I won't argue on the fact that some engines have faulty gaskets/wrong torque, I will say this : One backfire can take out gaskets ,as 2 mechanics found out at the John Deere shop i worked at yrs ago. Close to closing time on a saturday, a customer came in & bought a new tractor.. still in the crate,our procedure was to forklift the crate to just outside the shop door,pull the plastic off,after pulling most of the crating around the tractor,while still sitting on the crate pallet,we would hook up the battery,add gas & fire up the engine to warm up while undoing the rest of the packaging ,install the steering wheel,back tractor off the skid,drive into shop ,These guys were in a hurry,as it was closing time,drove into shop & the hasty,improper shut down blew gaskets & oil seals, NEW tractor. Now everyone stayed late to set up another new tractor,i do not remember the engine make,but all OHV engines are susceptible. By the time i get done typing you will probably have your tractor running ,& stuck in the snow in your lawn .Valves look real good after cleaning the oil gunk off. NOW, if only your tractor lives as long as that engine will,you will be set to mow for yrs.