I don't see anything that says they have to be covered during a delivery. -- JohnP
I agree it doesn't exactly say that in the USBC Rule book or the Equipment Spec. Manual.
But I've read in other posts/articles where they mentioned covering the hole's.
(One said it this way "By strict rule the thumb is a gripping hole if covered, weight hole if not covered. A second weight hole would make the ball illegal (or shot illegal) if thumb hole isn't covered.")
And after looking at the Manual I figured out where it came from.
In the Spec. Manual page 8 under "
Two-Handed Techniques--->Determining the Grip Center" (Which covers two handed and
thumbless bowling.) it says:
Note:
the ball must be in specification for balance and hole requirements as the ball rests in the bowler’s hand. A
ball may be in specification if oriented one particular way but not another. Compare Figures 11 and 14.
The example in Fig.14 is the not allowed orientation. It's a ball with 2 fingerholes, 1 thumbhole and a balance hole.
The ball is turned so that only the fingers are in the fingerholes. the thumbhole is away from you. In this example they consider the thumbhole a new second balance hole.
So it violates the rule for only having one balance hole and being able to demonstrate, with the same hand, that each hole can be used
simultaneously for gripping purposes (Because holding the ball that way you can't get your thumb in the hole).