${\mathrm{A}}_{\mathrm{bolt}}=\frac{(480 \mathrm{kN})(1,000 \mathrm{N}/\mathrm{kN})}{(4 \mathrm{bolts})(2 \mathrm{surfaces})(168.75 \mathrm{N}/{\mathrm{mm}}^2)}=355.56 {\mathrm{mm}}^2$

Determine the minimum bolt diameter from:

$\frac{\mathrm{\pi }}{4}{\mathrm{d}}^2=355.56 {\mathrm{mm}}^2 \therefore \mathrm{d}=21.28 \mathrm{mm}$