Most motors have a continuous torque rating. This rating was arrived at under some thermal test conditions at the supplier. Rated torque is least understood and the most corrupt parameter for any motor. Heat is the enemy of torque. Know the test conditions that a particular motor was rated at and make an engineering judgement whether you can use those ratings. We recommend against using any ratings unless the test conditions were exactly like your project (never the case).  Torque rating is a reference point only. Thermal modeling is difficult and needs empirical input to be useful. Many solid modeling software tools have a thermal FEA module this can help determine the likely thermal resistance of the system you are developing. Peak torque rating should not be used until you know how close the motor torque output is to your needs.