BKCJ-0402 - 12 tooth, XD-15 Ceramic Hybrid Bearing, Machined Delrin Derailleur Pulleys for 12 speed SRAM Road and Gravel Derailleurs (4mm bolt upper/5mm bolt lower)The ultimate upgrade pulley. Guaranteed to last forever against corrosion. Precision machined Delrin Derailleur pulleys featuring XD-15 Ceramic-Hybrid, radial bearings for enhanced shifting performance and longevity. Designed and manufactured for performance, these pulleys feature tight clearance bearings in the upper pulley to maximize shifting accuracy and loose clearance bearings in the lower pulley to allow for chain-line movement. 500 AF Delrin pulley material features a high mechanical tensile strength and rigidity, natural lubricity, high resistance to moisture, caustic chemicals and UV. This makes these pulleys highly durable, rigid and very smooth. Delrin also machines very cleanly and precisely. The inherent lubricative qualities of this Teflon-infused material greatly reduce friction when it interfaces with the bicycle chain. When compared to other pulley materials, 500 AF Delrin with Teflon offers significantly less drag than pulleys machined from aluminium or titanium.An Enduro Bearings exclusive, our XD-15 Ceramic-Hybrid bearings combine corrosion-proof, cryogenically treated nitrogen steel races with Silicon Nitride Grade 3 ceramic balls in a proprietary process that results in the ultimate high performance bearing setup. The deep-groove design raceways allow for maximum sized Grade 3 Silicon Nitride ceramic balls for increased contact area and improved performance and longevity. The 90% fill Kluber NBU52 grease only runs better and smoother with time and is highly resistant to being washed out. It is engineered to stick to the balls and continually be pulled back into the ball path where it is needed.This 12 tooth pulley set with 4mm upper and 5mm lower bolt-holes is designed as a direct fit upgrade for SRAM 12 speed road and gravel derailleurs. Enduro Derailleur Pulleys are available in a number of configurations and performance levels, starting with Ceramic-Hybrid, Black-Oxide treated Zero Ceramic as a step up, and XD-15 Ceramic-Hybrid at the top of the line. All are proudly precision manufactured in the USA.