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”