Proposed as a method of thread geometry standardization in the mid 1800’s, Whitworth threads – both Full Form and Modified- feature a 55 degree included angle. The notable differences in these forms are illustrated in the graphic.
The true specification for 55 degree Whitworth is “full form”, which has a radius crest and radius root on the thread.
The “modified” or “truncated” form, on the other hand, has a flatted crest and flatted root on the thread form and has been accepted in the Continental United States since the 1940’s. A “modified” internal thread, which is produced by the tap, will always accept a “full form” external of the mating part.
North American Tool manufactures both the Full Form and Modified Form Whitworth taps, stocking sizes 1/8” – 1 ½” diameters.
Please keep in mind, certain government agencies and applications within the US may not accept the Modified Whitworth form. If the parts being made are for export, or used in the Aerospace Industry, Full Form is usually required. If the requirement is unknown, it is always safe to furnish Full Form.