Explain 1/8" NPT Male Thread

A common question from our customers is how a 3/16" or 1/4" OD temperature probe can be used with an 1/8" NPT thread male compression fitting . Isn't a temperature probe with 3/16" or 1/4" OD too large for a compression fitting with 1/8" NPT male thread?

Our response is that NPT thread sizes are misleading. As an example, the 1/8” size is loosely related to the ID of Schedule 40 pipe (it's ID is really about 0.27"). Because of the pipe wall thickness, the OD of the pipe is larger (the OD of an 1/8” Schedule 40 pipe is about 0.40”). If you were to cut threads (27 per inch, tapered) on the OD of this pipe you now have an 1/8" NPT nipple.

If you want to tap a hole to thread this 1/8" NPT nipple into you’d use a 11/32” (.342”) drill bit.

The answer is that yes, temperature probes with 3/16" or 1/4" OD will easily fit into an 1/8 NPT male compression fitting.