This is a pack of ten 8x16x5mm rubber sealed "Speed" 1/8 wheel bearings. ProTek R/C "Speed" bearings are high quality steel/chrome bearings designed for the R/C racing enthusiast. 

These 8x16x5 rubber sealed bearings are the standard bearing for 1/8th scale off-road vehicles, both for internal bearings in the diff cases as well as external wheel bearings. Many racers prefer the rubber sealed bearings for better protection against dirt. Please check the manual that came with your vehicle for the correct size if you are not certain.