Individual 6x12x4mm rubber sealed "Speed" wheel bearings.
ProTek R/C "Speed" bearings are high quality steel/chrome bearings designed for the R/C racing enthusiast.

These 6x12x4mm rubber sealed bearings are the standard bearing for many monster trucks, such as the Traxxas T-Maxx, E-Maxx and related vehicles. Please check the manual that came with your vehicle for the correct size if you are not certain.