NPT threaddimensions PDF

Stack Exchange network consists of 183 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

We are a bearing manufacturer. I design all the new special products that come in the door. We use NPT grease fittings in quite a few of our products. We use a 1/8 & 1/4 NPT grease fittings. As our standard practice we generally use a certified thread gauge and set our cnc to tap so the thread gauge will have a 4 to 4-1/2 turns max engagment. This has always worked for us and have never had a return in the 60+ years we have been in business. I have discussed this issue with our old school bearing designers and have learned in my 12 years with the company that this is a good way to measure. We have tested many different grease fitting brands and actual lubrication system fittings which all work great. I don't remember a time when we ever had a leak using this standard.

1/2npt threaddimensions in mm

In my experience, the engagement depth on NPT fittings can vary wildly (depending on quality of machining, amount of teflon tape used, and the person wielding the wrench, etc etc). This inconsistency can cause other issues when it comes to actually assembling -- but that's another topic in itself.

I think many times we get wrapped up in a ASME standard and forget that these guidelines are vague for a reason, they are meant to get us in the ballpark and we are suppose adjust until it works for our applications.

NPT threadchart

I'll be the first to admit that I have always been somewhat confused by best-practice dimensioning standards surrounding pipe threads. While there is a decent amount of literature available online, (such as this) I still find the implementation to be a bit vague -- specifically regarding thread depth.

ASME B1.20.1 (AKA the NPT bible) makes me think that one should callout thread-features such that they run to the "vanishing point" or L4. However, L2 is designated as the "maximum thread engagement distance".