HI STUART , IVE DONE FEW, THE MAIN PROBLEM IS THE RINGS ,OIL ON THE PLUG ,DUE TO THE WRONG OIL OR OIL NOT CHANGED REG.IVE NEVER REMOUVED THE ENGING !. NEW HEAD GASKET+VALVE SEALS+RINGS AND OFF IT GOES .SOME TIMES VALVES GET BURNT, MAKE SURE USE ORIG VALVES IF IS THE CASE.ALL PARTS ARE AVAL.FROM MERC ,BUT THEY DONT GIVE ANY DISCOUNT ON ANY SMART PARTS!! GOOD LUCK . ALI