Hey Brunner,
Here's a list of what possibly could be causing your problem:
1) Bevel or edge of hole is too sharp
2) Span too long causing excessive rubbing..joined by a sharp edge(reason#1)
3) Hole is too large and is causing you to squeeze to hang on (very common)
These are the most common reasons that I've seen when a bowler has the blister at the base of the thumb. Hopefully one of these reasons helps you rectify your problem.
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Rick Leong - Ten Pins Pro Shop
Track Intl. - Amateur/Pro Shop Staff
Vise Inserts Staff
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