Dosing unit for use in an ambulatory infusion system, the dosing unit including: a dosing cylinder and a piston , the piston being arranged inside the dosing cylinder in a sliding displaceable manner along a displacement axis (A), wherein the piston is convertible from a storing configuration into an operational configuration, wherein a circumferential sealing member of the piston is mechanically relieved in the storing configuration and is in sealing and sliding engagement with a circumferential inner surface of the dosing cylinder in the operational configuration; and the dosing unit includes a configuration switch member , the configuration switch member being arranged, at least partly, within a bore or recess of the piston , the configuration switch member being movable relative to the piston from a storing position into an operational position by moving the piston in a proximal direction with a movement of the configuration switch member in the proximal direction being blocked by an interaction of the configuration switch member and the dosing cylinder , thereby switching the piston configuration from the storing configuration to the operational configuration, wherein via the relative displacement, the piston is radially extended, thus biasing the circumferential sealing member against the circumferential inner surface of the metering cavity and establishing the sealing, characterized in that the configuration switch member includes a stop member , the stop member being axially spaced apart from the piston in the storing position, wherein movement of the configuration switch member from the storing position into the operational position is associated with a movement of the stop member towards the piston , wherein the stop member hitting a piston surface stops movement of the configuration switch member in the operational position. wherein the piston is convertible from a storing configuration into an operational configuration, wherein a circumferential sealing member of the piston is mechanically relieved in the storing configuration and is in sealing and sliding engagement with a circumferential inner surface of the dosing cylinder in the operational configuration; and the dosing unit includes a configuration switch member , the configuration switch member being arranged, at least partly, within a bore or recess of the piston , the configuration switch member being movable relative to the piston from a storing position into an operational position by moving the piston in a proximal direction with a movement of the configuration switch member in the proximal direction being blocked by an interaction of the configuration switch member and the dosing cylinder , thereby switching the piston configuration from the storing configuration to the operational configuration, wherein via the relative displacement, the piston is radially extended, thus biasing the circumferential sealing member against the circumferential inner surface of the metering cavity and establishing the sealing, characterized in that the configuration switch member includes a stop member , the stop member being axially spaced apart from the piston in the storing position, wherein movement of the configuration switch member from the storing position into the operational position is associated with a movement of the stop member towards the piston , wherein the stop member hitting a piston surface stops movement of the configuration switch member in the operational position. <EOS>