American UN, UNC, UNF and UNR threads

American threads to ASME B1.1 are dimensioned in inches and designated by nominal diameter and threads per inch, for example 1 1/2-8 UN. UN stands for Unified National, the basic profile with a 60° flank angle. UNC is the coarse thread series (C for coarse), UNF the fine thread series (F for fine), and 8UN the series with a constant eight threads per inch, which is common for large flange bolts in pressure equipment and piping. UNR is not a separate thread but the UN profile with a mandatory root radius on the external thread. Inch threads, above all 8UN and UNC on stud bolts, are a regular feature of the enquiries RS Randack receives.

Why rolled inch threads are the norm

In thread rolling, the die forms the thread root with its own radius, so the root radius is defined by the die. In through-feed rolling, a UN die typically also covers the UNR profile; in in-feed rolling, the nominal diameter, pitch and radius of the die must match the thread exactly. RS Randack keeps rolling dies in stock for the common UNC, UNF and 8UN series and, where UNR is specified, inspects the profile of the thread root. A rolled inch thread also has a higher fatigue strength than a cut one, see Thread rolling or thread cutting.

Tolerance classes 1A, 2A and 3A

American external threads are made to classes 1A (loose fit), 2A (standard, also for coated threads with an allowance) and 3A (close fit). Internal threads have the corresponding classes 1B, 2B and 3B. For coated stud bolts, 2A is the usual choice because the tolerance leaves room for the coating thickness; in any case, RS Randack rolls threads that are to be coated to the lower tolerance limit so that the gauge still fits after coating.

Material grades B7, L7, B16 and 2H

The thread type says nothing about strength. That comes from the ASTM material grade:

• ASTM A193 B7: Quenched and tempered steel of the 42CrMo4 type for pressure equipment and flanges up to elevated temperatures, the most widely specified American grade.
• ASTM A320 L7: The same type of material, additionally with verified impact energy at –101 °C for low-temperature applications.
• ASTM A193 B16: Chromium-molybdenum-vanadium steel for higher temperatures, for example on turbines and high-temperature flanges.
• ASTM A194 2H: The matching nut grade in quenched and tempered carbon steel, usually as a heavy hex nut.

RS Randack manufactures stud bolts, hex bolts and nuts in these grades from European raw material sourced from approved mills, with the mechanical properties verified in our own laboratory and a 3.1 or 3.2 inspection certificate.

Stud bolts and heavy hex nuts

A stud bolt is a headless bolt, threaded at both ends or along its full length, that clamps a flanged joint together with two nuts. The standard combination in plant engineering is a B7 stud bolt with an 8UN thread and two 2H heavy hex nuts. Heavy hex nuts are thicker and have a larger width across flats than metric nuts of the same size; RS Randack produces them in nut machining with cut or turned internal threads, typically up to M80 or the equivalent inch size.

What to specify in your enquiry

• Full thread designation: nominal diameter, threads per inch, series and class, for example 1 1/4-8 UN 2A or 7/8-9 UNC 2A
• UNR if a radiused root is required
• ASTM material grade, or metric material and property class
• Length, thread length and, for stud bolts, the design (fully threaded or double-end)
• Matching nuts and washers, coating, certificate type and third-party inspection