North-American climbing holds have two different types of bolts commonly used. Both are imperial size bolts. The most common type for footholds are a martini-head in the 3/16”-16 size. For larger climbing holds, the prevalent style is socket head in the 3/16”-16 size. Martini head bolts require a 7/32” wrench and socket head bolts require a 5/16” wrench.