UNEF thread gaug is Unified National Extra Fine thread gauge
UNEF means the TPI should be big, (TPI is more,the thread if finer) For UNEF thread, the usual TPI is 18,20,24,28,32.
Extra-fine-thread series, UNEF, is applicable where even finer pitches of threads are desirable, as for short lengths of engagement and for thin-walled tubes, nuts, ferrules, or couplings. It is also generally applicable under the conditions stated above for the fine threads.
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