Mainspring Barrell Caps

Most barrel caps are friction fit into the barrel. Installing a barrel cap on a
mainspring barrel can be a frustrating experience if the cap does not fit snugly. Here
is one way to install barrel caps if they are loose. Many times you will notice
that the caps have been forced or gouged and distorted from mis-handling. More often
the barrel itself has been distorted where the cap fits on. This often happens when
the arbor is struck on the end opposite the barrel cap to remove it. This works, but
it will cause the cap to become slightly distorted so it won't fit tightly when reinstalled.
Most, but not all, barrells are put together this way, so it is necessary to use force to
get the cap off. If the person who removed it does not know about this situation, they
will probably try to tighten up the fit by modifying the edge of the barrel, causing
damage. There is usually a small groove with a shoulder on the inside of the barrel
that keeps the cap from sliding too far into the spring. If the barrel has been soldered;
expect the cap to be loose. If you see scratches and deep gouges around the edge; this
is another warning sign. If the cap has been removed and installed too many times, the
barrel and / or the cap will get warped and the cap will not fit tightly. The diameter
of the barrel cap must be uniformly between 2 and 3 thousandths of an inch larger than
the opening in the barrel, depending on the thickness of the walls of the barrel.

A barrel cap can be installed by clamping the barrel and the cap at one
edge snugly, but not destructively, in a vise with SMOOTH jaws, and then with a
hardwood mallet, the other edge can be installed with some sharp tapping.
Do not squeeze the barrel/cap combination tight enough to distort or destroy the
small edge on the barrel that holds the cap. I have also installed the caps by holding
one end of the barrel/cap in my hand and tapping on the cap with a wooden mallet
with the tooth side of the barrel sitting on a SOLID spot on the bench. This is faster
but a little bit more difficult. If the cap is loose and doesn't stay put DO NOT
START WITH TRYING TO MAKE THE BARRELL OPENING SMALLER. DO NOT TRY TO MODIFY THE BARRELL.
It is much easier to make a cap than a new barrel. Start with the cap. With a polished piece
of hard steel as an anvil; work the edges of the cap outward gradually by hitting the
cap evenly around the entire circumference of its outer edge with a polished hammer. This
is going to take some practice in order to get the edge even, but it can be done.
Stay as close to the edge as possible without actually hitting the edge. If the cap is
steel; this method will work; however it will take much longer to get the diameter
of the cap increased than if the cap is made of brass. If one is very careful the
diameter of the cap can be increased enough to make the cap stay put nicely. Be sure
to carefully check the barrel for cracks in the sides. As one can imagine; it will not
be possible to get the cap to fit tightly if the barrel is cracked. Again; do not try
to modify the barrel edge; the cap will hold if it fits snugly. Always check the barrels
before you take them apart. If there is evidence of butchery, plan on spending a lot of time
in trying to get the barrel caps to fit properly. Don't forget to check the cap to be sure
it is flat and not cupped before installing it on the barrel. Remember that when
you hit the metal of the cap in a spot where the metal cannot expand out it will distort
the cap. This is one of those times when knowing how to work with metal in the same
fashion as an artist will be very necessary. This is a classic example of why clock repair is
part technical, part artistic.

If the barrel is cracked soldering may be the only way to fix it. This is one of
those rare occasions where soldering may be acceptable. It is a judgement call. It depends
on how bad the break in the barrel cylinder is. Usually if it is a hairline crack you will
be the first to notice it, which indicates you are a good repair person. In this case If the
spring is a small to average size you will probably be successful with soldering. Do not just
put blobs of solder on the crack and call it good. The barrel must be cleaned COMPLETELY and
ALL oil residue removed before you start. Be sure to remove ALL the flux and clean and polish
the barrel after you are finished. If the procedure is done correctly, the solder will not be
visible and the barrel will hold just fine. Be careful when you do this, because many
barrells are soldered already where the cylinder attaches to the gear. If you get the
barrel too hot, the whole thing will fall apart on you. The best, and the most expensive,
way to fix this would be to have a new barrel made; however If I were doing this repair
for a customer, I would discuss it with them before proceeding. They may want to keep it as
original as possible. You must be able to know from experience whether or not the crack in
the barrel will hold by taking into account the strength of the spring, the design and size
of the barrel, and the extent of damage from the crack. This goes back, again, to the
clock repair essay and knowing the feel of things. After you have wound enough mainsprings
and worked enough metal you will know what decision to make about a cracked barrel cylinder
edge. If you are not sure, replace it. Only experience, or someone experienced who can
look at it, can provide an accurate answer.

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Copyright (c) 2002 David Tarsi. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being no invariant sections, with the Front-Cover Texts being no Front-Cover Texts, and with the Back-Cover Texts being no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".