This is an 8mm eccentric for use with the openbuilds wheels and 688Z bearings. These also have a longer 12mm insert sleeve so the aluminum plates can hold the eccentrics with less chance of deformation and the bolts is better supported in the middle of the support plates. As this is designed for heavier duty builds the assumption is a minimum of 1/2" plates for mounting the wheels.
8mm eccentric spacer 2017-10-25
Larger eccentric nut/spacer for more rigid builds