Assuming that a piston and a cylinder bore are geometrically cylindrical and that the reciprocating and rotational velocities of the piston are given, the motion of pistons for piston-type hydraulic machines is analyzed by perturbation method. It is shown that in general the motion of pistons is unstable. Hydraulic lock in spool valves is concluded to be fundamentally caused by the instability of the motion of spools (pistons). For the special case where the motion is stable, the effects of fluid pressure, the piston length, etc. on the radial displacement of pistons are numerically presented.