I am freshening up My 71 Tiger. I have a new patent plate and the 3 rivets to replace the old. Do I just drill out the old drive screws all the way, or will they twist out? How are they installed? They are blind holes, correct? I do not need to remove the cover to install the plate, do I? I have done LOTS of drilling and riveting on Aircraft, but never worked with the drive screws, as Triumph calls them. A# drill size would be helpful. Thanks!