The best glue to use is a quality Super Glue (Cyanoacrylate), Gorilla Glue (Micro Precise) or Resin Epoxy Glue. There is also the slight possibility of fine layer lines showing, but these are hard to see with the naked eye.
Industrial Piping - Industrial and waste - 1/35, 1/32 resin model
Industrial Piping (pipe) - all scales resin model
GV-A2 Inline Globe Valve 1/4 x 40 TPI Brass Valve for Live Steam Models
P7 Gas Can Valve Mini Steam Models Part Adapter Valve Fueling Steam Engine
M6x1 Live Steam Safety Valve Brass 15PSI Adjustable Valve
P7 Gas Can Valve 7/16 UNEF 28 TPI to 1/4x40 TPI Adapter Valve