The discovery five weeks ago that a customer of Reynols Metals Co. had been shipped defective aluminum plate has triggered a nationwide search for others with similar material.

Federal investigators are poring over records back to January 1977 to see if aluminum used to build everything from rocket engines to fuel tanks for the space shuttle might have come from suspected batches of "soft" aluminum produced by Reynolds at its McCook, III., foundry.

So far, Reynolds said in a statment released late yesterday from it Richmond headquaters, more than 20,000 parts have been inspected by customers "and 99.95 percent . . . passed inspection."

Investigators include teams from the Pentagon, the Federal Aviation Admintion and the National Aeronautics and Space Administration. Hardware being inspected includes the Pentagon's Titan rocket, most of the new airliners licensed by the FAA and just about every rocket used by NASA to launch satellites and spacecraft.

Under intense scrutiny were four huge tanks produced by Martin-Marietta Corp. to carry liquid oxygen and hydrogen for the returnable space shuttle. Built almost entirely of aluminum, these tanks are 154 feet long and weight 75,000 pounds when empty. Filled with more than 500,000 gallons of fuel, they weigh 1,625,000 pounds.

"We have a team of 10 checking every part and every piece of aluminum we bought for more than two years," said a spokesman for Martin-Marietta in Denver, where the fuel tanks are built. "We haven't found anything wrong yet but it will take us at least two to three weeks before we know for sure if we're out of the woods."

Space agency sources said some aluminum produced by Reynolds at McCook may have had some areas as much as 40 percent softer than other sections.

The reason is that the huge plates were not quenched with water uniformly when they emerged hot from the foundry. This could have produced aluminum that was hard on one side and soft on the other. Clogged water nozzles and defective pumps may have been responsible for the faulty spraying, sources said.

Milling and machining of the plates, plus test of finished parts, would have turned up the softest aluminum, sources said. Further tests of parts, tanks and rocket engines made from Reynolds aluminum would also have yielded defects.

Typical of the search was the NASA investigation of all receipt records from January 1977 to June 1979 for aluminum bought for the Delta rocket fuel tanks and engine that will be used tomorrow to launch a Western Union communications satellite from Cape Canaveral, Fla.

"Of 111 large pieces of aluminum we bought, nine came from Reynolds," said NASA's David Grimes from Cape Canaveral. "We put traces on those nine pieces and none of them were withdrawn from inventory to go into the Delta to be launched on Thursday."