Often used on earth movers, mining equipment, and other heavy machinery, these fittings have a T-shaped top that holds the dispensing tip in place so it won™t pop off when adding grease. An integral check valve prevents grease from leaking when pressure causes backward flow. The valve stays closed when not in use to keep out dirt and other contaminants.Zinc-plated steel fittings have mild corrosion resistance. 303 stainless steel fittings are more corrosion resistant than zinc-plated steel fittings and have good chemical resistance.PTF and NPTF (Dryseal) threads are compatible with NPT threads.