If clearances are to allow for heat expansion only, why when tappets are set hot do we still allow clearances? Surely they should be almost nil?
Because it's not just for heat, it's to allow an oil film to form otherwise you would be metal to metal contact, it allows for correct seating of the valve head in the seat and also for the valve and valve train to rotate in operation on many engines which is evidenced by the rotational wear patterns in many components, they often run off centre to achieve this.
