An extendable drilling tool, the extendable tool moving in a drilling direction and including a body with a longitudinal axis and at least one arm that is mobile relative to the body in a direction of movement that forms a first angle with the longitudinal axis that is non-zero, less than 45°, and open upstream according to the direction of drilling. The extendable tool includes a piston that is mobile in a direction parallel to the longitudinal axis, the piston being located downstream according to the direction of drilling relative to the arm, the piston including at least one bearing surface that works with at least one arm and that forms with the longitudinal axis a second angle that is non-zero, less than 90°, and open downstream according to the direction of drilling.