There is no need for replacing knuckles, nor does one have to renew
shafts, if they are in good condition. Removing hammers from the shaft
is much easier than removing shafts from the knuckles.
It is quite well possible to rebuild a mechanism, just replacing the
hammers, dampers, and felt. In fact, a well-trained technician is able
to renew anything in a mechanism, without exchanging any of the wooden
parts. However, this requires skilled hands, thus a lot of training.
If you are not skilled, you better should not even try to renew the
hammers. In the end you will have to pay a technician a lot in order
to have corrected the faults you made.
The advice is: go find a technician who is also capable to tune and
intonate (voice) your instrument. Let him check the instrument, and
follow his advice. If you think he is too expensive, go for a second
By the way, if you really want to stay with the original as close as
possible, you could just have the felt renewed on the original hammers,
instead of replacing the hammer totally. Costly, but sometimes the
best choice. Keep in mind that very often the life of hammers can be
prolonged by sanding them.
One can indeed buy hammers already glued to a shaft. This is because
new mechanisms are supplied with knuckles without shafts. This is done
because the piano building factories often prefer to glue shafts with
pre-glued hammers into the knuckles.
Another reason is that this allows factories like Renner to produce
and sell the same type of piano action mechanism to several piano
builders, which is cost-effective. Since it is possible to use several
lengths of shafts, the same mechanism can be used in several types of
pianos. (So a compactly built, small size piano might have the same
type of mechanism as a taller, more classic, model).
Pre-glued hammers are not advisable for restoration purposes. If you
remove the hammers from the shafts, you will have to work on the shafts
in order to prepare them for the new hammers. If you are able to
remove the shafts from the knuckles (which is not so easy), you will
have much more work to do to prepare the knuckles for the new shafts.
A short explanation might help. In order to remove old hammers you use
a pan filled with wet sand. You heat this up and put the hammers
upside down into the hot wet sand. The glue will then soften and you
can remove the hammers.
After that you will have to remove by hand the remains of the glue, and
before you will be able to place new hammers you will have to use a
rolling device, applied on the ends of the shafts, in order to
calibrate and prepare them for the new hammers.
This hot sand method cannot be used on knuckles since you do not want
to decompose the knuckle into its components. So you have to remove
the shafts mechanically, which is a lot of work, and requires special
clamping and drilling equipment. In fact, if a shaft is broken, most
of the time the broken parts just are glued together.
Hope to be of help.