A servo pulser used to actuate a pulser valve and create pressure pulses in downhole measurement while drilling tools is described. An electric gearmotor is used to rotate a shaft which in turn operates a servo valve. A facing portion of the shaft is compressed onto the face of a servo seat having passages connected to the inside of a drill collar and the rotating action is used to open and/or obstruct a fluid path through those passages. The shaft may include wear-resistant tips to obstruct the fluid path. Part of the torque-transmitting apparatus between the gearmotor and the shaft can be hydrostatically compensated, and part sealed against the operating environment. A magnetic torque coupler may be used as part of the torquetransmitting apparatus between the gearmotor and the shaft.