At 7:38 p.m. on Aug. 18, a half-hour after a Soviet diplomat's son flew out of Dulles to end one U.S.-Soviet confrontation, a Soviet embassy air force attache carrying 60 pounds of government documents walked out of a McLean office and into the arms of FBI agents.

So ended a 2 1/2-year effort by the FBI and Armand B. Weiss, an editor of technical publications and former government consultant, to trap Yuriy P. Leonov, a lieutenant colonel in Soviet military intelligence.

Four days later Leonov left the country. The incident became public this week only after the Soviets announced that they had expelled two Americans they said were spies.

The expulsions seemed to be the superpowers' latest muscle-flexing, with suggestions that high-level spies with vital national security information had been discovered in each other's countries. Yet Weiss, who the FBI confirms was the key player in the Leonov case, described a Soviet operation that would never make it into a John le Carre novel or a James Bond movie.

In interviews this week, Weiss said that Leonov never took any classified documents or vital secrets from their two dozen meetings. When Leonov pulled cash out of white envelopes to pay Weiss, he said, at times there were 100s and 50s, but in all Weiss got $1,800 for eight document deliveries.

The Soviets, Weiss said, were "skinflints."

In all their encounters, no lives were threatened, no guns drawn. When the FBI agents stopped Leonov, there was but a light push against Weiss' parked car.

Weiss said Leonov once took him to lunch at Evans Farm Inn, a hangout for CIA officials. The closest Leonov came to doing anything surreptitious was his habit of parking his car at Tyson's Corner in suburban Virginia and taking one or two buses to Weiss' office in McLean.

"It sort of exploded my myth of counterintelligence," Weiss said. "I always thought these guys would be slick and smooth and suave, and have a great command of the English language. As a person Yuriy was a nice guy. But he was not smooth."

Weiss said he was impressed with Leonov's perseverance. The Soviet colonel clearly was willing to spend years cultivating a source who once had a top-secret clearance and might still have access to information about U.S. strategic nuclear forces.

The "shopping lists" of documents Leonov requested, Weiss said, ranged from the cost-effectiveness of weapons to tips on how to prevent pilots from succumbing to air sickness.

Most of the material Weiss delivered is available at the Library of Congress and offices around town, though Weiss said he believes some of it should not be open to the public.

"I told a group just last night that they should be careful who they talk to, even about unclassified material," Weiss said yesterday.

Weiss' entry into the world of spies began just before Christmas, 1980, when Leonov walked into his office at Associations International Inc. and asked if he could buy a book entitled, "Force-on-Force Attrition Modeling." The Soviet spoke with a thick accent, but didn't identify himself.

A week later, Leonov left his name and address. On his third trip to Weiss' office, Leonov said he was an air attache at the Soviet Embassy, and had been a MiG 21 jet fighter pilot. Weiss said they then began discussing planes when Leonov asked him a highly technical question about radar wavelengths.

"It was not the kind of question the average guy would ask," Weiss said. "When he left I sat down and had a chat with myself." Weiss said it began to dawn on him that Leonov might be a spy, so he called the FBI.

Weiss said a "bored-sounding" telephone operator at FBI headquarters took his message. The next morning an agent arrived and asked Weiss if he would help.

Over the next two years, Weiss said he became Leonov's librarian.

The attache would bring in requests for government documents and private scientific papers on topics ranging from "A Model for the Study of High Energy Lasers in a Space Defense Role" to a "Catalog of Wargaming and Military Simulation Models" put out by the Studies Analysis and Gaming Agency of the Joint Chiefs of Staff.

"He said he needed help getting the material because Americans didn't like Russians and won't let us come to meetings or send us documents," Weiss said.

Gradually, Leonov began to ask for more sensitive documents, Weiss said. And the FBI told Weiss to lead him on. Weiss said FBI agents once told him to show Leonov a fancy briefcase containing unclassified MX missile reports they had stamped "Secret."

The only time Weiss gave Leonov a real "Secret" document was the last time they saw each other, Weiss said. Leonov had insisted that Weiss get him the three-volume proceedings of the "Twentieth U.S. Army Operations Research Symposium." One volume was classified.

The FBI had planned to detain Leonov on the night of Aug. 12, after Weiss had turned over that classified document with other material.

But the date and turning over of the "Secret" document had to be postponed after the State Department received word that Andrei Berezhkov, the Soviet diplomat's son, was seeking asylum in the United States. The Soviets insisted that the youth wanted to return to the Soviet Union with his family.

On Aug. 17, with the Berezhkov dispute still unresolved, Leonov called Weiss at home and said he would stop by Weiss' office the next evening. The following afternoon the youth held a news conference at Dulles International Airport, said he wanted to go home and flew out at 7:08 p.m. for Moscow.

In nearby McLean, Weiss said he turned over the "Secret" document and a stack of other technical reports to Leonov.

Weiss said, "He'd flip through it and then ask, 'How did you get this?' I said a friend lent it to me and I had to give it back in a few days."

He said Leonov handed him four $100 bills and two $50s and walked outside. A dozen FBI agents were waiting to retrieve the documents and, after a few minutes, let him depart emptyhanded.

Because he had diplomatic immunity and could not be arrested on espionage charges, Leonov was declared "persona non grata" in the United States the next day, which means he was expelled from the country.