Basically, there are two types of pressure plate assemblies: those with coil springs and those with a diaphragm spring. Both types have a stamped steel cover that bolts to the flywheel and acts as a housing to hold the parts together. Ln both, there is also the pressure plate, which is a heavy, flat ring made of nodular or grey cast iron. The assemblies differ in the manner in which they move the pressure plate toward and away from the clutch disc.