NASA delayed a key shuttle booster test Monday because of an unexpected problem discovered in a test last week that revealed hot gas made it past a small washerlike seal in a redesigned O-ring joint.

But Royce Mitchell, a top booster engineer at the Marshall Space Flight Center, said he was confident the problem can be resolved quickly and that it will not affect the schedule for the first post-Challenger shuttle flight, now planned for late August."It's a headache," he said of the problem. "We're being very cautious. We want to do the right thing, and we're taking the time to do the right things."

The rubber O-ring washer that failed was one of two used to seal up a leak test port, an opening in the joint that allows engineers to apply pressure to the joint's O-ring seals to force them into the proper position.

The leak port is closed prior to launch by large plugs. Small rubber O-rings are used to ensure a tight fit.

For a subscale booster test last week at rocket-maker Morton Thiokol Inc.'s Brigham City plant, engineers created a deliberate flaw that allowed hot gas to reach a nozzle joint's large secondary O-ring to prove the redesigned joints can tolerate major failures.