Explanation dimensions Tip Top guides. 
Very useful basic info which will help you to make the right choice.

For example - Tip tops with dimensions 10-2.8
- 10 = approx. the inner diameter of the frame so equal to the outer diametre of the ceramic ring incert
- this size dimention is an indication in mm. and can deviate a bit per brand.  

- 2.8 = is the inner diameter of the tube that will fit the top of the rod
- choose the tube equal to or a bit bigger than the tip - measure the tip with a caliper - glue with epoxy glue