Look for someone advertising "metal stitching" or similar terminology.
Here in Wisconsin/Upper Michigan, several of us have used an automotive engine rebuilding service that has the capability of repairing cracked engine blocks.
Metal stitch is the preferred technology -- particularly where stress will tend to open the joint. Taking the plate to a location that does the work is probably the cheapest but it can also be done on location. This repair is done on heavy mining equipment that must bear a lot of stress.
Splints also work in many situations. I have done a splint -- and then simply covered the work over with auto body putty so that it simply appeared to be an "extra thick" plate capo bar. The piano is perfectly stable after 35 years.
Welding is not particularly recommended. It "can" be done if the entire plate is put in an oven to control heating and cooling. The weld is not likely to break -- just a new break right next to the weld.
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Keith Akins
Akins Pianocraft
Menominee MI
715-775-0022
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Original Message:
Sent: 03-09-2017 11:16
From: Roger Gable
Subject: Have someone who needs a cracked plate repaired (Steinway "L")
Jeff,
I'm fortunate to have an excellent cast iron repair business close to me – that's all he does is cast iron repair. Every time I've used his service I could not see the original crack after his repair. He is a true craftsman. Maybe a Google search could turn up a similar business in your area. Try checking with a local machine shop, they might know where to direct you. Cast iron is tricky to repair because the entire cast iron plate needs to be heated before the welding process.
Roger