The master Bushing and insert bowls are casting with high-quality alloy steel, ranging from 17-1/2” to 49-1/2” and conforming to API. SPEC 7K Specification for drilling and well servicing equipment. There are two types, Pin Drive and Square Drive. API Standard rotary slips, drill collar slips, casing slips and roller. Kelly Bushings are suitable to work with these master bushings and insert bowls.