an 80-note stack with the
poppets on the aft side of the stack.  The stack mounts forward and
is almost entirely between the front legs.

Pre-1925 spool boxes usually have all mountings on the keyframe.
1925 and later have the spool box mounted to the piano, and not the
keyframe.  One cannot say the word "always" when speaking of the
Duo-Art.  Many examples have been found where the mountings do not
conform to the 1925 change over.

Usually, the 88-note stack has the cross valve seat system and the
80-note stack has smaller round valves seats.  Again, there are
exceptions.  Usually, the 88-note stack pianos have the Temponamic
control lever.

The best find would be an entire system that has been carefully kept
intact.  Many technicians have done this.  Paying more for an entire
system is money well spent.  Due to the complexity of the Duo-Art
system of control levers, its mounting brackets, unique shoulder
screws, and swivel connecting joints, getting the correct levers is
extremely important.

For example, if your piano is a straight 1927 piano, and someone offers
an entire mechanism from another 1927 Steinway piano, don't assume it
will fit.  If the removed mechanism came from an art-case piano such as
a Louis XV, it will likely be an exception having a large stack and the
Temponamic control knob.

One last exception: very early style pianos depart entirely from the
majority.  The expression box has the accordions pneumatics mounted
separately from the main box.  If your piano is from that era, start
looking for a different piano.  In likelihood, you will never find the
correct system for that piano.

Good luck.  It is a great project!

Bob Taylor
Missouri