I'm a licenced automotive & ( truck & coach ) mechanic up here in northern Ontario , Canada . ,which or governmment now can implement two diffferent classes ,of which we have to pay now twice the amount to practice our profession. To get back at your question reguarding preventive maintance ;if it's made in North America , buy the time you have followed their plan the rest of the unit is garbage;however a Mercedes follow Their PM maintance.My timing chain chain jumped , causing damage, the cause was a $14.00 item. PS a valve stem is the appendage from the valve face.