Under reamer

An under-reamer comprising:

• a body;
• extendable cutters mounted on the body, the under-reamer configured to be cycled between a first configuration in which the cutters are retracted and a second configuration in which the cutters are movable between refracted and extended positions;
• and
• a control mechanism configurable to prevent cycling between the first and second configurations and thus maintain the under-reamer in a selected one of the first and second configurations.

A downhole bore treating method comprising:

• running an under-reamer comprising extendable cutters into a bore;
• cycling the under-reamer between a first configuration in which the cutters are retracted and a second configuration in which the cutters are movable between retracted and extended positions;
• maintaining the under-reamer in a selected one of first and second configurations by preventing cycling of the under-reamer between the first and second configurations;
• and
• pulling the under-reamer from the bore.

A downhole device comprising:

• a body;
• a fluid actuated member mounted on the body and being configurable to provide a first device configuration and a second device configuration, the downhole device configured to be cycled between the first and second configurations;
• and
• a control mechanism configurable to prevent cycling of the downhole device between the first and second configurations and thus maintain the device in a selected one of the first and second configurations.

A downhole device operating method comprising:

• running a device comprising a fluid actuated member into a bore;
• cycling the fluid actuated member between a first configuration and a second configuration;
• maintaining the member in a selected one of the first and second configurations by preventing cycling between the first and second configurations;
• and
• pulling the device from the bore.

A downhole tool comprising:

• a body;
• an actuating piston;
• and
• a retainer piston operatively associated with the actuating piston and having one face configured to be exposed to external body pressure and another face configured to be exposed to internal body pressure, wherein the retainer piston is configured to generate an actuating piston retaining force when the external body pressure exceeds the internal body pressure.