A dual-purpose engine fitting is provided that may be used both during engine testing and during engine use, for vehicles with vacuum assisted brakes and for those without. The fitting has a body portion configured to fit within an opening in the manifold and defining a passage having opposing first and second ends. The fitting has a seal portion enclosing the first end of the passage. The second end of the passage is open to the manifold when the body portion is fit within the manifold. A thinned wall joins the body portion and the seal portion, establishing a predetermined fracture region at which the seal portion will separate from the body portion when sufficient torque is applied to the seal portion to thereby open the first end of the passage. A method of assembling vehicles is also provided.