A phase adjusting mechanism in which input and output shafts, rotatable at like speeds and directions, are positively intercoupled by helical spline members constructed and arranged so that a female spline member is shiftable axially relative to a male spline member to counter rotate the shafts through a limited range of relative adjustability and in which support of adjacent opposed ends of the shafts is effected by an internal backbone support extending coaxially between the shafts to provide thrust and radial bearing support therefor. Adjustment members for shifting the spline members are provided along with an indicator mechanism for indicating the degree of shaft phase adjustment. A locking mechanism is provided for securing the adjustment mechanism in selected positions.