This is an optional Exotek DR10 Aluminum HD Idler Gear V2, a hard anodized idler gear machined from quality 7075 aluminum alloy, designed to be stronger and more dependable than the stock supplied part in the DR10 Electric Drag Car Race Kit. The V2 model requires no shimming. 

NOTE: This is stronger than the stock idler gear, but is not indestructible. Use either black or white grease on the gears and re-apply frequently, as well as proper slipper adjustment to prolong the lifespan of this idler gear.