Apparatus for positioning a first part with respect to a second part is characterized by a cylindrical guide respectively attachable to each part, a helical surface being disposed on each guide, each helical surface including a right hand half and a left hand half. A piston-cylinder is disposed on the second part, the piston being extendable to engage the guide on the first part to axially align the guides. After alignment, the piston is retractable to bring the helical surfaces into contact to generate a torque and align the parts. In another embodiment, the guide for the second part is formed on the piston while the guide on the first part has an axial alignment member projecting therefrom to engage the guide on the second part to align the guides. Once aligned, the helical surfaces contact each other with the piston extended. The contact between the helical surfaces causes the rotational alignment of the second part with respect to the first part. The piston is then retracted, thereby causing respective mating elements of the first part and the second part to engage. The force of gravity is preferably used to bring the helical surfaces together and thereby cause a rotational torque to be applied to the second part relative to the first part.