The emerging use of metric-sized threaded fasteners in North American manufactured products has given rise to concern for inadvertent assembly of metric fasteners with dimensionally similar inch fasteners. To prevent the possibility of mismatched assemblies, it is necessary to determine which thread combinations can be assembled so that proper care may be taken with stock identification and handling procedures.

A computer program called THDFIT has been generated to analyze any mismatched combination of Unified National threads, ISO metric threads, and ANSI-OMFS threads for determining the maximum possible degree of thread engagement and the resulting assembly strength. The logic of the computer program is described and the results of the analysis are presented.