If the pipe threads were overtightened from the start, the fpt side could be distorted to the point where it will never seal. All good advise above from Del & everyone. The pump has a pressure rating too, so the shaft seal should be accounted for. If the fittings are overseas, lots could be wrong - the angle of each thread, the taper, etc.
Sure you don't have British Standard Parallel threads?
[URL]https://www.engineeringtoolbox.com/british-pipe-threads-d_754.html[/URL]
