Harmony just follows the San Francisco Giants these unexpected, first-place summer days.

Who should greet the Bay City Rollers when they arrived here but a hundred barbershop quartets, on convention at the same hotel?

No downtown street corner, drugstore queve or hotel lobby at 2 a.m. has been safe from the barbershoppers' impromptu melodies.

"You can hear that darn 'Down By The Old Mill Stream' at all hours of the night," joked Giant reliever John Curtis. "But it's the right mellow atmosphere for us. This team has a few lead singers, but it's the other guys singing the background music that makes us sound soooo good.

"I think barbershop harmony is the right image for us."

The Giant quartet that hums the sweetest is undoubtedly the pitching rotation of Vida Blue, Bob Knepper, John (Count) Montefusco and Ed Halicki - combined record 33-14 with a 2.88 ERA. Giant success devolves from them.

However, the Giants have a bullpen duo, a young outfield trio and a veteran infield quartet who go all but unnoticed. It is those melodious supporting players who make the Giants look like a team that will still be near the top of the National League West in late September.

Only one team in the majors enters the All-Star break with its three regular outfielders hitting over .300 as a group. That's right - Jack Clark (.310), Terry Whitfield (.302) and Larry Herndon (.292) of the Giants all of whom have batted at least 270 times.

You never heard of them? Well, Clark, currently on a tear, is third in the league in runs batted in with 61.

So what if he is a dead first-ball, streak hitter? Maybe he gets a poor jump on a line drive in right field and malingers occasionally on ground outs. But he is only 22, looks like a 6-foot-3 Bruce Springsteen a talks like a street-tough rock 'n roller, too.

"I'm a leader on this team," says Clark, who has speculated that he may someday hit .400. "When I'm going good, the whole team goes good. I just have a great time beating that ball.

"I can't get enough of watching Pete Rose. He never gets cheated of three good cuts."

Neither does the ripping-and-slashing Clark, who has 15 home runs, plus hitting streaks of 19 and 12 games (the latter still alive). Clark, along with Blue the Giants' only All-Stars, is the heart of the S.F. attack - present and future.

The entire Giantoutfield - ages 22, 24 and 25 - is fleet, with center fielder Herndon and Whitfield both slap hitters.

"I keep hearing about Montreal's great young outfield," snorts first base-third base veteran Darrell Evans. "Ours is already better."

If the Giant bullpen of lefty Gary Lavelle and Randy Moffitt (brother of Billie Jean King) is not overpowering, it is still stable and proven.

If the Bay basemen - Willie McCovey at first, Bill Madlock at second and Evans at third - are absolutely the least mobile group on any contenter in memory, they can hit.

McCovey has 51 RBIs on 55 hits, Madlock is batting his usual unspectacular .314, and Evans will probably manage at least 100 walks and 90 runs scored.

With these three wooden Indians struggling to move more than a step for a ground ball, the Giants desperately needed a slick shortstop.

"That was sure our trouble position," said one Giant pitcher, adding laconically, "Johnnie LeMaster just wasn't fielding enough to support his .200 hitting."

Classy Roger Metzger, acquired last month from Houston, has hit .385 in his first 20 Giant games. Even when ge gets back to his career .230, his glove will play a welcome tune to the ears of the Big Four.

Typical of the Giants' disguised versatility is their rash case of poison Ivie - Mike Ivie. Currently atop the league batting averages, at .333 in 159 times up, he has no regular position.

Ivie afield scratches his platooning way from first base to third base to catcher to the outfield. Ivie at bat has collected a half-dozen game-winning hits, second to McCovey on the team.

Every Giant claims to match Ivie's willingness to subordinate personal statistics to team welfare - and this is the man who refused, until disciplined, to move frome infield to catcher at one stage of his 1977 campaign with San Diego, insisting it would be bad for his career.

"This team reflects (Manager) Joe Altobelli's personality," said Curtis, "firm but low key. He has us feeling that first place is our personal responsibility. We've also picked up a little of that intangible team spirit that the old (Oakland) A's had. Vida preaches that gospel pretty good.

The Giant spirit needed that rebirth.

"Last year we had 'slump spirit' and selfish play. This year basically the same guys have totally different attitudes," says a Giant hurler.

"Winning sure changes people."

Just as losing can change them back - quickly.

"It's my job to help the players ignore the little daily pressures, remind them of how long a season is," said Altobelli. "Everybody else can get fidgety over one or two losses, but the manager is supposed to be the last one to show it.

"If you say 'pressure,' you must feel it. So I never say the word," said Altobelli, a man with only 257 career big league at bats but the confidence to have surounded himself with superb coaches - Dave Bristol, Jim Davenport, Tom Haller and Herm Starrette.

"Baseball isn't the world. It's just something I enjoy," Altobelli said, spreading his ham hands slowly, his cap turned around backwards a la Max Patkin.

"But, you know, pressure, that thing I don't mention, it must agree with me. I wouldn't want to wake up too many mornings in a row and feel that no one would put a difficult question to me that day."

For the Giants, those difficult questions posed by the Reds and Dodgers, those complex harmonies between good pitching, makeshift hitting, spotty defense and spirit, only make the first-placechorus sound sweeter.