A transmission coupling assembly for absorbing axial and/or angular displacement between an internal combustion engine and a downstream gear train comprises a resilient vibration damper (3;24) secured to the flywheel (1;26). A coupling member (4;23) secured to the vibration damper is rotationally coupled to a coupling shaft (7;21) by means of interlocking splines, the splines (4a) of the coupling member being substantially straight and the splines (5a) of the coupling shaft being crowned or spherically curved. At the other end of the coupling shaft (7;21) a similar splined connection is made to the input shaft (8) of the downstream gear train. Lubrication is provided at the splined connection between the coupling shaft and the input shaft of the gear train, with a Z-cross section seal extending between the coupling shaft and the gearbox casing to prevent leakage of lubricant fluid.