You have to grind off the peening to get the nut off that holds on the piston. It sounds like you already have the nut off so don't grind anymore. Race Tech has a special tool called a bullet that slips right over those threads & is tapered --just like a bullet and makes installing the new seal a breeze. You can also wrap the thread area with electrical or duct tape to achieve the same result. just wrap it up to the same diameter as the rest of the shaft [12mm], twist the top to a point, grease it & slide on the seal