Get a drill bit the diameter of the string anchor area you are wanting to use and see if it fits through the hole. If not, drill it or dremel it out. Notice I said the string anchor area. As the core is wrapped around the ball and secured, then the wraps applied over the core, this will increase the overall diameter of the string in the bridge anchor area, and will require a larger anchor hole than the nominal diameter of the string might otherwise indicate. I had to do that myself on my Ibby SRA305. I like to use either a GHS 138 (now discontinued) when running rounds, or a Chromes 132 when running flats. The 138 isn't a problem, but ironically, the thickness of the anchor area of the Chromes 132, especially with the added silk at the ball, was too thick to go through the bridge anchor; so I had to enlarge the hole. Be careful: on many bridges, the anchor hole is very close to the saddle adjustment screw, and if the anchor hole is not enlarged carefully, the drill bit or dremel bit can break through and ruin the bridge.