I discovered that the rack eyelets for a used touring bike I bought don't take the standard M5/0.8 pitch cap screw. Nor, as I discovered, do they take an M6, or at least not those with 1.0 pitch which is what my LHS had. My off-road bike uses M5 screws for the rack as well as for everything else, so I thought that was the standard (I'm influenced by computers, I think--I expect everything to be standardized)
My experience isn't extensive, so this might be a common variation that everyone knows about except me. Any idea what size/threading cap screw I actually need?