Crack Clamp with Rubber Jaws, 20 X 180mm, with jack screws, Violin/Viola Minimize slipping and possible damages to the fine edgework of the instruments by contouring to the shape of the edges. The use of rubber jaws makes this tool safe and even more usable for all precise restoration purposes. These clamps are ideal for repairing cracks in stringed instrument tops and backs. Bends to the curvature of the archings.