You might want to consider my route to battling the shrinking thumb problem which I fight with too. I use a series of preformed vinyl thumb sleeves (which are really designed to be glued into the thumbhole) to gradually shrink my thumb down before I bowl. I know what siae my thumbhole is, so I have to shrink the thumb down to that size before starting to bowl. Yes, it is a hassle to do this, but it does get the job done. One benefit of using the vinly preformed thumb sleeves (the hole is already there) is that they tend to give a little bit when you are trying to force the thumb into a tighter hole. If you try this same deal with a drilled urethane slug, the urethane slug will not give any, making it more difficult to get the thumb into.
However, if using a slug in your thumbhole, I would recommend that you use a urethane slug in the ball, since urethane slugs give you a cleaner release than vinyl slugs will.
Now you are sitting there wondering what the difference is between slugs and sleeves. Well, a slug is basically a solid piece of material (either urethane or vinyl) that is inserted into the ball.....the driller first drills a hole for the slug, then the solid slug is inserted into the ball, then the driller drills the thumbhole inside the slug. The purpose of using a slug is to get a consistent feel from ball to ball, and to prevent your thumb from grabbing the coverstock of the ball. Vinyl slugs are smoother and softer, but they will grab your thumb if your thumb is damp. Urethane slugs are harder, but they give you a cleaner release. Thumb sleeves are basically a piece of material (either urethane, vinyl, or plastic) that has a standard exterior diameter, and the thumbhole is already preformed inside of the sleeve. You can get sleeves in both round holes or oval holes, and the holes come from the factory.
Now I have you totally confused!
