These fittings are NPT threads (National Pipe Taper) and will compress against other Female/Male NPT threads creating a tighter and tighter connection. All NPT threads require thread sealant to create a watertight and gas-tight seal between the threads. We recommend using Yellow Gas PTFE Tape because it is rated for gases and is UL listed and it is NOT recommended to use white PTFE tape because it is not rated for gas service.