Steel Key Seat Clamps, 1" (25mm) Length, 7/16" (11mm) Wide, 7/64" (2.8mm) Slot Width. Key Seat Clamps convert steel rules, combination square blades and straight edges into key seat rules for laying out keyways and scribing parallel lines on round work. Clamps are 1" long x 7/16" wide (25 x 11mm) and have a 7/64" (2.8mm) slot width.