Customs duty cap for rock drilling bits limits excess duty payable above a prescribed ad valorem rate on imports. Exemption of imported rock drilling bits under Customs Tariff headings 8207.11 and 8207.12 exempts from customs duty so much of the duty as exceeds the amount calculated at the rate of 40 per cent ad valorem; the exemption is effected under the executive power in the Customs Act and comes into force on 28 February 1986.