[Steam-engine] Pipe fitting & threading

[Andy glines]

It will soon be time for me to start replumbing my
boiler.  I have the threading dies that I need for
1/2" & 3/4" pipe thread.  I need a little education on
how to use them.  Here is an example:  I have two
fittings that are 12" apart that I want to connect
with 3/4" pipe.  How long do I need to cut my piece of
pipe?  I know that there is a formula but I can't seem
to find it.  When I thread my pipe how far do I run
the die down before I stop?  To little and there is
not enough threads & to much could make the wall
thickness to thin.  
Thanks in advance.  

Andy Glines
Evansville, IN

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