What you have is a stripped drive gear. Remove the cover from the motor unit and you will find the remains. The part number is 41A2817 and is available at any good door opener service company. It comes with instructions and is available for around 20 to 25 dollars.

Listen, take my advice and get a new opener. The Craftsman garage door openers are the cheapest, horribly engineered, unreliable pieces of crud on the entire planet. Sound like an exaggeration? Not really. Just research this forum. There are more questions about those things than anything else period. Hundreds of questions. I have worked on them and they are just awful. If you got 14 years you are lucky. Just get a Genie Excelerator and be done with it. Quiet DC motor, twice as fast, two bulb light, reliable.

I can't comment expertly on if Sears garage door openers are junk or not, mine went 16 years before I needed to put a $40 gear kit in it, so I'm happy. It is actually a Liftmaster, and I think it may also be a Chamberlain. Sears just re-brands most of their stuff anyway.
I assumed the many problems listed on this forum for the Sears openers was because they were so ubiquitous.
Anyway, here are my pictures from doing the gear change you need to do, and at the end are the scanned in instruction sheets. This is the entire gear assembly replacement, which is easier than just the gear, and only costs about $10 more than just the gear. http://dl.dropbox.com/u/1347192/lifter/liftmaster_repair.html
I had one question though, how heavy should the door be when it is considered properly balanced, as this would certainly affect the wear of the gears. I put a bathroom scale under my 2 car steel door, and it read 35 lbs of weight on it. Is that too much?

Here is a link that might be useful: Sears garage door opener gear replacment with pictures and instruction sheets

Yes. Thirty-five pounds IS too heavy. A properly sprung door that is tensioned correctly will sit on the floor with as little as five pounds of weight.

An electric door opener is not a crane or winch. It is not designed to do any more work than what a child should have to exert in order to open the garage door. If you did not have an operator and your child slipped and fell while struggling to open the door, 35 pounds across his neck is enough to cut off his air supply and kill him. Your door needs to have a spring adjustment and this is best done by a PRO who knows the risks involved.

Chamberlain is the manufacturer and they were and probably still are, the largest operator manufacturer in existence. Sears either owns them outright or owns a controlling interest in them. Liftmaster is the brand of operator that Chamberlain themselves marketed. However, Chamberlain made electric operators for a large number of chain stores which were sold under the proprietary brand of those chain stores.

One of the reasons why you see many people asking about Craftsman operators is because of the huge number that have been sold. People trust the Sears name. I've been out of the door biz for twenty-years but when I had my company, I sold LiftMaster. I found them to be a fairly well-made unit compared to the Stanley operators of the day. As for Genie, those were the worst piece of crap we ever worked on back then. I can't speak for them now because I have no hands-on experience. Perhaps they've improved. My fave was "Automatic Doorman by Lynx Industries". Simple, easy to repair back then and highly reliable. Unfortunately, logic boards crept into their design too.

Ugh....give me relays and micro switches any day.

Thanks, I'll have the professional repair guy check the springs. In the past they gave me the generic "can your wife lift the door?" as the check for the proper weight. The door will sit fine at the half up point, not going up or down, but like I said, it weighs 35 lbs when fully down. Seems like if the springs were tightened to make that a lot less, it would fly up once it was raised 2 or 3 feet though.
I'll have him check it. I watched them tighten the springs once, looks dangerous alright.
Thanks again, Don

When a door is manufactured, the complete door face including all of the metal hardware that is attached to the door face.....that moves as a unit when the door is raised and lowered is put on an accurate scale and weighed.

Taking into consideration the width of the door, the height of the door, the weight of the door and the cable drums selected, charts are consulted to calculate the correct springs. The inside diameter of the spring, the diameter of the wire used to wind the spring and the length of the spring are then determined.

There are other considerations such as how much "lift" or vertical travel the door face must make before it begins to become horizontal. Also, the life of the spring is also determined and the life is spoken about in "cycles". One cycle represents the door opening and then closing fully. Many residential door springs have an estimated life cycle of 6000. High usage industrial doors may use springs that will live for 100,000 cycles of operation.

How well a door is balanced is relative to how good the person who calculated the springs was. It is also relative to how good the installer was when he put the door in. All too often, doors are slapped in on a flat rate basis. As long as the door goes up and down, the installer just rushes to the next job because the more doors he slaps in per day, the more money he makes.

IF......the springs on your door were calculated properly and the door was installed properly, then the door should sit on the floor with about 5 to 10 pounds of weight max. If you lift the door 1 foot and let it go, the door should stop right there. It should not try to rise up nor should it try to fall lower. You should be able to do the same thing at any point in the vertical travel of the door.

When the door is fully open, it should not rush up in the last foot or two of travel. It should sit just above the opening in a position that allows full clearance of the opening. In other words, if your opening is seven foot high, then the bottom edge of the door should be slightly more than seven feet off the floor. The door should not creep back down into the opening either.

The door should move smoothly up and down. It should not rub the tracks, squeak, squeal or bind anywhere. Torsion springs should have a light coat of motor oil on them to prevent rust and keep the coils from binding on each other.

The springs should have the exact same number of turns on them because they work in unison. If one spring is wound tighter than the other, then that shortens the life of that spring. All springs will settle in after installation and lose a bit of tension. Installers often put extra tension on the springs when they first put them in to compensate for this tension loss so they don't have to return on their own time to do a door adjustment under warranty.

Over time, the springs will continue to lose a bit of tension but the biggest loss is within the first 6 months.

And yes, if you don't know what you are doing, spring adjusting can be very dangerous.