This is a pack of ten 5x10x4mm rubber sealed "Speed" 1/8 clutch bearings. ProTek R/C "Speed" bearings are high quality steel/chrome bearings designed for the R/C racing enthusiast. 

These 5x10x4mm rubber sealed bearings are the standard bearing for the 1/8th scale nitro clutches. Some racers prefer the rubber sealed bearings for better protection against dirt for their clutch. Please check the manual that came with your vehicle for the correct size if you are not certain. 

These are also used as wheel bearings in the Kyosho Lazer ZX-5 1/10th scale 4WD electric buggy.