Employees were assembling a crawler crane, using an assist crane to lift and set a drum. After the drum was set, an employee climbed to the top of the boom section and began to slide the drum forward with a pry bar to set the pin. The pry bar slipped, causing the employee to lose his balance and fall from the top of the boom. His head struck the deck of the assist crane before he fell to the ground, and suffered a fractured skull.