Rules:
In order to insure that every ball drilled in your shop adheres to USBC specifications for total weight, diameter, side weight, finger/thumb weight, top/bottom weight, hole sizes, and the number of holes, you need to know what those guidelines are.
USBC specifications pertaining to the weight, size and balance of bowling balls provide that no ball shall exceed 27 inches in circumference (8.59” in diameter) or shall it be more than 16 pounds in weight.
Balance:
The following tolerances shall be permissible in the balance of a bowling ball used in sanctioned competition:

10.0 pounds or more:
 Not more than three (3) ounces difference between top half of the ball (finger hole side) and the bottom half (side opposite the finger holes)
 Not more than one (1) ounce difference between the sides to the right and left of the finger holes or between the sides in front and back of the finger holes.
 A ball drilled without a thumb hole may not have more than one (1) ounce difference between any two halves of the ball.
 A ball drilled without any finger holes or indentations may not have more than one (1) ounce difference between any two halves of the ball.
 A ball used without any hole or indentations may not have more than one (1) ounce difference between any two halves of the ball.

For a ball weighing less than 10.0 pounds to 8.0 pounds:
 Not more than two (2) ounces difference between top half of the ball (finger hole side) and the bottom half (side opposite the finger holes)
 Not more than three quarters (3/4) ounce difference between the sides to the right and left of the finger holes or between the sides in front and back of the finger holes.
 A ball drilled without a thumb hole may not have more than 3/4 ounce difference between any two halves of the ball.
 A ball drilled without any finger holes or indentations may not have more than 3/4 ounce difference between any two halves of the ball.
 A ball used without any hole or indentations may not have more than 3/4 ounce difference between any two halves of the ball.

Less than 8.0 pounds:
 Not more than 3/4 ounce difference between top half of the ball (finger hole side) and the bottom half (side opposite the finger holes)
 Not more than three quarters (3/4) ounce difference between the sides to the right and left of the finger holes or between the sides in front and back of the finger holes.
 A ball drilled without a thumb hole may not have more than 3/4 ounce difference between any two halves of the ball.
 A ball drilled without any finger holes or indentations may not have more than 3/4 ounce difference between any two halves of the ball.
 A ball used without any hole or indentations may not have more than 3/4 ounce difference between any two halves of the ball.
Definitions:
Weigher: Position of individual facing ball and scale during weighing procedures.
NOTE: The designated ball areas (front  finger wt, etc.) illustrated here are based on the position of the ball when placed in the scale.

Front: The one half segment of the ball which includes the finger holes (see sketch #1)
Back: The onehalf segment of the ball which includes the thumb hole (see sketch #1)
Finger or Thumb Weight: The difference in balance between the front and back portions of the ball. Example: if a thumb hole segment is one half ounce heavier than the finger hole segment, the ball has one half ounce of thumb weight. 

Right Side: The one half segment of the ball determined by a plane equally dividing the finger and thumb holes through the center of the ball. The right side is determined by placing the thumb hole toward the weigher. (see sketch #2)
Left Side: The one half segment of the ball opposite the right side (see sketch #2)


Top: The one half segment of the ball which includes both the thumb and finger holes (see sketch #3)
Bottom: The one half segment of the ball which is opposite the side in which the holes for gripping the balls are drilled. (see sketch #3) 
Procedures:
The scale shown here represents the most sensitive and accurate of this type. The front beam is for balance while the back beam is used for gross weight. Varying types of balances scales may have different amounts of graduation on both the front and rear beams from 1/16 ounce to 1/2 ounce respectively. The diagrams demonstrate the theoretical method of centering bowling balls in the scale ring as the starting point for weighing and balancing.

In diagram #1 the line AB represents the distance between the centers of each finger hole. Point E is 1/2 the distance of AB. A line CE is then drawn from the center of the thumb hole to the midpoint of the finger hole line. Point D is 1/2 the distance of line CE and is the center of the ball and the starting position for weighing. Do not at any time be influenced by the drilling angle or pitch of the holes, or by the location of the name of the manufacturer. 

Diagram #2 indicates the method used in determining the center of the ball with more than 2 finger holes. The dotted line ABD connects the center of the 3 finger holes. Points C and E are the midpoints of line AB and BD respectively. Point I is the midpoint of line BF. Points G and H are the midpoint of line CF and EF respectively. Points G and H are connected forming line GH. Point J is the midpoint of line GH. Point I is connected to Point J and the midpoint of this line is the center of the ball. This is a theoretical aid to help in finding the center of the ball and does not allow for the curvature of the ball. 

Position #1: Determine Gross Weight by placing the ball in the scale as in sketch “A” and Position #1, with the thumb hole towards the weigher, the holes to be centered on a line from front to back. Place the poise (sliding weight on front beam) at zero, the poise (sliding weight on rear beam) is adjusted to obtain the gross weight. The poise on the front beam must be at zero to obtain gross weight. If the right and left sides of the ball are in perfect balance the gross weight from this test is the accurate gross weight. 

Position #2: Proving Weight and weighing right side of ball:
 Rotate the ball 180 degrees as illustrated in Position 2 with the finger holes toward the weigher and centered on a line from back to front.
 If the scale is not in balance, the poise on the front beam should be moved accordingly until the scale balances.
 If the front beam poise is moved one/half ounce to the right, for example, this signals that the right side of the ball is onehalf ounce heavier than the left side. It also signals that the ball weighs slightly more than the result obtained in test #1
 Since there was a difference of onehalf ounce between test 1 and 2, the gross weight equals the total of the weight determined in position 1 plus onehalf the difference between the tests. In this example, onefourth of an ounce more than the weight determined in position 1.

To double check the weight an accurate gross weight scale should be used.
If a gross weight is not available or if either test 1 or test 2 shows the ball to be slightly over 16 pounds, the ball may be passed, provided onehalf the difference does not bring the weight over 16 pounds and it balances in conformity with ABC/WIBC specifications. Let us assume in test 1 the ball weighed 15 pounds, 15 3/4 ounces, while in test 2 it weighed 16 pounds 1/4 ounce. Taking one half of 1/2 ounce, or 1/4 ounce and adding it to 15 pounds, 15 3/4 ounces would total 16 pounds and the ball would be acceptable providing it is otherwise within balance limits. The allowable difference between the left and right side of the ball shall not exceed one ounce.

Position #3: Determine the difference between the top (drilled area) and bottom of ball.
Place the ball as shown in Position #3. With the poise on the front beam set at zero, move the poise on the back beam so the scale is in balance. 

Position #4: Rotate the ball 180 degrees so the finger and thumb holes are to the right as shown in Position #4. If scale remains in balance, top weight and bottom weight are equal. If the scale is not in balance, the poise on the front beam should be moved until the scale balances. A move of the poise on the front beam to the left to balance the scale indicates ball top weight in the amount shown on the front beam scale. If the poise on the front beam was moved to the right, this indicates bottom weight as shown on the scale. The difference should not exceed 3 ounces. 

Position #5: Determine Thumb or Finger Weight. Set the ball in the scale as shown in Position #5, place the poise on the front beam at zero. Move the poise on the back beam until the scale is balanced. 

Position #6: Rotate the ball 180 degrees to Position #6. The thumb hole is now to the right of the weigher. If the poise on the front beam needs moving to the right in order to balance the scale this indicates finger weight in the amounts shown on the front beam. A move of the poise on the front beam to the left indicates thumb weight and again the amount would be read on the front beam. The difference must be one ounce of less to comply with ABC/WIBC specifications. 