Designed to enable safety wire holes to be drilled accurately through bolts without drill breakage. Bolt is threaded to the required depth, then the hole is drilled through on a perfect center. All steel construction. Fits bolt sizes 8-32, 10-32, 1/4-20, 1/4-28, and 5/16-24. Use 3/64″ drill bit part number CD364 sold separately.