Upchurch: Threads

HPLC employs several thread sizes. Most common are 1/4-28, 10-32 and M6. The first two are U.S. Customary System measurements while the third, the M6, is an International Metric System designation.

U.S. Customary System

The first number indicates the diameter of the threaded portion of the nut (not the tubing diameter). Thread diameter numbers range from 1 (.073") to 12 (.216"). Beyond .216" the thread description is the thread per inch count. Thus, a 1/4-28 male nut [Figure 7] has a 1/4" (.250") diameter thread barrel and 28 threads per inch. A 10-32 male nut [Figure 8] has a 10 (.190") thread barrel and 32 threads per inch, a narrower nut with a denser thread count.

International Metric System

The Metric System uses a one number thread description. Metric nuts in HPLC typically have one thread per millimeter and the M simply stands for metric. The single number refers to the diameter of the thread barrel in millimeters. Thus, an M6 nut [Figure 9] is 6mm in diameter and has 1 thread per millimeter.