I used a 3/8in 4ft long thermostat bit. It only has about 3in of bit and the rest is smooth it is real flexable so it follows the marro easy. Had to drill from both sides because of the bend. Another guy i know drilled as far as he could one way and the other and took a visene dropper and put some water in it, put some drops in the horn and it softened it up enough to clean it out with a coat hanger.