I actually posted this in another spot then saw this forum..lol anyway I drilled my ball and use a switch grip I removed my thumb insert and drilled my weight hole through the inside of my thumb at an angle to remove side weight and make it legal I did this as not to damage my cover stock cause I like to redrill my stuff and try different things anyway I was told today that you can not put a weight hole inside on a thumb insert and just want to be sure also why can't I?? If anyone knows I would greatly appreciate it the place I bowl the people are miserable  to say the least and love to bash anyone thats bowling good for any reason.. thanks fior any and all help.

Holes
The following limitations shall govern the drilling of holes in the ball:

3: No more than one vent hole to each finger and/or thumb hole not to exceed ¼ inch in
diameter. USBC considers a vent hole to be any non-gripping hole that intersects with a
gripping hole at any depth. Any hole intended for use as a balance hole that intersects with
a gripping hole will instantly be considered a vent hole. This hole may not exceed ¼ inch at
any point throughout the depth of the hole.

Also

Devices
Removable devices shall be permitted provided:
5. No voids shall be permitted under the device

the weight hole would be a void under the side of the insert

It's similar to pitching the weight hole and you can't use the maximum hole size of 1-1/4" when pitching. By drilling a weight hole in the side of the thumb you're expanding the actual size of the drilled hole over the 1-1/4" max. Just my $.02, Bruce