The Magnum's allow you to streamline your printer setup - eliminating 2 of the shoulder bolts entirely and allow you to "stack" your smaller toothed idlers with the Magnums. The Magnum Pulleys also simplify the kinematics of the belt system for your printer. In additional to all of that, they look super awesome!
They have been several iterations of these idlers, as well as numerous design tweaks and quite a bit of testing! We hope you enjoy them!
If you are using the "china" idler pulleys, you can use the existing 20mm shoulder bolt, but if you are using the "Gates" idlers, you will need to purchase 25mm shoulder bolts.