Polyethylene Key Bushing Cauls
One of the easiest improvements you can make
to a piano is to replace worn key bushings. The
secret to obtaining neat, accurate results is the
use of precision bushing cauls that correspond to
the key pin size of a given piano. When left in
place until the glue hardens, these cauls act as a
form, molding the bushings to the correct size
and greatly reducing the need for key easing.
Our high density polyethylene key bushing cauls
are the most economical on the market. We
machine them to ±.001" tolerances for a wide
range of key pin sizes. This material is extremely
durable, impervious to glue, and dimensionally
stable. All cauls are color coded for size identification.
Caul Thickness: Our stock Final and Double Shouldered cauls are made .002" larger than the
key pin, ±.001". With proper cloth thickness and gluing, and after leaving the cauls in place 6-8
hours until the glue hardens, these slightly oversized cauls result in keys that need little or no
easing. Non-oversized cauls are available on a custom-order basis.
Caul Width: We cut all of our stock caul sizes 11/32" wide, rather than 3/8". This narrower
width allows these cauls to be used in a much greater range of keys, since many cauls have
mortises less than 3/8" wide (front to back dimension). Custom widths are available by special
Single Shoulder vs. Double Shoulder Cauls
Our standard stock sizes include the Single Shoulder types specifically made for the two-caul
method of bushing one side of the mortise at a time. In this method, an Intermediate Caul
holds the first piece of cloth in place, then a Final Caul is used to clamp the second piece. This
method is extremely fast and accurate due to economy of motion and tool usage, as explained
in our Tech Tips section and in our publication Efficient Key Rebushing. The single shoulder
design also allows for making flush cut balance rail bushings, using the flat side of the caul, or
shouldered bushings, on front rails or some balance rails, using the shoulder side of the caul.
We also manufacture the conventional Double Shoulder Cauls (usually used for one-caul
methods where both sides of the mortise are bushed at once).
Double Shoulder cauls
Single Shoulder cauls
Single Shoulder cauls
How Many Cauls Do You Need?
Since the desired result of key rebushing is properly sized bushings, efficient work requires enough
cauls to clamp all bushings until the glue is fully hardened. The bushings will then be properly sized,
with only a minimum of easing required to achieve the final fit. If using a one-caul method, a
minimum of 88 cauls will allow bushing one key rail at a time.
For the two caul method, you need at least 45 Intermediates and 90 Finals. You can insert the 45
Intermediates, then pull the first few out and reinsert ahead as you work down the rail. If you do a
lot of rebushing it is worthwhile having 90 Intermediates since this will speed up your work. Since the
Finals need to stay in place until the glue hardens, you should have 90 of each pin size you are
working with. Most pianos have .146" pins on both rails, so it is most convenient to have 180 of size
#2 so both rails can be bushed at one sitting.
Packaging & Quantities
All bushing caul sizes are sold in boxes of 45 or 90.
Mortise Sizing Cauls
These special cauls further improve the consistency of your
bushing jobs by uniformly sizing the key mortises after removal of
the old bushings. Just steam or soak out the bushings, insert a
Sizing Caul into each mortise while the wood is still damp, and
allow to dry. The mortises will then be very evenly sized with
smooth, flat sides as shown below. Bushing stability will also be
improved because the moisture-swollen wood will be compacted
back to its original dimension, not left in a soft, expanded state.
The top two keys at left had Mortise Sizing Cauls
inserted after bushing removal.
The lower two keys were left to dry empty.
We manufacture these special cauls in three sizes:
For keys with .125" to .133" pins- use Mortise Sizing Cauls MS-125, sold in boxes of 90
For keys with .146" to .152" pins- use Mortise Sizing Cauls MS-146, sold in boxes of 90 or 180
For keys with .162" pins- use Mortise Sizing Cauls MS-162, sold in boxes of 90
Note: For .087" key pins you can use our regular key bushing cauls size #4 or DS-162 as mortise
Note: .137" pins are usually used in balance rails with hard beech key buttons. These mortises
do not tend to swell or distort during bushing removal, so mortise sizing offers no benefit for
this pin size.
Tip: Since many pianos have .146" pins on both rails, you will probably want to have 180 of the
MS-146. But .125", .131", .133" & .162" pins are found on only one rail, so you will only need 90
of the MS-125 and MS-162.
Go to FAQ topic
NOTE: There are many
exceptions to the pin
sizes shown, so always
measure key pins on
each job before
Key bushing cauls now sold exclusively by Erwins Piano Restoration, Inc.