How to Drill a Bowling Ball for Curving

Examine the bowling motion. Before drilling a ball, it is extremely important that the bowler's motion be studied as this will dictate how weight should be removed from the ball. Balls may have a late or early curve, depending on the motion, which can be enhanced or balanced by changing the weight distribution as needed by the bowler. A different ball should be used to study the motion, ideally one with a weight and core similar to the undrilled ball.

Determine the center of gravity on the ball. Typically a punchmark on the ball designates the center of gravity. Directly underneath this mark, within the ball, are 2 ounces of weight to balance out the weight removed from finger and thumb holes.

Decide on how to distribute the ball's weight. Hole placement shifts the center of gravity in either a positive or negative direction. Positive means the center of gravity is shifted toward the side of the ball which curves into the pins (for right-handed bowlers, this is the right side of ball when held with the finger holes up and the thumb hole down, for left-handed bowlers it's the left side) while negative means the weight is shifted in the opposite direction. Balls with a positive shift curve later and more, while those with a negative shift curve sooner and less. Before drilling, determine whether the ball should have a positive or negative shift to augment the bowling motion to provide either a positive or negative shift.

Mark where to drill the finger and thumb holes to the specifications of the motion and the bowler's grip. Measure the distance between the fingers and how deep the holes should be for the grip. Typically finger holes are deep enough to reach the second knuckle, but some bowlers prefer shallower holes. Determine the grip for the ball before proceeding. Ideally, drilling the three holes are equidistant from the center of the gravity (evenly spaced around the punchmark) keeps the ball balanced. Placing the finger and thumb holes further further down any side of the ball shifts the center of gravity in the opposite direction, as the added weight at the punchmark is there to account for the weight removed by drilling. For right-handed bowlers, drilling to the right of the punchmark creates positive shift, while drilling to the left creates negative shift. The opposite is true for left-handed bowlers.

Be aware of the weight distribution rules for competition. Most competitive leagues, such as the American Bowling Congress (ABC) and Women's International Bowling Congress (IWBC) place restrictions on the weight distribution of balls. Balls more than 10 pounds can't have a difference between top and bottom weight of more than 3 ounces or a difference between are the areas to the right and left of the finger holes of more than 1 ounce. Take these rules into account when positioning the holes.

Drill the holes. Once the location for the holes has been determined, place the ball into the drill press and make sure it is securely locked. Select the correct drill bit, the size of which should be similar to the bowler's thumb in diameter and length, and drill the thumb hole. Do the same thing for both finger holes. Make the holes slightly larger than the fingers and thumb as the skin tends to swell during games. Place a sanding tool into the drill and sand the holes down as soon as they are done.

Decide on the use of a balance hole. Balance holes are used to remove weight from the heavier side of the ball, the side without the finger and thumb holes. If the ball shift on the ball is either too positive or too negative (curving too late and too much or too soon and too little), drill another small hole on the opposite side to remove a little weight to even out the shift. Be sure to track how much weight is removed to keep the ball within regulations and start with a small balance hole as more weight can be removed later.