UNF Unified America Thread

American Fine fastener threads

Threaded fasteners, bolts, nuts, screws and fastenings in UNF



                     d    TPI                  pmajor diametereffective diameterminor diameter external threadsminor diameter internal threads
1 1/4"120.08331.12501.07091.02281.0348
1 1/8"120.08331.25001.19591.14781.1598
1 3/8"120.08331.37501.32091.27281.2848
1 1/2"120.08331.50001.44591.39781.4098






























Americans experienced the same problem from lack of thread standardization that Britain did. The challenge was taken up by William Sellers, a member of an eminent family of American engineers Williams grandfather had made the plates with which the Continental Congress printed its currency. To William, we owe the colour “machinery grey.” When others were decorating their machinery, he insisted on painting his a uniform light grey, in order not to obscure the functions of the parts. Sellers specified a thread form and a graded series of nuts and bolts that used it.

In 1864 the Franklin Institute committee recommended the adoption of Seller’s system of screw threads. The thread form became known as the “Franklin thread,” or, more commonly “Seller's thread,” and later as the “United States Standard Thread.” In May 1924 it was designated the “American Standard Thread.”

The main difference between Seller's and the Whitworth's thread for that predated it is that the tops and bottoms of the threads (the crests and roots) are flattened. The flattened roots was a bad choice for Sellers. Such angular joins in metal concentrate stress, and the process of manufacture results in high stresses at the roots of threads anyway. The result is cracks and broken fasteners.

This problem was not so noticeable in Seller’s day for two reasons. One was that most machinery was stationary and the weight of a bolt rarely mattered. If a bolt broke it could be replaced with a larger one. The second reason was that thread roots tend to be rounded anyway as the tools that make the bolts become worn.

The Unified thread form was developed after World War II by representatives of Great Britain, Canada, and the United States of America, to prevent recurrence of the wartime difficulties in supplying fasteners and tools in both British Standard Whitworth and US Standard configurations. In 1949 the American National Standard Series was replaced with the Unified Inch Standard Series. In the end there were three base reasons identified for the change. The first reason was to provide interchangeability with Canada and United Kingdom. The second reason was to allow for interchangeability in the growing global marketplace. The third reason was to correct certain thread production difficulties.