From what I read in other posts, if you have a 1.5 inch MIP pipe coming out of the wall, you don't normally put a trap adapter on that, but just use the nut and washer.
I did not know that, and I ended up chasing down that elusive 1.5 FIP/SlipJoint trap adapter. I got a 1.5 NPT die. I tried to chase threads. And I ended up cracking the cast iron santee applying a lot more torque to the copper trap adapter than needed.
I tried to fill the crack with J-B Weld. Seems everybody likes that but me. Did not stick to the cast iron. Leaked.
But probably for the best, because the galvanized below that santee was pretty clogged up. I ended up running plastic with a coupled of couplings. Sure would have been easier and cheaper if I just did that to begin with.