I have a 5 1/8" span, and have used it since around the mid-90s.  It's on 8 of the 11 15-pounders I have.  In 2003, I switched from 16# to 15#, and the driller decided I needed to change spans at the same time.  The first three 15s I had drilled had 5-inch spans.  I ended up struggling mightily with them and gave up on 15s until the 16s started making my shoulder hurt every week.  In 2005, I got two new 15s with the 5 1/8" span and have used it ever since.

 

 

This year, I'm using a ball that I normally don't use because of the house pattern.  I've played with the thumb hole since August to match it to the two I like to throw the most, but no matter what I do I can't stop one thing from happening.  At the end of most nights, you could prick my thumb several times and I'd never know it.  I can wiggle it and move it around just fine, but I can't feel anything with it until the next morning.  It was aggravated during a demo day using a 5-inch span with a tighter slug than I'm used to, so now it's taking 2-3 days to start feeling normal again.

 

I'm not sure how to go about fixing this.  I've tried opening up that northern third, but the next week I almost always end up needing to put in tape.  Is it possible to change the pitch of the thumb hole to make it stop?  Do I pull out the three original 15s with the 5" spans and make the top 1/2 inch or so ridiculously wide open?  For that matter, is there something else I can try?

 

Any help would be greatly appreciated. 

Sounds like you are pinching a nerve most likely at the base to the thumb.  Usually this is caused by pitches and or the front edge of the thumb hole resting on the nerve. This will not get better until you make a change somewhere.  I would try a little bevel on the front.  If it gets worse you may have too much reverse and be squeezing the ball. A 5 inch span is a large span.  If your fingers are set at zero you may still be pulling the thumb out of the thumb hole and rubbing the nerve.  I very rarely ever drill fingers at zero because very few people can actually bend their fingers 90 degrees. 

 

Smash49

 

 Yep...see a physician and go to BC.net and get a performance fit.  Your driller is living in the 70's it seems.  Do know that you need to give real measurements to get correctly fit.