No one here can figure out Jackie Beard, the 5-foot-4 heart of the U.S. boxing team, a contingent that has won more medals in these Pan American Games than any other team.

In the ring Beard looks like a veteran approaching 30 with his wide variety of styles and complete confidence.

In winning a unanimous decision over Luis Pizarro of Puerto Rico in Roberto Clemente Coliseum tonight, Beard put on a seasoned demonstration of a variety of styles: going inside with uppercuts and hooks, like Joe Frazier; dancing and jabbing, like Muhammad Ali; throwing quick combinations, in mid-ring, like Sugar Ray Robinson - even fighting southpaw.

The U.S. team's other three championships - featherweight Bernard Taylor, light welterweight Lemuel Steeples and light heavyweight Tony Tucker - also won unanimous 5-0 decisions, but it was Beard who made the ring his canvas.

Beard, however, is only 17 years old, just graduated from high school in Jackson, Tenn., last month. If any U.S. fighter here has an unlimited future - both in the Olympics and later as a pro - it would appear to be the bantamweight Beard.

More than being a stocky, rugged talent, Beard actually has turned into the team's spokesman, saying repeatedly, "When we're finished, people back home will say we're No. 1 in the world, not Cuba.

"Our goal is to win more gold medals in Moscow in '80 than the '76 team did in Montreal (five). We want people to know that the U.S. has some tough young fighters coming up."

The U.S. team thinks it delivered its message tonight, although Cuba certainly will disagree. The United States had the most medals and beat Cuba five time in eight head-to-head matches. Using an arbitrary scoring system of three points for a gold, two for silver and one for bronze, the United States would come out first.

Nevertheless - a big nevertheless - all Cuba's five finalists won gold medals tonight. Jose (The Animal) Gomez, a ferocious middleweight, and heavyweight Teofilo Stevenson led the Cubans with crushing first-round knockouts.

At any rate, the U.S. squad couldn't be more delighted.

"We're getting tougher all the time," Steeples said. "At the World Cup in '78, the U.S. only won two bronzes. Now...look out in Moscow. Maybe it'll be all 11."

If the banty rooster Beard had his way, that's what would happen. Beard is not as slick and quick as a Sugar Ray Leonard, nor as handsome and glib, but he has a sawed-off courage, a look of determination and leadership that is unnerving - and the U.S. team needed such assets.

The United States started last night's finals with light flyweight Richard Sandoval losing a controversial decision to Cuba's Hector Ramirez. The crowd hooted loud and long.

The second fight was far worse, as Dominican Pedro Nolasco was robbed by an awful 5-0 hometown decision that gave flyweight Alberto Mercado Puerto Rico's first gold medal of the Games.

Beard walked into this caldron of crazy biased judges and calmed the waters. Against his Puerto Rican opponents, he did not try to go for a knockout, instead showing such mastery of the local fighter that the crowd was utterly silent and even cheered his winning decision.

The atmosphere of fierce partiality was broken. Taylor, Steeples and Tucker could fight one man, not 12,000 fans. Beard's confidence seemed to have set the tone. In fact, he even stood in the crowd near ringside for moral support for other fighters.

"We get together back in that locker room," Steeples said, "and we pass the spirit on."

Beard's confident mastery seemed to have set the tone. In fact, he even stood in the crowd near ringside to watch Taylor's fight, as though his will were there if necessary.

"When people meet me, they seem to think I'm in my mid-20s," he said, smiling. "I don't understand that. Maybe the ring has just made me grow up.

"What I try to do is confuse the other fighters," Beard said. "First, he thinks I'm a slugger, then I look like a boxer. Then, all of a sudden, I'm left-handed.

"He thinks, "This guy is all mixed up, but I can't hit him and he's hitting me.""

Just last month it was Beard who felt confused.

"My high school graduation was the same day as the Pan-Am trials," he recalled. "I said, "I don't want to pass up what I worked 12 years for. Pan Am just ain't worth it. You only graduate once." But then I had an idea."

The idea, one that worked, was for Beard to fly from Jackson, Tenn., to the trials in Toledo, Ohio, for his first fight on a Wednesday, then fly home for his graduation on Thursday and return to Toledo for another fight Friday.

"I'm sure glad I did it," said Beard, who has a 94-7 career record and will enroll at Jackson State next year, planning to major in criminal law.

"I was lying in bed the other night thinking about what I'd accomplished.Not bad for a little guy."

SAN JUAN, Puerto Rico, July 14 (AP) - The deep, overwhelming forces of the United States concluded their domination of the VIII Pan American Games by smashing the record for total medals tonight.

Yachtsmen, track and field athletes, women softball players and boxers piled up 26 medals, including 13 gold, on the last full day of competition and pushed the American count to 263 total medals in these two-week hemispheric celebrations of sport.

That broke the total medal record of 247, set by the United States in the 1975 Games in Mexico City.

The United States' team of almost 700 competitors, largest ever sent to any international competition, fell just short of another standard of superiority, however.

With a single event yet to be decided - individual show jumping Sunday - the Americans had 126 gold medals, compared with the record 128 they won in the 1967 Games in Winnipeg, Canada.

In women's softball, the United States took the gold with a one-hit shutout by Barbara Reinalda in a 2-0 victory over Puerto Rico. Belize, a 6-3 loser to Puerto Rico earlier in the day, got the bronze.