There is a big difference in thread nomenclature that really confuse people who have never worked with plumbing or pipe fixtures. Normally American SAE threads are measured by determining the actual diameter of the bolt and then counting how many threads per inch i.e 1/4 / 20. This bolt has a diameter of a 1/4 inch, and 20 threads per inch. Same goes for metric, excpet the pitch is the actual distance measureed from one thread to the next i.e 10 x 1.0 . This bolt would meausure 10 mm thick with 1mm between threads. Pipe threads are diferent. They measure or refer to the inside diameter of the pipe, because in in fluid dynamics you are concerned with what volume a certain pipe will carry not its outside diameter. So a fitting for a 1/4 National Pipe Thread (NPT) is actually going to be nominally closer to 1/2 inch or so after you accomodate the wall thickness of the pipe. Add to this, that the common pipe threads NPT are tapered. This is so that as the fitting is tightened they get wedged together and form a seal. There are also National Pipe Straight (NPS) threads that are the same thread pitch as NPT threads except they are straight and dont do the taper thing. this brings us to Zerk fittings. If you look at a common zerk fitting you can see it is a tapered thread with 20 threads per inch (TPI). I searched one time thru every industrial supply catalog I could find, as well as online and I could never find a tapered pipe tap that small. Every rescource I could find said to use a 1/4 20 SAE tap. It works fine. Zerk fittings do come in different sizes too so you'll have to get the right ones.....You probably knew all this, but maybe it'll help someone in the archives someday.....Tim
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