Methods and apparatus for releasing a lead mill of a BHA from a whipstock in a wellbore include slideably releasing the BHA from the whipstock without relative rotation and without destruction of a retractable bolt. A system includes: a bias mechanism; a retractable bolt at least partially disposed in the bottom hole assembly and biased to a retracted position by the bias mechanism; and a retraction actuator capable of selectably opposing the bias of the retractable bolt. A method includes: coupling a whipstock to a BHA with a retention system, including: a retractable bolt biased to retract into the BHA; and a retraction actuator configured to resist the bias of the retractable bolt; and after the whipstock and BHA have been disposed in a wellbore, activating the retraction actuator so that retraction of the retractable bolt ensues.