(Updates with end of spacewalk)

By Irene Klotz

HOUSTON, Nov 24 (Reuters) - Two shuttle Endeavour astronauts finished the last of four spacewalks outside the International Space Station on Monday, completing repairs and maintenance meant to restore the outpost to full power.

Stephen Bowen and Shane Kimbrough returned to the station's Quest airlock at 7:31 p.m EST (0031 GMT) after a six-hour, seven-minute outing.

It was the third spacewalk for Bowen and Kimbrough's second since the shuttle arrived at the space station on Nov. 16 for what was scheduled to be an 11-day visit.

NASA managers decided on Monday to extend Endeavour's stay by one day in hopes of achieving one of the mission's prime goals: producing water samples from a urine purification system in time to return them to Earth with the shuttle.

The device, which was carted into orbit aboard Endeavour and installed in the station's Destiny laboratory, has been shutting down due to a suspected mechanical problem.

While Bowen and Kimbrough worked 225 miles (360 km) above the planet outside the station, crew mate Don Pettit and station commander Mike Fincke made a second round of modifications to the urine recycler's centrifuge.

The device is needed to separate solid particles from liquid as part of the distillation process. The purified urine is treated and combined with water recovered from the air and other sources to become drinking water.

NASA needs the new system operating before it expands the space station's crew size from three members to six, a milestone slated for May.

Astronauts on Sunday removed the centrifuge's rubber dampers, which had been installed to mute noise and minimize vibrations, and bolted the device directly into its rack for increased stability.

The centrifuge had been becoming unbalanced as it spun and shut down before its intended four-hour cycle was complete. The refurbished mount worked a little better Sunday night, but still shut down early.

The astronauts planned to add more bolts to make the centrifuge even more stable.

"We'll run the urine-processing assembly again and see if that helped," said flight director Brian Smith.

Bowen and Kimbrough, meanwhile, worked on two rotary joints in the station's exterior truss which are needed to pivot solar wing panels to face the sun for power. One joint was contaminated with metal filings and was cleaned, lubricated and repaired during three previous spacewalks.

The astronauts installed one remaining new bearing in the degraded joint, then lubricated the other joint in hopes of preventing a similar problem.

Kimbrough also installed GPS antennas and retract a latch on Japan's Kibo laboratory. The work will prepare the station for the arrival next year of Japan's new cargo ship and an exterior platform that will be mounted to Kibo for science experiments.

With an extra day in orbit, Endeavour's return to the Kennedy Space Center in Florida was rescheduled for Sunday. (Additional reporting by Chris Baltimore in Houston)

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