A drive mechanism suitable for use in drug delivery devices is disclosed. The drive mechanism may be used with injector-type drug delivery devices, such as those permitting a user to set the delivery dose. The drive mechanism may include a housing, a dose dial sleeve having a helical thread of a first lead, and a two-part piston rod. The two-part piston rod may include an outer part having a helical thread of a second lead and an inner part having a helical thread of a third lead. In such an embodiment, the first lead of the thread of the dose dial sleeve may be equal to the sum of the second lead of the thread of the outer part of the piston rod and the third lead of the thread of the inner part of the piston rod.