Abstract: A centralizer for a well casing, and the method of producing a centralizer, for assembly in the field. The centralizer comprises two collars and a plurality of spring bows, with a bow receiving section in each collar for each bow. Aligned channels are formed in the collar defining a passage for receiving a bow end, and each bow end has a hat section for positioning in the aligned channels. After a bow end is inserted in a collar passage, one of the channels is compressed into the bow hat section to lock the bow in place.

Abstract: Individual clamping structures are described which require only two main members, a pivoted handle member and a clamping member journaled on an integral boss on the handle member. Several types of cam structure are described for positively breaking the pressure seal of a closure during relieving movement of the clamp, thereby insuring the release of any residual pressure while the clamp is still limiting closure movement. A further aspect of the invention directs escape of pressurized gas away from the operator by arranging multiple clamping devices so that clamps near the operator remain closed until all other clamps have been relieved. An opening is thereby provided for blast release on the side of the closure opposite to the operator.