I'm still confused about this calibrating thing. I've calibrated diaphragm gauges in the past, but those were gauges themselves, not diaphragm sending units with remote gauges. The diaphragm acts directly on the gauge needle and you just zero it with a small screw. Piece of cake.
I can comprehend the way a diaphragm sending unit works, but I don't understand how moving the whole sending unit could change the pressure on the diaphragm. When the oil pump is not turning, there's no pressure and so the diaphragm should be completely "relaxed." I can't see how that would change at all by rotating it, even if you hold your mouth just right and chant incantations in ancient Coptic pentameter. Still not going to be any pressure acting on the diaphragm. Even if you screw the **** thing all the way in until the threads jam, any pressure exerted no the oil is just going to displace it to the sump against no pressure.
Now, if the diaphragm has some internal adjustment to affect the pre-load on the resistance element, I could see how that would allow one to zero the unit. I just can't see how moving the sender would do that - but I'm only familiar with the kind you just screw in and hope it seals so it doesn't leak. I never knew there was any other kind. I'd really like to see a cutaway illustration of one to see how it works.
Rich