Workers look over the replacement liquid hydrogen recirculation line (left) and the just-removed damaged one (right). Photo NASA/KSC

The replacement of a damaged liquid hydrogen recirculation fuel line aboard space shuttle Discovery has gone according to plan, NASA officials said Saturday.

The original six-foot, 40-pound line with its foot-long dent was removed from the shuttle's engine compartment Friday. A spare line was installed on Saturday, completing a job never done before.

"The work continues to proceed very well at launch pad 39B," NASA spokesman Joel Wells said.

A test to look for leaks in the shuttle's plumbing was scheduled for Saturday.

If no problems are found, the engine compartment will be closed for flight on Monday.

The three-day launch countdown would then begin on Tuesday at 1:30 a.m. EST (0630 GMT), and leading to liftoff on Thursday at 9:18 p.m. EST (17th at 0218 GMT).

Discovery's mission, running two months late because of an assortment of wiring and other technical problems, will last 10 days and feature four spacewalks to repair and upgrade the $3 billion Hubble Space Telescope.