Finger inserts line the gripping holes, while a thumb slug forms the surface of a drilled thumb hole; bowlers may also use tape inside a gripping hole to fine-tune fit. USBC rules say plugs, inserts, tape and slugs may not extend beyond the ball's surface, and its equipment manual allows tape in a gripping hole. Have a pro shop match the fit and check the current rules before changing a ball.
Inserts and slugs are part of the fit between a bowler and a drilled ball. They should support a relaxed grip while keeping the ball compliant with the rules for your league or tournament.
The parts in a drilled ball
| Part | Where it goes | What to know |
|---|---|---|
| Finger insert | One or both finger holes | Replaceable material matched to finger size and feel |
| Thumb slug | Thumb hole | Creates the surface of the thumb grip; not the same as every interchangeable thumb device |
| Grip tape | Inside a gripping hole | Adds or changes contact and fit; use only in the gripping hole and within the current rules |
USBC’s current playing rules state that plugs, inserts, tape and slugs may not extend beyond the ball’s surface. The equipment specifications manual says tape in a gripping hole is allowed. Because equipment rules can be revised and a particular tournament may set procedures, check the current USBC rulebook and ask the league or tournament official before making an in-competition change.
Fit comes before material
Start with a pro-shop fitting for span, pitch and hole sizes. Inserts can be changed when their surface wears, but a different material or shape is not a substitute for a correct span. A grip that requires squeezing can affect release consistency; looseness can also change how the ball leaves the hand. Do not cut, glue or reshape a part without the driller’s instructions.
When buying a first ball, follow the beginner equipment order and choose the correct ball weight. Once the fit is set, ask the pro shop how a pin-to-PAP layout applies to your ball.
If you are comparing external wrist support, see wrist supports and positioners; those devices do not replace a correctly fitted span and grip.
Browse grips and accessories.
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Common questions
What do bowling ball finger inserts do?
They provide a replaceable contact surface in the finger holes. The pro shop selects the insert size and fit for the bowler's fingers and release.
What is a thumb slug?
A thumb slug is a material installed in a drilled thumb hole to create its gripping surface. A slug is different from a switchable thumb system, so ask the driller which part the ball uses.
Can I put tape in a bowling ball hole?
USBC's equipment manual allows tape in a gripping hole, and its playing rules require that plugs, inserts, tape and slugs do not extend beyond the ball surface. Check the current manual for the event you bowl.
Should inserts feel tight?
They should fit the bowler's grip without forcing the fingers or requiring a squeeze. Have a pro shop adjust a fit that slips, pinches or changes your release.