2.5 Wheel Spacing and Adaptation Requirements
2.5 (1) A wheel spacer fitted to a low volume vehicle between the wheel and hub assembly, other than that fitted as original equipment by a high volume vehicle manufacturer, must:
(a) have a maximum spacing of each wheel away from the hub surface of 20mm; and
(b) be set-screwed or attached by another secure method to either the wheel or hub face; and
(c) be manufactured from a solid block of suitable material; and
(d) have two machined or die-cast surfaces that are parallel, and contain minimal indentations or irregularities; and
(e) be fitted as to ensure the wheel locates snugly over the hub spigot so that the hub carries the weight of the wheel assembly instead of the wheel studs, or where there is a mis-match between the hub spigot and the wheel center, a close tolerance center bore locator must be provided; and
(f) maintain, with the spacer fitted, not less than the minimum required amount of wheel stud or bolt engagement specified in 2.4(4).
Read more
here