An apparatus for adjusting the force in a door operator or closer for closing a door, comprising: a housing having a longitudinal axis and a sidewall having a slot extending along at least a portion of a length thereof; a spring within the housing and connected to the door operator or closer, the spring being compressible to different positions to vary force applied by the door operator or closer; and a spring collar (40, 40') non-rotatable about the longitudinal axis of the housing and adapted to move linearly within the housing, the spring collar including a tab received within and slideable in the housing slot for preventing rotation of the spring collar about the longitudinal axis of the housing, the spring collar bearing on the spring to vary the spring compression and thereby vary force applied by the door operator or closer, the spring collar tab being at least partially visible from the exterior of the housing through the sidewall slot to indicate compression of the spring; characterized by: an adjusting screw extending along the longitudinal axis of the housing and passing through the spring collar (40, 40'); and a nut threadably engaged at an end of the adjusting screw distal from the door operator or closer and rotatable about the longitudinal axis, the nut bearing on the spring collar during rotation, wherein the adjusting screw has a non-circular cross-section and the spring collar (40, 40') comprises an opening having a complementary-shaped non-circular cross-section to permit sliding linear movement of the spring collar along an outer surface of the adjusting screw during rotation of the nut while prohibiting rotation of the adjusting screw about the longitudinal axis of the housing. a housing having a longitudinal axis and a sidewall having a slot extending along at least a portion of a length thereof; a spring within the housing and connected to the door operator or closer, the spring being compressible to different positions to vary force applied by the door operator or closer; and a spring collar (40, 40') non-rotatable about the longitudinal axis of the housing and adapted to move linearly within the housing, the spring collar including a tab received within and slideable in the housing slot for preventing rotation of the spring collar about the longitudinal axis of the housing, the spring collar bearing on the spring to vary the spring compression and thereby vary force applied by the door operator or closer, the spring collar tab being at least partially visible from the exterior of the housing through the sidewall slot to indicate compression of the spring; characterized by: an adjusting screw extending along the longitudinal axis of the housing and passing through the spring collar (40, 40'); and a nut threadably engaged at an end of the adjusting screw distal from the door operator or closer and rotatable about the longitudinal axis, the nut bearing on the spring collar during rotation, wherein the adjusting screw has a non-circular cross-section and the spring collar (40, 40') comprises an opening having a complementary-shaped non-circular cross-section to permit sliding linear movement of the spring collar along an outer surface of the adjusting screw during rotation of the nut while prohibiting rotation of the adjusting screw about the longitudinal axis of the housing. an adjusting screw extending along the longitudinal axis of the housing and passing through the spring collar (40, 40'); and a nut threadably engaged at an end of the adjusting screw distal from the door operator or closer and rotatable about the longitudinal axis, the nut bearing on the spring collar during rotation, wherein the adjusting screw has a non-circular cross-section and the spring collar (40, 40') comprises an opening having a complementary-shaped non-circular cross-section to permit sliding linear movement of the spring collar along an outer surface of the adjusting screw during rotation of the nut while prohibiting rotation of the adjusting screw about the longitudinal axis of the housing.