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El_sabor
gbj_tester
I think the lumps of copper in Copaslip are big enough to be a problem given the geometry and tolerance of the mating parts. Whereas a coarsely toleranced screw thread will effectively pump the grease out of the flank contact area, the mating of the Hirth joint is, to a 50μm blob of copper, more like a testicle getting squashed between two bricks - it cant run away fast enough, and there is no relative motion between the faces to drive it. You end up with small flattened copper plates stuck between the steel teeth.
You don't need any kind of protection against galvanic corrosion at the joint interface, since there is no dissimilar metal contact.
If you use a lubricant on the teeth which can be squeezed out (oil or clear grease), it won't be doing any lubricating when lubrication would have any effect. If you use a lubricant which can withstand the very high pressure, it's stuck between the mating faces compromising the joint.