Thanks all for the ideas and chance to talk it out. The problem was not solvable by hitting it harder, the diameter of the cap was larger than that of the mating hole. I tried in one small area indenting the rim of the cap and quickly realized that this would create a mess. After sleeping on, it occurred to me that I could take a dremel cutting wheel and cut 1/8" deep reliefs into the cap at 90 degrees to the edge of the rim (parallel to the axle), every 60 degrees around the circumference of the rim. This allowed space for the circumference to contract enough to fit the mating hole. This worked great!