ASTM A193-B7 Thread Pitch Explained
“My plant uses hex head cap screws, ASTM A193-B7 as standard. We typically stock the constant thread series 8 TPI fasteners above 1 inch diameter. However, many tapped items like expansion joints come with UNC thread counts, like 7 threads per inch (TPI) for 1-1/4″ diameter bolts. This causes a problem sometimes when fasteners don’t fit, or bolting with different thread spacing gets intermixed. Any advice? What does most of the Chemical Process Industries (CPI) use on these larger fasteners. Thanks for any advice or history on this issue.”
Answer: Marc, thanks for the question. Most ASTM specifications for construction fasteners use Unified National Coarse (UNC) threads (7 TPI on 1-1/4″ diameter for example). Consequently, most female threaded items (expansion joints) are produced with National Coarse threads. However, ASTM A193 is one of two exceptions (A320 being the other) in which 8UN threads (8 threads per inch) are standard for fasteners larger than 1″ diameter unless otherwise specified. With that said, A193 bolts can be ordered with UNC threads if that is specified when they are ordered. If a thread pitch is not specified, most fastener suppliers/manufacturers will supply this grade of bolt (A193 grade B7) with 8 threads per inch on bolts above 1″ in diameter. Most good suppliers will ask what thread pitch the customer wants when an order is placed.
Neither the A193 or A320 specifications address thread pitch directly, however they do require compliance with ASTM A962, titled “Common Requirements for Steel Fasteners or Fastener Materials, or Both, Intended for Use at Any Temperature from Cryogenic to the Creep Range.” . Section 12.4.2 of ASTM A962 states that fasteners 1″ and under shall be coarse thread, and those 1-1/8″ & larger shall be 8 thread pitch series, unless otherwise specified.
In your situation, you will need to know the thread pitch of the insert, expansion joints, nuts, etc. prior to ordering the bolts and then specify the matching thread pitch when the bolts are ordered. One solution would be to purchase both the male and female threaded components from the same supplier and make them responsible for matching the thread pitches (if that is possible).