Abstract: Disclosed herein is a drill bit for earth boring, including a lubrication system that eliminates necessity for a complex system to accomplish pressure compensation. Since the temperature of the earth generally increases with depth from the surface, a drill bit experiences increasing temperature with increasing depth. The coefficient of thermal expansion of lubricant inside a bit is greater than that of the metal defining the walls of the lubricant cavities. Internal bearing lubrication pressure control is achieved utilizing thermal expansion of the lubricant, seal displacement, and a pressure relief valve.