Abstract: An earth boring bit has features particularly for use in drilling holes in which compressed air is used as a drilling fluid. The bit has three rotatable cutters, each having a cavity for mounting on a depending bearing pin. The cutter and the bearing pin have a thrust bearing in the lower portion of the cavity and a roller bearing in the upper portion of the cavity. The bit has air passages for discharging through the roller bearing gaseous fluid pumped from the surface. A seal seals the nose area from the roller bearing area. Liquid lubricant is located in the nose area for lubricating the bearing surfaces in this portion of the bit. A lubricant chamber is in communication with the nose area. The lubricant chamber cavity has a sliding piston for applying pressure to the lubricant. The sliding piston is driven by the compressed air in the air passages.