The built-up mainshaft has a plurality of eccentric elements secured to an elongated shaft element in end-to-end angular off-set relationship to each other. Each of the eccentric elements has integral cylindrical body and axially extending sleeve-like portions and an axially extending bore therethrough to receive therein the elongated shaft element. Each eccentric element is scored to render the cylindrical body portion thereof deformable in an inward direction toward the shaft element. A clamping means is provided for each eccentric element to coact with the scoring to effect deformation of the associated cylindrical body portion and secure the latter in torque transmitting relationship with the shaft element.In a narrower aspect of the invention, the scoring of each eccentric element comprises a radial slot extending axially the length of the bore and in the plane passing through the axis of the bore and radially, from the bore, through the wall of the sleeve-like portion and into the cylindrical body portion. A chordal slot is provided to extend in the sleeve-like portion at or adjacent the juncture of the latter and the cylindrical body portion and intersecting the radial slot.