Are you sure they are actually 20 TPI and 16 TPI ? These are very close to
a pitch of 1.25 mm and 1.5 mm respectively. Just a left field idea, but is
it possible the pipes are specified for their nominal bore size rather than
OD, similar to the BSP system. Someone in Italy must know.
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Regards,

Chas.

(To email me, replace "xxx" with letters tango papa golf.)