Replace an original screw with a longer one only if it has the same
number of threads per inch.  Changing the pitch of the threads will
tear out the original threads in the wood like mincemeat.  The new
screw might hold once, but when it is removed again, the hole will
be stripped again.  Cross-sectional photographs were included in a
recent article in "DMM", the Journal of the GSM (Gesellschaft fuer
Selbstspielende Musikinstrumente, the German automatic music collectors'
society), clearly illustrating the damage caused by substitute screws
of different pitch.

Avoid replacing a screw with a fatter one, which will not only tear out
the original threads, but also might crack the wood.  We once restored
a large Pierre Eich orchestrion in which the screws for most of the
components had been replaced with larger diameter screws over the
years, from size 6 to 8, then 10, and then 12, resulting in most of the
wood being cracked so badly that we had to make a new pneumatic stack,
register boxes, pump and reservoirs.  (Editorial comment: We always
prefer to restore original parts over making new ones, but in too many
cases the old parts have been butchered beyond repair.)

In all cases, the grain of the wood should be 90 degrees to the length
of the screw.  In some cases this involves using a plug (with the grain
crosswise) and in others, a dowel (with the grain running lengthwise).

1. The best choice, where possible, is to drill the hole 90 degrees to
the screw hole and use a dowel.  You will leave enough of the original
screw hole to show exactly where to drill, and the screw won't be able
to pull the dowel out of the hole.  Examples: most screw holes near the
edge of a board, such as the perimeter screws holding a pouch board to
a valve chest, a valve board/pouch board sandwich, etc.

2. Where #1 isn't possible, as where a screw hole is in the middle of
a board, far away from an edge, drill in from the back side (the side
opposite from the screw hole), don't drill the hole all the way through,
and use a plug.  In this case you'll also be leaving a little of the
original screw hole to show where to drill, and the screw won't be able
to pull the plug out of the hole.  Examples: large pumps and reservoirs
that have large flat wooden plates screwed to each other everywhere,
not just around the edges.

In #1 and #2 above, if you can drill the holes with a drill press or
milling machine, use hot glue or liquid hide glue for gluing the plug
or dowel.  If you must use a hand drill, and the hole is not perfectly
round due to unsteadiness of the drill, leaving a gap between the plug
and surrounding wood, use epoxy.  It will fill the gap.  Go to the
extra expense of using professional epoxy such as Gougeon's "West
System," supplied by boat-building shops, not the inexpensive epoxy
from the local hardware store.

3. As a last resort, where your only choice is to drill straight down
into the screw hole, use a plug, orient the grain parallel to the
surrounding wood, and glue it in with good quality epoxy, which will
help to lock the porous end grain of the plug to the end grain of the
surrounding wood.

We have spent weeks plugging screw holes in several of the largest
known Hupfeld orchestrions (home to thousands of screws), where
previous restorers had glued cross-grain hardwood plugs down into
bored-out screw holes with yellow glue, and the screws had then pulled
them back out of the holes like hundreds of little corks.  Yellow or
white glue can allow the plug to creep; hot glue can crack and let the
plug come loose due to its brittleness.  In this case, epoxy will lock
the plug in -- not as well as a cross plug (#1) or a plug from the back
side (#2) but better than other types of glue.

In our restoration we spend so much time repairing stripped and
poorly-repaired screw holes that we pre-cut maple and beech plugs (for
American and German instruments, respectively) in various diameters and
lengths by the hundreds, and keep them "in stock" so we don't have to
cut 10 or 20 new ones every time we need them.

Art Reblitz