When Davey Moore hit Roberto Duran with a flashy combination in the second round and Duran merely flashed his steely, terrifying smile, the night was all but over. Duran is washed up no mas.

Duran, who was considered the most intimidating figure in boxing before he quit his welterweight championship bout with Sugar Ray Leonard in November 1980, scored a technical knockout of Moore in the eighth round tonight at Madison Square Garden to gain his third title, the World Boxing Association junior middleweight championship. A veteran of 80 professional fights, Duran has also held the welterweight and lightweight titles.

"I was sure I'd beat Davey Moore," Duran, who celebrated his 32nd birthday today, said through an interpreter. "I'm taking things very seriously now." Promoter Bob Arum said Duran may challenge middleweight champion Marvelous Marvin Hagler. Thomas Hearns holds the World Boxing Council's junior middleweight title, but Arum said a Duran-Hagler fight would be more lucrative.

Moore, 24, who was previously unbeaten in 12 professional fights, appeared stunned from the first by Duran's slugging. If Duran's smile was intimiditaing, his punches were worse. And they never stopped. Not until referee Ernesto Magana stopped the fight with 2:02 gone in the eighth and Moore struggling with gravity. Moore's handlers threw in the towel to stop the fight, but Magana said he put a halt to the bout without ever seeing the corner's signal.

Holding a towel filled with ice to his right eye and with his left eye bloody and his face lumpy with welts, Moore said, "I don't believe it.

"In the first round my eye got thumbed but it didn't have much effect. I just couldn't get off. He was tougher than I thought. He was very, very strong."

Asked if he wanted a rematch, Moore managed a weak smile and said, "Maybe when my eye gets better."

Although Moore is a well-regarded puncher, his biggest mistake may have been impatience. From the first round on, he tried to match Duran's charges. The decision proved hasty and wrong.

Duran fought poorly following his loss to Leonard, losing to Wilfred Benitez and Kirkland Laing, but he has recently proved himself as intense as he was when he began his professional career 17 years ago. Duran had previously had difficulties with his weight, but he came into the ring nearly two pounds below the 154-pound limit. Moore was forced to spend part of the afternoon in a steambath to lose three-quarters of a pound.

With the crowd chanting his name, Duran came after Moore with hard right hooks to the body and combinations.

Duran quickly proved his experience in the second round when he fought his way off the ropes and turned Moore around. He hit Moore with a number of hooks and uppercuts that drew blood from Moore's mouth and forehead.

Moore occasionally threw some effective counterpunches in the third and fourth rounds, but Duran continued to score more effectively on the exchanges, keeping his head down and his fists in Moore's face and solar plexus.

Duran laughed off Moore's increasingly feeble blows in the fifth round and began to get stronger as Moore began to fade. After a relatively slow sixth, Duran went for the knockout in the seventh and almost scored it.

Begining with a tremendous hook that buckled Moore's knees and sent him reeling, Duran's onslaught lasted nearly the entire round. With 16 seconds left in the seventh, Moore went to the canvas for a count of nine after Duran rapped him with an overhand right. Moore struggled to his feet and slumped against the turnbuckle.

The minute between rounds did little to revive the champion. With both eyes swollen, Moore was defenseless and Duran duly relentless, driving numerous hooks to Moore's temples. Through the wild cheering, a few were heard to call for the fight to be stopped, and the referee finally complied.

Duran, a champion once more, was exuberant after the fight. Although his nemesis, Leonard, is retired and out of reach, the man with the odd grin and the unquestioned punch seemed happy with his new station and renewed reputation.

"I wasn't surprised," said Duran.

In winning, Duran becomes only the seventh fighter in history to win championships in three weight classes. The other six: Bob Fitzsimmons, middleweight, heavyweight and light heavyweight; Tony Canzoneri, featherweight, lightweight and junior welterweight; Barney Ross, lightweight, junior welterweight, welterweight; Henry Armstrong, featherweight, welterweight and lightweight; Wilfred Benitez, junior welterweight, WBC welterweight and WBC super welterweight (junior middleweight), and Alexis Arguello, WBA featherweight, WBC super featherweight and WBC lightweight . . . The gate was the biggest at the Garden since the second fight between Muhammad Ali and Joe Frazier on Jan. 28, 1974. Moore received 40 percent of the gate and Duran received 25 percent, according to promoter Bob Arum. The gate was put at $964,305 and the crowd was a sellout of 20,061 . . .

In a preliminary bout, Alfredo Escalera, who lost his WBC 130-pound title to Arguello in 1978, won a unanimous 10-round decision over Gene Hatcher.