I am not sure exactly what my pitch is on my thumb.  However, the back (or knuckle side) of my thumb starts to feel "raw" after I bowl.  I do have my stuff drilled with thumb slugs, and I ussually have a peice of black tape on the back side of the thumb.  Is this a sign that more reverse pitch could be used on the thumb.  I just bought a used ball and was going to experiment with the pitch a little to fine tune the fit before trying it on a new ball.  The pad side of my thumb feels fine and I have no joint or interrior pain in the thumb.

To rub the back of your thumb you are generally bending it some.

I would first try to tighten the hole up by using tape to where you can’t knuckle the ball.

There are several reasons for knuckling at thumb. The hole can be too big or you can have too much reverse making you feel like you are going to loose the ball unless bend your thumb to hold on are the main reasons.