Well, I was going to refer you to the Unofficial BR FAQ because there was an article there on calluses, but it is not there now - since the update it looks some info is missing. Luckily, I had a copy of it, so here it is:
These tips are from coach Bill Hall and Jayhawk Bowling.
Making changes such as adding wrist devices or other types of gloves can have a
great affect on how your hand fits into the bowling ball. Also making adjustments to your physical game can have a great affect on how your hand holds onto or releases the bowling ball. It is your responsibility to make sure your ball driller is aware of the
changes you are trying to make in your game.
Thumb:
1. Callus on back of the thumb. This is a great indicator that you are knuckling the ball and holding the ball more with the thumb than you are with the fingers. You either have too much reverse in the thumb, causing you to squeeze it with the thumb, or it could be that you are out of rhythm. It can also mean your span is too long.
2. Burn mark on tip of the nail of the thumb which may turn black after a while. This means you have too little reverse or a span that is too long. It is also a sign you are hitting up on the ball at the release point.
3. Calluses on both sides of the thumb. This is usually not a grip problem but more of a sign that you are turning the ball early at the bottom of the swing and probably turning the ball more with the thumb than you are with the fingers.
4. If you have callus buildup on the inside part of the thumb, it only means that you are coming over the top of the ball, which is usually caused by wrapping the ball behind your back during the swing.
5. A burn mark on the front side of the tip of the thumb, that often turns brown. This is a sign that you have too little reverse in the thumb, too long of a span, or are having your swing wrap around behind your back. It can also mean that you are trying to grab the ball at the point of release in an effort to try to put more revolutions on the ball than your physical game is set up to deliver. This is common with bowlers who wear wrist devices.
6. Callus on the front side of the thumb at the base and another on the outside edge of the thumb. Now this is just slight wear from constant repetitions of the ball coming of the thumb, and not any large callus, or blister, or burn mark, or black mark. That is all it is nothing more.
Fingers:
1. Calluses on both sides of the fingers. This is usually a sign of a couple of different things, one being that the finger holes are way too tight. When this happens it causes the skin to spread out and touch the side walls of the holes causing the skin to be irritated. At first there are blisters, soon followed by callus buildup.
The second thing that usually causes these calluses to appear is when a bowler comes over the top (over-turning) at the point of release.
2. Callus buildup on the inside part of the fingers. This is a very common sign that a bowler is turning the hand too early and then trying to correct it at the point of release by trying to turn the hand back behind the ball as the release is completed.
3. Callus buildup on the outside part of the fingers. This is a rare on to see and usually not a flaw in the physical game but a flaw in the grip having too little side pitch.
4. Burn mark on the nails of the fingertips. This is very common sign when the grip is too long or the pitches are tucked too far under and don't accommodate the flexibility or lack of flexibility of this part of the hand.
5. Moving to the front side of the pads of the fingers. Callus buildup is at the tips of the fingers and not centered on the pads of the fingers. This is without a doubt a span is way too long. It is something you see a lot with bowlers that are trying to get more revs on the ball. Of course, it doesn't work out that way and just causes these bowlers to hit up on the ball at the point of release and to get out of the ball late.
6. If the callus is in the center of the finger tip pad. There is nothing wrong with this type of callus as long as it is not overly thick (just normal wear).
If the center of the pad callus turns into a burn mark or brown mark. it is mostly caused by hitting up on the ball at the point of release or having the fingers pitched too far under, causing them to drag as they exit the finger holes.
Whatever you do, do not just leave it up to the ball driller to keep track of where your grip started and the changes that you have made along the way. Request a copy of each of your drill sheets and keep them in order by dating them so that you know what changes were made and when.
The hand is a complex part of the body and many times it just takes small adjustments with the fit to be able to have a comfortable feel, so be sure to discuss them with your ball driller to help solve some of the problems.
The previous tips are from coach Bill Hall.
Normal other than excessive callusing.
From Jayhawk Bowling.
Fitting Tips
Getting just the right fit is the most important aspect of running a pro shop. If a ball doesn’t feel right, the bowler won’t relax his hand in the ball and won’t be able to roll it properly.
Diagnosing problems with balls that a customer brings to you is an important aspect of making a customer feel confident in your ability to drill his new equipment and possibly gain you a new customer for life!! Here are some tips for checking a bowlers hand and calluses:
The Importance of Checking the Bowlers Hand:
Any bowler who bowls too little, or too much, is bound to have problems with his or her bowling hand. The infrequent bowler does not give the hand a chance to become accustomed to the friction caused by the ball leaving the hand.
No matter how good the fit, there must be some degree of pressure on the fingers and thumb. If the bowler does not bowl often enough, blisters may form at the spots of contact. These usually are minor and develop when the occasional bowler bowls too many games in one session.
Normal Callus:
An average bowler will form a callus - a hard, thickened area on the skin - wherever the fingers and thumb have contact with the ball. It is formed because of the weight bearing of the ball on certain parts of the hand. This may be good for a bowler.
Pathological Callus:
Excess pressure, an ill-fitting ball, and improper use of a properly fitted ball may cause what is known as a pathological callus. This is one to worry about because it is painful and greatly hinders a bowler. The pathological callus is a hard mass of skin surrounded by an inflamed rim and it may have a deep central core like a corn. These areas do not stretch when the thumb or finger are flexed. The result is a burning sensation.
