The camshaft phasing unit for the belt drive has, in principle, the same internal construction as the phasing unit for the chain drive. An additional requirement to be fulfilled here is for external oil seal integrity. This phasing unit therefore has internal seals, oiltight sintered materials, a seal surface to the rotary shaft seal on the camshaft and a closing plug on the end face. This is fitted after the central fixing screw for the camshaft. The camshaft phasing unit is of a modular design and allows the use of various toothed belt gears and numbers of teeth with the same internal construction.