The key differences in dimensions between NPT (National Pipe Tapered) and NPSM (National Pipe Straight Mechanical) threaded pipe nipples are:
NPT Thread:
Tapered thread design - the diameter decreases along the length of the thread.
Nominal pipe size corresponds to the inside diameter of the pipe.
Threads are designed to create a leak-tight seal when tightened.
NPSM Thread:
Straight (parallel) thread design - the diameter does not taper.
Nominal pipe size corresponds to the outside diameter of the pipe.
Threads are not designed for a leak-tight seal, threaded pipe nipple dimensions but rather for mechanical strength.
The impact on dimensions:
NPT nipples have a slightly larger outside diameter to accommodate the tapered thread.
NPSM nipples have an outside diameter that matches the nominal pipe size more closely.
NPT nipples are typically a bit longer than NPSM for the same nominal size, to allow for the tapered thread.
For example, a 1/2" NPT pipe nipple may have an outside diameter around 0.840", while a 1/2" NPSM nipple would be closer to 0.840" OD.
The choice between NPT and NPSM threads depends on the application needs - NPT for leak-tight sealing, NPSM for mechanical strength. Understanding these dimensional differences is important when selecting the right type of pipe nipple.
