[Tidybuoy]

I'm wondering if the threads on copper pipe fittings are the same as brass.

I'm asking because I'm plumbing for my new Tankless Water Heater and the unit came with a brass fittings kit (i.e., parts that have valves and connect to household plumbing). I purchased 3/4" male threaded fittings that will be soldered to the copper input/output piping but they don't screw into the brass fittings very easily and start resisting after about 1-1.5 turns. Seeming like it's not a match, I tried threading in a 3/4" brass fitting and it went in fairly easily and started resisting after 5+ turns.

Anyone have experience with this? I'm wondering if I should be using a brass fitting with a sweat to copper connection? I can't find the installation manual for this and having trouble locating on net.