Abstract: A clamping apparatus for locking tools in a well is disclosed. A first piston sealably engages a hollow body member to form a chamber, which is filled with a working fluid. A bias member or spring moves the first piston, extending a clamping arm connected to the first piston against the well wall. A second piston is sealably engaged in a bore in the first piston; the bore is in fluid communication with the chamber. The second piston is driven into the bore by a motor, increasing the pressure in the chamber through a passage in the first piston between the bore and the chamber. This increased pressure further urges the first piston and locks the clamping arm against the well wall. The hydraulic pressure is released, and the clamping arm is unlocked, by opening a valve in a line between the chamber and a reservoir. The clamping arm is retracted by driving the second piston into the bore into contact with the first piston, forcing the first piston back against the spring.