HEY PAUL THE CRACK IN THE HOT PLUG WILL NOT CAUSE A RUNNING PROBLEM UNLESS YOU CAN SHINE A LIGHT AN SEE THRU THE CRACK , FACTORY TOLEARENCE IS 3/4 INCH LONG, AS LONG AS IT IS NOT ALL THE WAY THRU, THE PROBLEM WITH THSES PLUGS IS THEY ARE SO HARD IF IT SHOULD DEVELOP ANOTHER CRACK CLOSE BY THIS ONE IT COULD POSSIABLY BREAK OF THE PIECE IN BETWEEN AND IT WOULD MOST LIKELY END UP IN THE TOP OF A PISTON, NOT GOOD , THE CRACK BETWEEN THE VLVS , CAN BE REPAIRED BY A GOOD MACHINE SHOP BY DRILLING AND PLUGGING,IF THE PLUGS HAVE TO BE REPLACED, HOWEVER I HAVE SEEN THESE ENGS WITH A TONS OF MILES ON THEM AND THE CRACK BETWEEN THE VLVS DOESN,T SEEM TO HURT THEM , HAVE NEVER SEEN ONE CRACK ALL THE WAY THRU TO WTR JACKET , BUT ON A MAJOR REBUILD I WOULD GO AHEAD AND FIX THEM, THOSE HOT PLUG HAVE TO BE GROUND FLAT AFTER THEY ARE REPLACED, SO THAT THEY WILL NOT CUT INTO THE HEAD GASKET , THEY CANNOT BE MILLED , THEY ARE TOO HARD JERRY