"It is recommended that you get approximately 13.7 N-cm (= 0.137 N-m or 1400 gf-cm or 19.4 ozf-in or 1.21 lbf-in) of holding torque (or more) for RepRap axis motors to avoid issues, although one stepper with less has been used successfully. For Wade's Geared Extruder (most widely used one as of 2012) it is suggested to use motor that is capable of creating a holding torque of at least 40 N-cm." -- http://reprap.org/wiki/StepperMotor#Holding_Torque
