Re: On a boiler tap, what does NS stand for?

Hi John and Jim:
Some additional comments on boiler taps. The NS probably does stand for "National Standard" as described in older Machinery's Handbooks. Pipe Threads are designated NPT for "National Pipe Thread". Boiler threads, be they straight (staybolts, patchbolts, etc.)or tapered (washout plugs, studs, etc.) belong to either the 12 pitch series or the 14 pitch series. The Unified National Standard (in my Eighteenth Edition) allows for various standard thread pitch series that utilize the same thread pitch regardless of diameter. 12 pitch and 14 pitch are standard thread pitch series and can be found in Machinery's Handbook's Threads Section.
I have seen both 12 and 14 pitch threads so be careful -- check your threads with a thread pitch gauge so you don't screw a 14 pitch washout plug into a 12 pitch hole. That is a recipe for disaster. Also be careful with tapered washout plugs. Fine pitches like these are easy to cross thread, especially tapered threads. More than one unsuspecting boilerman has been killed by blown washout plugs that were cross threaded into their holes.
The reason standard pitches are used in boiler work is the fact that boiler sheet thicknesses don't vary much and you always want the maximum number of threads engaged to develop the same joint strength as the tensile strength of the bolt or washout plug.
I hope this helps. Bill Petitjean