When Elvin Hayes stepped to the foul line with 3:20 left in the third quarter, he didn't need the encouragement of the Capital Centre crowd to tell him he was only two points short of 20,000 for his career.

"I'd be lying to you if I told you I wasn't keeping track with a computer in my mind," said Hayes. "And was it something that the points came on foul shots?"

With the 7,621 in attendance standing and cheering, the Bullet forward, sank both free throws to give him 34 for the game and become only the 10th man in NBA history to hit the 20,000 figure. Only four players-Wilt Chamberlain, Oscar Robertson, Kareem Abdul-Jabbar and Elgin Baylor-have done it in less time than the 11-year veteran.

Hayes who has been plagued with foul-shooting problems much of his career, added two more free throws before the end of the period to finish with 36 and sat out the rest of the Bullets' 134-117 romp over the San Diego Clippers. It was a marvelous individual performance that also included 14 rebounds, 12-of-20 shooting and four blocks.

"This is very gratifying," said Hayes. "I was surprised to it on free throws. I guess I would have liked it more on a turnaround. But it was very ironic that I did it against San Diego, the city where I began my career."

The coach of the current San Diego team, Gene Shue, helped engineer the deal that brought Hayes from Houston to the Bullets for Jack Marin. Shue modestly described the deal last night "as one of my better trades but I thought he'd be a good complement to Wes Unseld and it's worked out that way."

Hayes was a center when he came to the Bullets but he has developed into a vintage forward, especially in the last three years under Coach Dick Motta.

Both old and new Hayes were on display in this game. He showed all the aspects of his offensive game - the turnaround jumper, the dunk, the twisting baskets off rebounds - plus the latest refinement: deliberate, well-executed passing.

Once, he tossed a length-of-the-court pass to Greg Ballard off a missed San Diego free throw. He picked up two assist on crisp, short tosses, inside to wide-open team-mates.

"That's what pleases me," said Motta. "I didn't know if he could make the transition from center to forward after the trade, but now he's passing so much better and his court awareness has grown in the last 2 1/2 years. And, hell, I never thought he'd hit 20,000 on free throws.

Hayes wanted to reach the 20,000 plateau before the home crowd and he came out as intense as he has been all season. He made three of his first 11 to score 22 by intermission.

"I knew San Diego had played the night before and might be tired," he said. "I really tried to come out running and get into the game quickly. You don't score a lot of points usually in the second half so I wanted to get as much as I could in the first and then let the second take care of itself."

His teammates, well aware that he needed 34 points to reach his goal, helped out.

"We read the paper and we knew what was going on," said Tom Henderson, who had a 23-point effort overshadowed by Hayes' achievement. "We wanted him to get it and we wanted to win. It's a great tribute to him to score so many points."

Anytime Hayes came free, he got a pass, although at times the Bullets tried too often to force the ball to him. The closer he came to 34, the quicker the tempo became. He reached 30 midway through the third on a 12-foot jumper, then hit 32 off a fine baseline pass from Mitch Kupchak.

The crowd didn't need to be told it was about to witness history. Immediately the fans started chanting "EEEE" and the Bullets' next possession, Hayes said, perhaps remembering all the unfortunated foul shots from the past. "I was glad when they both went in."

When the second foul shot swished through the basket, the game was stopped and players from both teams shook Hayes' hand as the partisan rooters stood and applauded.

"The fans were keeping me aware of where I was even if I knew," said Hayes. "They kept saying, 'Ten more, eight more, six more.' You never dream about scoring 20,000 because it seems out of reach. But what made this so nice is that I knew my team-mates wanted me to get it. They were pulling for me and getting me the ball.

Hayes is the sixth man in NBA history to amas 20,000 points and 10,000 rebounds. He will add heavily to both figures before he reitires, as he plans to play at least three more years.

"Yeah, I may play until I drop," said Hayes, who asserts he is 31, not 33 as listed in the record books. "I feel great and I think there is a lot more basket-ball left in me. I've avoided any major injuries and God willing, I don't see any reason why I shouldn't keep going."

How about to age 50?

"Hey," he said, "that's not a bad idea. Not bad at all.' CAPTION: Picture, Evin Hayes of Bullets blocks a shot in close-in action against Clippers. Hayes passed the 20,000 mark in career scoring to lead win. AP