I have a z31 so our cars are different but are you talking about the stock POV, or emergency relief valve as Nissan calls it. If so I've never heard of anyone drilling on it. What did you drill? Mine has 3 tabs indented near the top of the valve that you need to bend out to take it apart. After you add springs or washers you put it back the way it was and tap the tabs back in.
what you have to do is one of 20 things...either drill a small hole in the shaft and thread the hole and use a screw to hold on the cover (the best way) or you can drill a small hole in the chaft and use a pop rivet to hold the cover on...but then you have to drill out the rivet anytime you want to switch out the washers...I suggest the screw method.