A solenoid type magnetic trip assembly (22) for a molded case circuit breaker (10) includes an armature (80) biased against an adjustable stop (138) by a tension spring (116) to set the initial gap (220) for the magnetic trip, so that the spring bias remains constant for the full range of the initial gap (220). The armature (80) includes an elongated armature element (82) mounted by a frame (84) to slide longitudinally along a pair of guide rails (122). The frame (84) defines a trip surface (104) axially aligned with the elongated armature element (82) which engages a trip arm (160) on a trip bar (24) to trip the circuit breaker (10) in response to a predetermined level of overcurrent. A bimetal (168) providing a thermal trip function is cantilevered from a support spaced from the trip bar (24) by the armature (80), but has a terminal portion (174) at the free end (172) projecting toward the trip bar (24) and through which the elongated armature element (82) of the armature (80) extends. A radially enlarged slug (88) on the free end of the elongated armature element (82) of the armature (80) is subjected to a magnetic force opposite to the force generated by load current tending to pull the armature (80) into the solenoid coil (74). This opposing force increases as the initial gap (220) increases, placing the slug (88) closer to the magnetic frame (78), so that a greater range of trip currents can be selected despite limited room for armature travel. A gap (228) in the magnetic frame (78) prevents short circuiting the magnetic field where the few turns of a large gauge coil wire produce an unsymmetrical winding. A magnetic shield (198) protects the bimetal (168) from deformation during high current short circuits. A non-magnetic spring clip (230) firmly retains the magnetic frame (78) in a recess (232) in the circuit breaker housing (12).