Just like the Hyper-Rev Gears, these gear bosses are the first to be mass produced with 'through' to reduce the rotational mass & improve performance! They're made from a high quality delrin for durability as well as having a low coefficient of friction. The Gear Boss is mirrored on both sides, so it does not matter how you install it. Of course, they're designed to work with my Hyper-Rev Gears & Pinions, but will work well with Jag Pinions, Tyco Pinions, BSRT Gears & Pinions, Wizzard Gears, and Quicker Gears.