After trying Liquid Nails with no success, I ended up taking your advice, dck. The only reason I didn't right from the git-go was that I didn't know how I was going to find a tap the right size. So I took one of the cracked derailers and used my moto-tool to cut in and find out what I was dealing with. Turns out there is only 5mm of thread in the Delrin holding the screw. So I found a fine thread screw with a slightly larger diameter that was shorter than the original, but just long enough. I also found that the Delrin has a relatively low melting point, so I heated up the screw and it went in nice and firm. I think the problem is fixed. The derailer now has a non-old school screw in it, but I'm putting this one up on the Big Board as a win; me-3, Delrin-5. I'm still down in the count, but I'm not out. Thank you for your assistance.