More on "metal stitching," as first mentioned by Mark Kinsler in the
071115 and 071116 MMDs.

A number of years ago I attended a seminar on this very topic at a
Piano Technicians Guild national convention.  The host was a metal-
lurgist specializing in cast iron and its repair. It included a mar-
velous and detailed photographic presentation of both successes and
disasters.

It was explained in the following way.  Gray cast iron is seriously
saturated with carbon.  The advantage of this is that the molten metal
flows beautifully into castings, doesn't flex, has an exceptional
amount of tensile strength, and as far as pianos go, it is acoustically
dead, preventing any unwanted metallic ringing.  On the bad side, it is
unforgiving when it comes to bending, and therefore prone to breaking.
It is also highly sensitive to expansion, when heated to high tempera-
tures, as in welding.

The problem with welding is that it involves heating a centralized
location.  In doing so, the heated spot dramatically expands, while the
remaining parts of the plate remain rigid.  Remember: cast iron would
rather break then bend.  So, if you are expanding an area, you are
forcing the rest of the plate to endure tension that wasn't there
before.  Therefore, a welded plate, brought under tension once again,
is now even more likely to break.  There have of course been success
stories, but the odds are against you, when heating cast iron hot
enough to perform a weld.  New cracks could even appear where there
were none before.

Okay, now the good news.  There is an excellent cold repair method that
really works.  I speak from experience.  It really blew me away when I
tried this.

There is a company called Lock-N'-Stitch that can provide you with a
complete kit (http://www.locknstitch.com/).  Machinists call the tech-
nique "metal stitching".  There are several different methods depending
on the type of crack.  In the case of piano plates, using "stitching
pins" is often the solution.  These are threaded and look something
like a screw, but are far from being screws.  First, they are made of
an alloy that resembles the characteristics of cast iron.  Second, upon
close examination, you will see that the threads are sharply cupped
upward.

The kit provides a bit to drill a hole at the crack and then a special
countersink for the top.  Then you use a special tap, which is pro-
vided, to cut threads in the plate.  As the screw is turned into the
hole, the cupped threads grip the sides and quite literally pull into
the crack.  When the screw becomes tight, the head intentionally snaps
off as the tightening reaches a specific torque.  Then another screw is
inserted next to the first the same way.  Once the crack is lined with
screws you start over again inserting additional screws between the
others, overlapping the first ones.  This sounds and looks strange, but
when you are finished the entire original crack has been replaced by
all new material provided by the pins.

The repair has amazing strength and it involves no heat.  Next, fill
the repair area with a little glazing putty, a couple coats of sealer
sanded smoothly, and then a fresh coat of gold paint, followed by a
clear coat, and it will look as if the crack and repair were never
there.

If you have a plate that has serious cracking problems and feel it is
beyond this type of repair, the company can also repair it for you,
provided you can ship it.  They have a welding technique that first
involves placing the entire plate into an oven and heating it so that
it has uniformly expanded.  At that point, welding can be safely per-
formed without risk of uneven expanding.  I urge you to visit their
webpage and read up.  There is a world of information provided on how
to properly repair cast iron without risk of self-destruction.

One final word: the plate probably didn't break on its own for no
apparent reason.  Before reinstalling the repaired plate, be certain
that everything is where it is supposed to be.  If the plate is rocking
and has to be forced flat when tightening the screws, or if the area
where the pin block contacts the top plate flange is uneven, you need
to fix the problem or you will be repeating the same scenario that
caused the crack in the first place.

Rob Goodale, RPT
Las Vegas, NV