an excellent job in holding clock movements to the case: simple,
inexpensive. Unfortunately, the screws are significantly harder than
the wood normally used for backboards. As a result, the wood (not the
screw) wears a little each time the screw is removed.
Over the short term, this
usually doesn't pose a problem, but a clock that is 150~200 years old
may have had the movement (and screws) removed a number of times
creating cumulative wear on the wood.
Another factor adding to
wear is that the screws originally used were mild steel with no
plating as is used today. The original screws quickly oxidized and the
tiny rust pits act like sandpaper when
the screws are removed. The result is more wear to the wood.
Eventually, the screw holes in a clock's backboard will accumulate
enough wear that the original screws no longer hold the movement
securely. A number of remedies have been used for this problem.
Some repair methods are effective while others are less so and may
even devalue the clock.
Install Larger Screws:
While it may be effective to install larger diameter
screws in the worn holes, this solution poses several challenges:
1. It will likely
take a significantly larger diameter screw to
effectively grip the worn hole. A screw large enough to work
may be too large to fit through the holes in the mounting tabs.
This would require drilling (modifying) the movement.
2. Locating the
correct type of screw in a large enough diameter
may be difficult. A bright cad-plated Phillips head screw
just not do in a proper restoration. A non-plated
screw was original equipment and should be used.
3. Using a larger
screw does not correct the problem. It only
works around it. Eventually the larger screw will also wear
the hole. Then what?
Drill New Holes:
It is not uncommon to find that some previous repairer has moved the
movement mounting tabs and drilled fresh holes.
While effective, this repair is generally discouraged as it is a
permanent modification to the backboard and is often a sign of a
"marriage" between movement and case.
Moving the mounting tabs and drilling new holes usually has a
measurable devaluation effect on the clock.
Repair the Worn Holes: In the long term, the best
solution to correction of worn screw holes in wooded surfaces is to
find a method to restore the worn wood.
allows the original screws to be retained and the backboard does not
end up with a number of unused holes in it.
Having said that,
it must be noted that "repair"
can take on a number of different forms. Some good, some not so
common method of repairing a damaged screw
hole involves the use of
common household toothpicks.
The points of several toothpicks are inserted into the hole.
The original screw is then inserted into the hole and the tapered
toothpick pieces are wedged into the hold effectively reducing its
While not pretty, this repair is usually effective and avoids
unnecessary modification of the movement or backboard.
Unfortunately, the wood tooth picks or wood slivers tend to fall out and get lost the next
time the movement is removed. The wedges also tend to press on the
original wood and actually enlarge the hole further
Dowels: A step up from
toothpicks is the use of a wooden dowel to create an "insert" for the
worn hold and provide fresh wood for the screw.
smallest diameter dowel that will fully fill the worn hole is
selected. 1/4" diameter usually does the trick.
A drill bit of
the same diameter as the dowel is used to create a clean hole in the
backboard where the worn hold existed.
A section of
dowel of the correct length is cut and glued into the hole to provide
a fresh area of wood for drilling a new screw hold.
One of the main
drawbacks of this type of repair is that the dowel is made such
that its grain is perpendicular to the grain in the backboard. As a
result, the new screw is inserted into the end-grain of the dowel.
This is the weakest area of the dowel and
splitting is common.
crops up in trying to match the color of the dowel to that of the
backboard. The dowel not only has grain running differently from
the backboard, it is also a different type of wood.
dowel to the backboard becomes quite challenging.
Custom Plugs: An alternative to
the dowel approach that takes only slightly longer involves cutting
and installing custom wooden "plugs" to renew the worn hole.
Custom plugs address
all of the shortcomings of dowels such as grain direction, wood type
and stain matching.
Custom sized plugs
are cut using tools called (obviously enough) Plug-cutters. They are
available in a wide range of sizes from woodworking specialty stores
and are reasonably priced.
available to make both straight and tapered plugs. Tapered plugs work
exceeding well for hole repair.
The first step in
repairing the worn hole is to measure the thickness of the backboard.
Next a drill bit
is selected that is large enough to just clean up the old, worn hole.
Usually, a 1/4" bit is large enough for the job.
drill can be used with a "stop" installed.
you have a drill press, a "Forstner" bit can be used to produce a very
Regardless of the
drill bit type, it should be set so that the drill penetrates only
about 3/4 of the way through the backboard.
Use the depth rod on
a vernier caliper to determine the final, actual depth of the hole.
Next locate a scrap
of wood that matches the type, tone and pattern of the backboard as
close a possible.
Scraps from old
clock cases often work perfectly for this. If none are available, Pine
makes a good choice.
Trim a section of
the scrap to a width that is exactly the same as the dept of the hole.
Install the plug
cutter of the correct size in a drill press and cut several plugs.
Examine the plugs
and select one that is free of chips.
A little hide
glue is brushed into the hole and onto the sided of the plug.
Carefully place the
plug into the hole and align its grain with that of the backboard.
A dowel works
well for pushing the plug into the hole and flush with the surface of
Remember to wipe
away any excess glue once the plug is pressed into place. A damp cloth
works well for this.
If the wood for the
plug was carefully selected and installed, it becomes almost
If a piece of new
wood was used for the plug it may be necessary to "age" the plug to
better match the older backboard. See the how-to on Aging Wood for
If a 1/4" plug was
used, the tabs for most movements will completely cover it making a
totally undetectable repair.
The steps to create and
install custom plugs to repair a worn backboard may appear to be
extensive, but in fact the entire process usually takes less than 1/2
hour to complete for four holes.
quality and durability of the repair makes it well worth the time it
takes. Give it a try and I think you will agree.