One of the weakest links in the Tetrix kit of parts are the standard 3.75mm shafts used in the set. For the purposes of FTC, they cannot be reliably counted on for any application requiring transfer of force through the shaft and they become marred easily during use. 6mm and larger shafts are much preferred, but what to do about the alignment issue? As far as i know, they don't make bushings or bearings with an 8mm OD and 6mm ID that would fit the set. So i present....
Two different designs: 1) the original 2 part which takes up less space on your channel but takes more hardware and volume 2) the 'LoPro' version, takes up more channel real estate, but much sleeker and easier to mount. Both are designed to use the 6mm bearings available from Andymark or Servocity. 1/4" bearings from Servocity should work as well. Now you can confidently mount bearings on your Tetrix frame and worry less about getting the shaft (insert pun husky) from your robotics parts. Enjoy, use, and please comment to let me know if they're working well or if you have ideas for improvement.