TorlonŽ polyamide-imide (PAI) has the highest strength and stiffness of any thermoplastic up to 275°C (525°F). It has outstanding resistance to wear, creep, and chemicals— including strong acids and most organics—and is ideally suited for severe service environments.
As taken from this website.... http://www.solvayadvancedpolymers.co...ogle&WT.srch=1
The reason for using those check ***** is that they are lighter and much less abusive than the Steel ***** used by the factory. It is not unusual for the steel ***** to hammer their way through the valve body plate over time.
Hope that helps.