Installing a Delta R18222 shower valve in master bath. Both the outputs are solder connections, yet the inputs are threaded. Shouldn't I solder these as well since they will be enclosed?
Most of those valves will allow either a threaded connection, or you can slide the pipe into the opening and solder it there. If you remove the cartridge and clean and flux the pieces, a soldered connection is probably better. Either one can work if done right, though. Don't solder a threaded connection since, if you ever do want to disassemble it, it will be a major pain.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013