Point taken on the NPT sizes( http://en.wikipedia.org/wiki/National_pipe_thread#Pipe_thread_sizes )...I had a steel an-6 laying around and its threads engage the ID of the tube pretty well per http://en.wikipedia.org/wiki/AN_thread that would mean something in the 9/16" ID.  Which would correspond to the drill size for 3/8 npt( http://ww2.mne.ksu.edu/classes/ME300/Reference/npt%20tapdrill.htm ).  However, 3/8npt seems perilously  close to the OD of 11/16( measured using the poor mans calipers, a crescent wrench ).

Thus, if everything is same on our blocks perhaps something of the order of a 5/8x18 or x20 would be more appropriate?

Thoughts?

-Scott
(Hoping clear weather holds for another 14 days...)






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