17mm. It's usually always the bolt that is 17mm. Did you replace the gasket and clean the bolt? You should be ok this time if you didn't but make sure you do next time. Mine was like pancake and needed a flathead to get it off the bolt. Also, don't for get when you change the filter to take some of the new oil and dab you finger in it and rub the gasket on the filter, so it doesn't bunch up on ya. You probably already knew this tho. 