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Woodscrews do 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 corrosion 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 will
    just not do in a proper restoration. A non-plated straight-slot
    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.

This approach 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 good.......



 


Toothpicks: A 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 size.

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.

 

 

 

 

Generally, the 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.

 

 

 

Another problem 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.

Matching the 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.

 

 

Cutters are 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.

A brad-point drill can be used with a "stop" installed.

 

 

 

 

Alternatively, if you have a drill press, a "Forstner" bit can be used to produce a very precision hole.

 

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 the backboard.

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 invisible.

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 some tips.

 

 

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.

The improved quality and durability of the repair makes it well worth the time it takes. Give it a try and I think you will agree.

Regards,

Tom Temple