Two crew members were involved in maintenance on a BOP. The task was to remove / free up the hinge pins on a stack mounted accumulator bank. To remove or free up the pins a 1.5 tonne SWL chain lever hoist was attached to the top of the pin by a shackle. The hoist was used to apply tension on the pin while it was being driven up with a hammer. Two hinge pins had been successfully released in this manner when the men set up on the third. While taking weight on the manual hoist the load chain circa 3ft from the hook parted. No injuries were incurred. The lever chain hoist was not overloaded and would have been supporting no other weight than the pin (circa 2-3 kg) and the friction forces due to corrosion etc. The hoist had been independently inspected in October 2005. The crew had visually inspected the tool before use observing no apparent defects.