Active Autowerke M50, S50, S52 High Performance Thermostat Housing Cover | E36 325 328 M3 Z3MSwap plastic for aluminum. And forget about cracking, forever. Here are your choices: you can wait for the plastic factory thermostat housing cover to break and cause major overheating. (Even with a new stock cover, it won't take long.) Or you can install a thoughtfully engineered all aluminum thermostat housing cover from Active Autowerke and never experience this type of failure again. Sure sounds like a no brainer to us. Currently there is no