i had a sheared bolt in one of my coil pack from when i purchased my zed, it lasted for about 4 years and not a single bit of trouble it wasn't wedged in or anything just sitting there with the resistance against the plug to hold it in, I had to change the cam cover due to an oil leak at christmas so bolted it back in properly then and its still fine, so it being an earthing fault isn't the case I'm afraid. the cover itself is about £250 including a new gasket and its just plastic so it will be easier and a lot cheaper to replace that than to attempt to drill it out.