Hammer flanges, like all other wooden parts, shrink away from the
screw heads and allow the hammer to drift to one side or rattle. While
the action is in the piano, strike the keys to see if there are any
that strike improperly. Mark the keys so as to indicate just what the
trouble is, so that you will know how to remedy it when the action is
out. If the hammers are set so close that they rub against each other,
you may have to cut off a slight shaving of felt, but this is rarely
necessary; for if properly placed, there is generally room for all;
yet sometimes the expansion of the felt or warping of the shank makes
cutting necessary.

7. _Dampers_.--The dampers in the square action depend entirely upon
their weight for their efficiency in damping the strings and returning
after being raised by the key. Often, after the key is struck, the
damper will not return to its place and the string is undamped. This
is generally found to be caused by the wire sticking in the hole
through which it passes, the wire being rusty or bent or some foreign
substance being in the hole round the wire. The bushing cloth in the
hole may be in such condition as to retard the free passage of the
damper wire, in which case the wire may be heated with a match and run
up and down a few times through the hole, which will free it. The
damper may not fall readily on account of a sluggish joint in the
flange. Work it back and forth as far as it will go a few times; if
necessary, take it off the damper rail and look for the cause of the
trouble.

Damper flanges get loose on the damper rail and work to one side,
causing defective damping and rattling. See that they are all tight,
and in their places.