Bowling balls have three primary points that influence the reaction of the ball after drilling: the locator pin, the mass biasand the centre of gravity. Most ball manufacturers place extra weight under the CG to balance out the weight that's removed when the finger and thumb holes are drilled; this is called top weight.
More advanced bowlers often opt to remove some of the top weight to customise the reaction of the ball when it's in motion. This requires drilling a fourth, balance hole into the ball.
Determine if it's necessary to remove top weight. Consider your bowling style and the lane condition. Oil-heavy lanes and bowlers who bowl at high speeds should use low top weights. Keep in mind that lower top weights cause earlier breaking points and smoother hooks. A lower top weight also makes a ball change direction quicker, arc more and roll earlier.
Measure the distance from the pin to mark where the balance hole should be drilled. Consult bowling ball manufacturer layouts for measurement specifics. These are available online or can be accessed by contacting the manufacturer directly. Each layout affects a bowling ball in a unique way. Select the option that suits your style and customises the flare and axis rotation the way you want.
Set the bowing ball into the drill press with the CG, as indicated by a punch mark, on top. Be sure to clamp the ball tightly in the press; a poorly locked ball can lead to uneven drilling.
Drill the balance hole. Put a drill bit into the bowling ball drill and adjust the position and angle to match the specifications appointed by the layout. Follow a weight removal chart to choose a drill bit size that corresponds to the number of ounces to be removed. Drill the balance hole 2 1/2 to 3 1/2-inches deep for maximum effect. Small, shallow balance holes only slightly change ball reaction and are most appropriate for slow ball-speed bowlers. Large, deep balance holes suit fast ball-speed bowlers best.
Sand the drilled balance hole and surrounds to remove excess dirt and debris. Uneven sanding can lead to inconsistencies in play.
According to the American Bowling Congress, only a single additional hole can be drilled for balance purposes, and this hole cannot be more than 1 1/4-inches in diameter. Abide by this rule if planning to bowl competitively. Drilling bowling balls is by no means a simple process. Technicians spend years learning how to properly complete the process and are constantly adapting new techniques. Bring the bowling ball to a professional for best results.