> Also, the old fittings had to be drilled out of the smaller drain. Now
> it's
> time to cut threads into the 1/2" hole I've created. Also, it's only 1/2"
> deep too. Hopefully, the walls of the fitting are thick enough. I've got
> two 1/2" taps: a 13 tpi-NC and a 20 tpi-NF. Which would be better?
>
Paul:
Neither of your 1/2" taps are of any good for this duty, for a couple of
reasons. First, both taps will cut standard parallel threads, which are
suitable for bolts but not for pipe fittings. Second, a 1/2" tap will not
cut a thread in a 1/2 hole, no matter how hard you try, assuming that you
really did drill the fittings out with a 1/2" drill. If not, then what
diameter was the drill that you used to drill out the fitting? If you can
tell me this, I can tell you what size pipe thread is the closest and what
size tap drill you must use first before tapping. You should use a pipe tap
for this job because a pipe thread is tapered - as you tighten the thread,
the clearance in the thread is taken up by the taper which aids in sealing
the thread. Any fitting you screw into this tank fitting will have a male
pipe thread.
If, in fact, you did drill out the old fitting with a1/2 inch drill, you
will have to tap it with a 3/8" pipe tap. The tap drill size for this tap
is 19/32", so you would need to run this size drill through the hole first
before tapping. The one potential issue here is the wall thickness of the
tank fitting, as you pointed out.
Michael Marr
1960 TR3A
2000 Jag XK8
Naperville, IL
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