The thrice-postponed second flight of the space shuttle Columbia was rescheduled for Thursday morning after technicians at the Kennedy Space Center in Florida removed two clogged oil filters, replaced them with fresh filters and drained and flushed the lines that carry the oil through the filters to the spacecraft's hydraulic lines.

The shuttle's auxiliary power units also underwent an oil change yesterday after technicians found that five quarts of lubricating oil in two of the three power units that control the shuttle's hydraulics were contaminated with a wax that forms when the oil comes in contact with moisture and hydrazine, the fuel that drives the power units that control the shuttle just after liftoff and at landing.

"Getting to a Thursday launch is a very tight schedule," a statement by the National Aeronautics and Space Administration said yesterday afternoon, "but one which the mission management team feels can be met."

Shuttle operations director George F. Page reset the countdown clock to start at 8 a.m. Tuesday, a decision that was passed on immediately to astronauts Joe Henry Engle and Richard H. Truly, who were at the Johnson Space Center in Houston.

Engle and Truly spent almost 12 hours in the cockpit of the shuttle last Wednesday when they came within 31 seconds of being launched into space with two troublesome power units.

Shuttle engineers had no idea the oil filters would give them any trouble until the onboard flight computers stopped the countdown 31 seconds before Columbia was due to take off.

At the 31-second mark, ground computers handed over control of the countdown to Columbia's four onboard computers, which were confused by low-pressure readings in the oxygen tanks of the three spacecraft fuel cells that supply the astronauts with their light, heat and air.

When the countdown resumed almost two hours later, the oil pressure in two of the three pumps that feed lubricating oil through filters and into the auxiliary power units rose from 60 pounds per square inch to 100 pounds per square inch in 15 seconds.

This told flight directors that the filters in those two units were clogged, that the oil was struggling through a bypass line built into the unit so that oil will continue to lubricate the machinery even if it's contaminated and has clogged up the filters.

Working around the clock Thursday and Friday, technicians first sampled the lube oil in the third power unit, the one whose oil pressure stayed steady last Wednesday morning.

This power unit had been replaced after the first shuttle launch last April. The two troublesome units, their oil and their filters had not been replaced since last April's flight.

The lube oil in the trouble-free unit was found to be uncontaminated, but when the oil was drained from the two other units it was found to be contaminated with a wax that forms from a chemical reaction of the oil and the hydrazine fuel that drives the unit. The filters in both these units were removed and found to be clogged from deposits of the wax.

The filters were replaced, the pencil-thin lines carrying the oil were then flushed with six gallons of fresh lubricating oil and the empty lines then purged with nitrogen gas to remove any traces of the wax. Each unit was then filled with 2 1/2 quarts of new oil.

Had the filters been clogged with a different kind of contamination, technicians would have removed the power units and replaced them with new units, a laborious and time-consuming procedure that would have delayed the launch at least another week.

As things stood yesterday, Engle and Truly will now take off at 7:30 a.m. (EST) Thursday on the first leg of a five-day flight that will take them around the world 84 times before they land, late on the morning of Nov. 17, at Edwards Air Force Base in California. The two men are about to become the first astronauts to fly a spacecraft that has been flown before.