I have linked to this article so many times I thought I'd make it a topic on iTopSpin.com:
Balancing Plastic Snap Cap Spin Tops
by Herman Lau
Many people have a problem learning the spin top because the tops they use are not balanced. The tops spin very roughly, vibrate or wobble making it impossible to do many of the tricks. As a result, many people mistakenly believe that the tops are not well made. The following method of balancing a top applies to any plastic spin top with a snap off cap. These type of tops are still being made by several manufacturers and readily available.
1. Find the cap position that gives the least amount of vibration. Spin the top and note how smooth or rough it is. Then take the cap off, rotate it 180 degrees and replace. Spin the top again. Is it smoother? If not, return cap to its original position. Occasionally, for some tops, that is all that's necessary to balance your top. In the vast majority of cases however, the top will still need balancing. Proceed as follows remembering to keep the cap in the same position throughout the remaining steps.
2. Get some Blue-Tac. They are also known as Stic-Tac, E-Z Tac, Fun-Tac and other names. It is a sticky clay-like substance used mainly as an adhesive. They can be obtained from discount drug stores, stationery stores and supermarkets. They are usually blue, but they do come in different colors. Break off a small piece the size of a pea. Knead it into a ball.
3. For clarity and ease of explanation, look at the inside of the spin top cap as if it was the face of a clock with the two prongs of the cap at the 12 and 6 o'clock position. Stick the ball of Fun-Tac at the 3 o'clock position on the inside rim of the cap next to the edge. Replace the cap and spin the top noting how smooth of rough it spins. Next remove the ball of Fun-Tac and stick it at the opposite side ( in this example with the blob of Fun-Tac at the 9 o'clock position). Replace the cap and spin. One side will always give a smoother spin than the other. Observe which side gives the smoother spin.
4. At this point, if the vibration has not been eliminated, it should be considerably dampened. In most cases you can still fine tune the top by moving the pellet of Fun-Tac up or down along the side you found in step 3 that gave the smoother spin. The top should now be balanced enough that you can use it for any top tricks from the simple to the most difficult. It is at this point that you've found the point on the cap perimeter where proper weight would give a perfect spin.
5. If the spin is not perfect, i.e., if there is still a slight toughness while it is spinning, then the final step is changing the size ( and therefore the weight) of the Fun-Tac by adding more or using a smaller pellet at the some point you found in step 4. Once balanced, the point of a perfectly balanced top should be absolutely still when spinning on a hard surface or on the palm of your hand. In fact, if the top is perfectly balanced, you will definitely feel a pronounced eerie lightness to it while it is spinning on your palm.
6. Additional tip: Make sure the side of the point is smooth. If not, scrape it smooth with a knife, sand it down, or replace the tip. Otherwise, even though the top may be perfectly balanced it will feel slightly rough while spinning on your palm. With practice, you should be able to balance any plastic snap cap spin top with the above method in less than 5 minutes.