Abstract: At 9:27 p.m. on Monday, February 15, 2010, the second shift five employee oil workover rig crew removed the master valve and then inspected the tubing hanger locking screws and packing glands at an Occidental of Elk Hills, Well Number 627-13Z. High pressure from the well casing bore lifted the 70,700-pound hanger string releasing high pressure gas to the atmosphere. Two of the ELCO ET Style Lock Screws were not removed or installed according to the manufacture instructions & procedure and the Lock Screw Pins did not engage and secure the tubing hanger casing it to lift from the well casing bore flange. The pressure blast injured Employee #1, #2, #3, and #4, who were working at the well head. Employee #1, #2, and #3 were hospitalized for more than 24 hours with broken right legs and related leg injuries. Employee #4 was treated and released for an injured ear drum. The incident was reported to DOSH at 1:33 a.m. on February 16, 2010.