Motion transmission group for capping heads for screw caps, wherein said capping heads are provided with means for axially engaging the caps, wherein said group is intended to be operatively associated with a capping machine and comprises a main structure extending along a longitudinal axis between a coupling end for a capping head and a drive end, axially opposite to the coupling end, at which the group is configured to receive in input from a capping machine translation movements along said axis and rotation movements around said axis to be transmitted in use to said capping head, wherein said group comprises a device for applying in use a predefined axial load to the axial engagement means of the caps, with which a capping head associated in use to said coupling end is provided, wherein said device in turn comprises at least one axially pre-loaded compression spring so as to generate in use said predefined axial load, wherein said device comprises a support structure for said at least one compression spring, said support structure being suitable to keep said compression spring arranged coaxially to said axis in a predefined pre-loading condition by means of a first and a second axial positioning portion, associated with a main body of said support structure in axially different positions, wherein said support structure is shaped in such a way that said at least one compression spring is engageable directly or indirectly from the outside of said support structure to allow in use the operational coupling with the axial engagement means of the caps, with which a capping head associated in use to said coupling end is provided, and in that said device is separable from the main structure of said transmission group as a single body, with said at least one spring kept associated to said main body of said support structure in a pre-loaded condition by said two axial positioning portions (11b, 11c), to allow the replacement of said device with a structurally similar device, but suitable to generate a different axial load in use, wherein said device is separably associable to the main structure of said transmission group by reversible connection means (5; 25), wherein said device is separably associable with the main structure of said transmission group at a first axial end of said support structure not engaged by said spring by means of said reversible connection means (5; 25) and wherein said support structure is shaped so that said at least one compression spring is directly or indirectly engageable from the outside of said support structure at a second axial end (11") of said support structure which is opposite the first axial end and in use is arranged near the coupling end of said transmission group, characterized in that said device comprises a movable abutment body which is axially slidingly associated with said support structure and is interposed between one end of said spring and the second axial positioning portion, wherein said second axial positioning portion is positioned near said second axial end (11") of said support structure, said movable abutment body facing externally to said support structure near said second axial end (11"), in use said spring being intended to exert said predefined axial load on the axial engagement means of the caps via said movable abutment body, and in that at least one of said two axial positioning portions (11b, 11c) is axially adjustable relative to the main body of the support structure to adjust the pre-loading of said compression spring. wherein said device is separably associable to the main structure of said transmission group by reversible connection means (5; 25), wherein said device is separably associable with the main structure of said transmission group at a first axial end of said support structure not engaged by said spring by means of said reversible connection means (5; 25) and wherein said support structure is shaped so that said at least one compression spring is directly or indirectly engageable from the outside of said support structure at a second axial end (11") of said support structure which is opposite the first axial end and in use is arranged near the coupling end of said transmission group, characterized in that said device comprises a movable abutment body which is axially slidingly associated with said support structure and is interposed between one end of said spring and the second axial positioning portion, wherein said second axial positioning portion is positioned near said second axial end (11") of said support structure, said movable abutment body facing externally to said support structure near said second axial end (11"), in use said spring being intended to exert said predefined axial load on the axial engagement means of the caps via said movable abutment body, and in that at least one of said two axial positioning portions (11b, 11c) is axially adjustable relative to the main body of the support structure to adjust the pre-loading of said compression spring.