I ran into a puzzling problem with my kitchen sink drains. One sink drain looks like this: and the other looks like this: I had no idea why the second one was cobbled together like that. It keeps coming apart and leaking at the top slip joint, so I thought I could improve it by replacing that mess with a straight tail piece. So I went to Lowe's and bought one that looks like the one in the first photo, except without the dishwasher drain. When I tried to install it, the plastic nut at the top of the tailpiece did not want to thread onto the drain flange. After a lot of futzing around, and trying to thread the metal nut I removed onto some plastic pieces to see if the threads would line up, it seems that there really is a difference in the threads. That might explain why somebody cobbled it together like that, because they couldn't thread a plastic fitting onto it. Is it possible that there are different thread patterns for metal vs. plastic fittings? Does it make sense that my two drains would have two different thread sizes? Is there something obvious that I am missing?