It is typically meaning the outlet size of submersible pump thread. BSPT Rp 6 – Thead Dimensions Basic Thread Dimensions * The below values are nomoinal (without tolerances) and are valid for both external and internal threads. Property mm Inch Nominal Pipe Size (Ø) 6″ … Pitch (TPI) (P) 11 … Pitch (Distance) (P) 2.309 0.0909 … Continue reading what is meaning of Rp6 of water pump