Qest compression fittings by Zurn can be used to connect gray polybutylene, PEX, CPVC or copper tubing in any combination. These are one of the few fittings that can be used with gray polybutylene plastic tubing. Polybutylene was often used in manufactured homes.
Fitting sizes listed are for the inside diameter of the tubing. IE: The 1/2 Qest coupling is for 1/2 inside diameter tubing which is 5/8 outside diameter.