A portable power grinder comprises a housing (11), a rotation motor (10) with a rotor (13) having a rear end portion (27) extending out of the motor (10), an output shaft (22) which is drivingly coupled via a reduction gearing (23) to the rotor shaft (13) and intended to carry a grinding tool, and a speed governor (18) for providing a constant motor speed. The reduction gearing (23) comprises a gear (24) integral with the rotor shaft (13) and another gear (25) integral with the output shaft (22). Alternative rotor shafts (13) with gears (24) of different sizes may replace each other for obtaining different gear ratios and different output speed levels, and the length of the rear end portion (27) of the rotor shafts (13) varies with the size of the integral gear (24). The housing (11) has a detachable lid (15) carrying the speed marking (2) of the tool and having an internal projection (28) of a size directly related to the speed level readable on the speed marking (21). The rear rotor shaft end portion (27) has a shape directly related to the maximum speed level provided by the actually fitted rotor shaft (13), and the lid projection (28) matches such rotor shaft end portions (27) only that correspond to gear ratios providing output speeds equal to or lower than the speed marking level.