Abstract: An oil well drilling rig assembly having a main drilling mast which receives and installs sections of drill pipe. A pipe supply mast is pivotally mounted in a spaced relationship from the base of the main mast to provide a safe catwalk area therebetween. The supply mast is movable between a horizontal position for transferring pipe sections between an adjacent pipe supply rack and a vertical position adjacent the main mast for transferring a section of pipe therebetween. The supply mast is raised and lowered by hydraulic cylinders and is telescopically adjustable to various lengths. A combination shock absorbing bumper and alignment arrangement is provided between the mast drill mast and the pipe clamps on the supply mast. The supply mast is provided with a plurality of pipe clamps for releasably clamping a section of pipe on the supply mast. The clamps are provided with adjustable jaws which maintain the axial centerline for clamped pipes of various diameters.