Pneumatic Fitting Tips

Properly identifying pneumatic fitting and component sizes, especially on older equipment, is easier with these tips.

Kevin Kakascik, Technical Marketing Engineer at AutomationDirect, wrote an article for the June 2020 issue of Fluid Power World titled Cracking the Code on Existing Pneumatic Fittings. Here’s a summary, click on the link above for the full text.

Pneumatic equipment is rugged and reliable, but it does not last forever. A problem with a $2 fitting can take down an entire production line, but when there is trouble a knowledgeable technician can save the day. Following are a few tips for sorting out the types and sizes of pneumatic fittings.

Sizing Up the Situation

At a glance, fittings for solenoids, manifolds, cylinders, valves, air preparation units, and more all look quite similar.

Following are the basic steps to identify these fitting sizes.

Step 1: Determine the gender, male or female
Male fittings have threads on the outside, female on the inside.

Step 2: Determine if the threads are straight (parallel) or tapered
Tapered threads have a smaller outside diameter at the first/deepest thread than at the final/shallowest thread; straight threads have the same diameter at all points. Note that there are many thread designs:

  • National Pipe Tapered (NPT)
  • National Pipe Straight (NPS)
  • British Standard Pipe Taper (BSPT), also known as R thread
  • British Standard Pipe Parallel (BSPP), also known as G thread
  • Metric, Tapered or Straight

Step 3: Determine the thread pitch
Thread pitch is measured as distance from thread to thread, or how many threads per inch. It can be measured with a caliper or a fine ruler and some math, but a dedicated pitch gauge is best.

Step 4: Determine the thread diameter
Calipers used at the largest thread (first thread for female fittings or last thread for male) will identify the thread diameter.

Step 5: Determine the fitting size
Use the measurements in conjunction with charts published by suppliers like AutomationDirect to determine the fitting size and associated part numbers.

Hose and Tubing Connections

Hoses are usually specified by inside diameter, and they are connected using barbed fittings which do not really have any industry standards. Flexible tubing is measured by outside diameter, which is critical for the push-to-fit fittings typically used. A drill bit gauge can help identify an outside diameter, and a drill bit can do the same for an inside diameter.

Rules of Thumb

Some good rules of thumb (which should always be verified with measurements) for pneumatic equipment are as follows:

  • NPT threads are commonly used in machinery built in the U.S.
  • If the tubing side of a fitting is metric, then the fitting is probably BSPT or BSPP
  • BSPT is commonly found on Chinese and Japanese machinery
  • BSPP is commonly found on European machinery

Another tip for common pneumatic machinery, the following imperial and metric tubing sizes match very closely and are interchangeable:

  • 5/32” is the practically the same as 4mm
  • 5/16” is the practically the same as 8mm

Avoid Downtime

With the right tools and knowledge, users can properly determine pneumatic fitting and tubing sizes, and quickly order the parts they need from the AutomationDirect website.