keep in mind that if you use these gears (parma) The axle bushings will have to be replaced with bronze, and that requires soldering. Any crown with the shoulder on the back side will not allow the pinion to mesh with the supplied axle bushings. because of the narrow area between the axle uprights.
these generic gears are a close repop of the COX, and don`t have the shoulder on the back side.
http://www.ncphobbies.com/generic-1/24-gears-page-2/