See the shot attached. Theese are the magnets, need a rubber band to keep the balls. Some magnets have some force, others have no force. Go away from here.
The screws provided are M4 and the inner diameter of the tubes is freater than 4mm so is very difficult to center the ball without a special jig or attachment not included in the kit. NOT RECOMMENDED.
Metal ball surface not good quality. Also carbon tubes are very tiny and consequently slightly flexible, where is highly requested to have a stiff support. And i purchase a 300 mm length, imagine the 500 mm....!! This kit in reality is substandard. Better to do not purchase this kit as value of money is not there.