Two tiny plastic inserts worth less than a nickel apiece and a magnetic sensor no bigger than the head of a pin failed in the $2 million spacesuits worn by two astronauts in flight last month, forcing cancellation of their planned spacewalk the day before they returned to Earth on the latest mission of the space shuttle Columbia.

So minuscule were the causes of the double-suit failure that the National Aeronautics and Space Administration said yesterday it will reschedule the canceled spacewalk for the next flight, which is due to take place the first week of February.

Planned for just three days, the next flight will be extended to five days to accommodate the spacewalk.

The walk in space will be made by astronauts Story Musgrave and Donald H. Peterson, who together with commander Paul J. Weitz and pilot Karol Bobko will make up the four-man crew aboard the space shuttle Challenger, which will be flying into space for the first time.

Previous flights have been made with the first shuttle, Columbia. The spacewalk will be the first American spacewalk in nine years.

Inspection by technicians at Houston's Johnson Space Center showed that the device that was supposed to regulate the oxygen pressure inside astronaut William B. Lenoir's spacesuit was missing two tiny plastic inserts that help to hold a pair of screws against a metal plate.

Documents showed the inserts were made last spring, removed during a test last August and never re-inserted to reseat the screws.

Further inspection of the spacesuit worn by astronaut Joseph P. Allen revealed that one of two tiny magnetic sensors that run a fan feeding oxygen into the suit apparently failed just after Allen put on the suit. No bigger than a pinhead, this sensor may have been damaged prior to flight.

Its failure was enough to shut down the fan that circulates cooling water through the suit at the same time it supplies oxygen. The motor that runs the fan has no magnetic "brushes" like most electric motors because of the risk of a spark in the pure oxygen circulating through the spacesuit, developed in a program costing $236.4 million.

At a news conference yesterday in Houston, Lenoir said he was not as sick as the news media suggested the day flight directors postponed the walk one day to give him time to feel better. Lenoir conceded he vomited the morning of the scheduled spacewalk but said it was not like being sick on Earth.

Said Lenoir: "It was more like a wet belch. I never really felt vertigo or motion sickness. I could have done the spacewalk."

Sources at the Johnson Space Center said the seven-man panel investigating the double-suit failure would not release their findings until later this week, in part to double-check the work of the 20 inspectors who pored over the suits and in part to make sure there were no contributing factors to the suit failures.