What is causing my 1/4 NPT fittings to not fit together properly?

[axemaster]

Ok, so here's a weird problem. I am trying to make a 1/4 npt fitting with two pieces of brass pipe.

The tap says:

Vermont
1/4 NPT 18
HSGG M21

The die says:

1/4 NPT
Baystate -5

So I threaded the pieces of pipe, expecting them to fit together nicely... and they didn't - far from it. I found that they were screwing in at an angle, and stopping after a short distance.

Needless to say I'm mystified by this. I've tried threading several pieces, being careful to make nice straight threads, and still this problem remains. I suspect it must have something to do with that "-5" on the die, but I haven't been able to find any references explaining what it means.

Please help!

 

[boneh3ad]

I would imagine because it isn't a 1/4 NPT 18 die. I don't know what "baystate -5" means and I don't think it has to do with the die size (manufacturer maybe?). Regardless, your die probably just doesn't have 18 threads per inch like the tap does.

 

[axemaster]

I thought that might be the case, but if you look at
http://www.engineeringtoolbox.com/npt-national-pipe-taper-threads-d_750.html
you will see that 1/4 npt only comes with 18 threads/inch. There don't seem to be any other variants, which I assume explains why the die doesn't have a tpi noted on it.