UN Thread Types

UN (Unified) is a specification of the shape and dimensions of the thread for a given size (diameter) of fastener and number of threads per inch of fastener length. UN bolts and machine screws are the type of fastener in everyday use in the US, UK, and Canada. UN, UNR, and UNJ thread dimensions are measured in Imperial units. The M and MJ threadforms are similar but are measured in metric units and are manufactured in metric sizes.

UNC (Unified Coarse) is a special case of UN. For each UNC thread size, the UNC specification also indicates how many threads per inch the fastener must have. Any hardware store in the US, UK, or Canada will sell UNC fasteners.

UNF (Unified Fine) is the same as UNC except that for each size of bolt, the number of threads per inch is higher, so the thread is finer. UNEF (Unified Extra Fine) is the same as UNF except that the number of threads per inch is even higher for each fastener size. UNF and UNEF fasteners are generally available only from industrial fastener suppliers.

4UN, 6UN, 8UN, 12UN, 16UN, 20UN, 28UN, and 32UN are thread series in which the number before the "UN" indicates the number of threads per inch.

UNS (Unified Special) refers to combinations of size and threads-per-inch that are not in any of the standard series mentioned above. It is generally recommended that UNS sizes be avoided, but for some applications some designers may like the idea of deliberately using parts that are hard to replace and incompatible with common parts and tools. Even among UNS fasteners, some combinations of sizes and threads per inch are considered more standard than others.

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