Abstract: The mast of a portable earth drilling rig is mounted for pivotal movement between a horizontal transport position and selected drilling positions. A series of radially spaced apart lateral projections on each side of the lower end of the mast are provided with apertures for engaging laterally extending projections disposed on a pair of telescopic braces mounted on the rig frame. The braces may be selectively positioned for automatic engagement with a predetermined pair of projections on the mast whereby the mast, for drilling, may be positioned in the desired angular attitude.