A metering mechanism includes a body mounted in a sleeve covering the top and an adjustment nut covering the bottom, the adjustment nut threaded to the sleeve, the screw connection then causing rotation of the adjustment nut with respect to the body of the metering mechanism, the body being extended by a valve seat that can be closed by a valve, able to receive a plunger of which the reciprocating motion permits intake at the valve, the travel of the plunger depending on the screw position between the adjustment nut and the sleeve. The metering mechanism further includes a rotation lock between the adjustment nut and the body of the metering mechanism to prevent the adjustment nut being screwed or unscrewed on the sleeve. Also disclosed is a method for locking and unlocking such a mechanism, and to a proportional metering pump incorporating such a mechanism.