The key to putting these back together is to get everything super clean
and add some lube on the inner piece's tapered outer surface before
reassembly. Tighten the two opposite screws, alternating sides until
it's good and tight. A good finishing touch is to use a hammer and
non-maring rod or block and strike the inner portion sharply to force
the fit even tighter. This will allow the screws to tighten up one
more time. Thread lock should not be needed in ordinary use, but
perhaps in rock mills, etc?

We sometimes used these without the key to allow a certain amount of
"slip" in the case of accidental shock to the system. Kind of like a
shear pin in a boat prop and for the same reasons. Mounting shaft
dimension (etc.) was, of course, more critical.

dennis
in nca