Thumb Slug (Mounted)

A thumb slugis a solid insert, typically made of either urethane or vinyl, which is placed into a bowling ball. Instead of drilling the thumb hole directly into the bowling ball, a hole is drilled and the solid slug is inserted. Then, the thumb hole is drilled into the slug.

Is this something you should be using in your bowling balls? Generally speaking, the answer is yes.

The Purpose of Thumb Slugs

The main reason that bowlers use thumb slugs is because they create a consistent feeling in the bowling ball. Modern reactive bowling balls are made with several different materials, which can create different textures in the thumb hole the farther down it is drilled. The coverstock, weight block, and core materials all have different densities and textures. Additionally, different bowling balls have different cover thicknesses and weight blocks of varying sizes and shapes, meaning that thumb holes will feel different from ball to ball.

Thumb slugs eliminate these inconsistencies by providing a smooth, even texture all the way down the thumb hole in each bowling ball.

Thumb slugs also make it easier to make adjustments to the thumb hole. Tools like a bevel sander will remove material from the thumb hole at different rates based on their densities. The core material of a bowling ball tends to be less dense than the coverstock, which can often make it very difficult to evenly sand out a thumb hole that lacks a slug.

Other Thumb Hole Options

Another option is the Vise’s Interchangeable Thumb (IT) they give you the ability to have a thumb slug that can be switched from one bowling ball to another, thus ensuring the exact same feel regardless of what ball you are using.