@ A coordinate measuring machine (10) having a vertically movable probe (41) supported from the bottom of a probe arm (42), the weight of which is offset by an improved pneumatic counterbalance (50). The pneumatic counterbalance is adapted to use a source of pressurized air acting upon a spherical piston (54) which is connected to probe arm (42). Spherical piston (54) fits with a close tolerance in cylinder (52). A rigid bracket (58) is connected to the top of probe arm (42). Piston rod (56) connects spherical piston (54) to a spherical bearing (53) in rigid bracket (58). A precision pressure regulator (70) having a relatively small capacity is provided to insure only a small change in the counterbalance force as the spherical piston (54) moves within cylinder (52). Regulator (70) is adjusted to provide make up air for the air which leaks past spherical piston (54).