Grade 5 silicon nitride ceramic.

These silicon nitride bearings are 40% harder than the original fitment steel ball and spherical to within five one-millionths of an inch!

Replace your worn steel ball and get lower noise and longer service life at the same time. 

The third photo shows a microscope image of an original steel ball removed from a good used TD124 MK1. The wear on the original ball is very hard to see with the naked eye and most people don't realise just how badly worn the ball gets from normal use. Under high magnification multiple sets of score marks are clearly visible on the ball's surface! The pictured example is absolutely typical and not a particularly severe case.

The original steel ball is retained by a brass cap which is held in place with four crimps. Carefully pry off the brass cap with a sharp blade and then use a small blob of grease to hold the new ceramic ball in place whilst you re-fit the spindle.