How to Measure Malleable C-Clamps

Open the screw of your malleable C-clamp as far as it will go by turning the handle at the top of the threaded rod counterclockwise until you can no longer turn it with normal pressure. Do not force the rod.

Measure vertically from the top of the base where the ferrule hits the clamp frame to the bottom of the ferrule. This is referred to as the opening or jaw size. Leave the clamp open.

Measure horizontally from the inside end of the "C" frame to the outer edge of the base where the ferrule hits the clamp frame. This is the throat size.

Remove any rust and debris from the clamp rod with chemical rust remover or a wire brush and clean the rod with a soft, clean cloth. Follow the manufacturer's instructions and safety recommendations if you use chemical rust remover. Clean the inside of the jaws of your caliper with a soft, clean, dry cloth.

Open the wheel of your caliper and hold it straight around any point of your clamp's threaded rod. Tighten the caliper wheel around the rod so that it grasps the rod securely but do not turn the wheel so hard that you damage the threads of the rod. Read the caliper scale or digital readout to find the diameter of the rod.

Repeat for any other clamps in your toolbox or collection. Mark or note the sizes of each so you know which one to use if assembly or repair instructions for a project call for a C-clamp with a particular jaw, opening or throat size or rod diameter.