I prefer to build cues without weight bolts, but sometimes one is needed to achieve a certain weight. If you want to change the weight bolt (or weight screw) in the butt of the cue first make sure there is one. I do not typically drill and tap a slot for a weight bolt. It is always preferable to have an experienced cuemaker or repair person do the work. Do not attempt to drill and tap a slot for a weight screw unless you have the right equipment and some experience. It is very easy to mar the finish, cause the ring work (if there is any) to rise and generally screw up the cue if you don't know what you are doing. The same is true if you know what you are doing but don't have the right equipment.
I generally use a 1 or 2-ounce bolt if one is needed. If you insert a medium sized Phillips screwdriver into the slot the 1-ounce bolt will be about 2.5 inches in. If the bolt starts just past the bumper it is the 2 plus ounce bolt. The slot is about 6 inches deep and will accept 2 plus ounces. I typically use a 3/8-16 galvanized flat head bolt that is available at any hardware store. I grind the head down so it is flush with the threads. That way it fits deeper into the butt. An alternative way to handle changing the weight is to just cut the head of the bolt off and use a hacksaw to cut a slot in the end. Then use a slot screwdriver to position the bolt. That will allow you to weight the cue to fit your preference. It shouldn't require saying but, I can't be responsible for work that is improperly performed.