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These United Scientific 3‐prong extension clamps are made using pressure die‐cast zinc alloy with a chrome plated finish. The prongs open to about 3" and have a recommended maximum grip size of about 2.25". It also has double keyed spring‐type adjustment screws. The boss head clamp can swing freely, is lockable at any angle up to 360°, and can be used with rods up to 19mm.