The J series thread is defined in specification ANSI/ASME SAE AS8879 (formerly MIL-S-8879) and in ANSI/ASME B1.15.
UNJ vs. UN
UNJC vs. UNC
UNJF vs. UNF
UNJEF vs. UNEF
UNJS vs. UNS
UN Bolt (external) into UN Nut (internal) = Compatible
UNJ Bolt (external) into UNJ Nut (internal) = Compatible
UN Bolt (external) into UNJ Nut (internal) = No assembly issues; functional problems are possible.*
UNJ Bolt (external) into UN Nut (external) = Compatibility Issue – In most cases an interference fit at the thread minor may have minor diameter interference and functional assembly issues.*
*The two threads are not designed to be used as an assembly connection. There are dimensional differences in the threads which may create issues in extreme cases.
*Dimensional differences related to shape and thread minor diameter may impact the strength of the material including stress cracking and loading failure.
*The above factors are product design related and impossible to determine without conducting an extensive testing and evaluation of the design.
Per SAE AS8879, UNJ Thread profiles are not interchangeable with Metric system MJ profiles.