Abstract: A blowout preventer of the cross ram type has at least a first and second pair of rams which are in the same plane or on the same level. The rams are reciprocable in insertable sleeve-like ram guideways which are replaceable to accommodate different types of rams; for example, a pipe ram or blind ram, or different configurations of rams from different manufacturers. This provides a blowout to preventer which has flexible functions and a low stack height and low weight. A spherical shape also provides lower weight, ease of guideway insertion, and reduction of stress concentrations.