A wonderful thing happened to the Los Angeles Dodgers last season. The defending World Series champions blew the pennant.

As a result, the Dodgers were at last able to get rid of Steve Garvey and Ron Cey, aging cornerstones who had turned into millstones.

Now, with newcomers Mike Marshall and Greg Brock in the core of their lineup, the Angelenos are finally free to grow from a perennial contender -- with only one World Series championship since 1965 -- into a true dynesty.

Just as the flinty hearts who ran the L.A. front office were looking for a chance to sweep out the oldsters who had helped them gross millions, along came the perfect excuse for a housecleaning. In '82, the Bums did a first-rate fold, blowing a 3 1/2-game lead in the NL West in the last dozen days as they lost eight games in a row.

This was a short-term disaster. However, the long-green Dodgers never think short. For 25 years, since ditching Brooklyn to move to L.A., they've stood for continuity and conservatism, long-range planning and attention to dollar detail. If the batboy gets the hiccups, the Dodgers do an in-depth study.

The L.A. brain trust decided it would cheerfully write off '82 if it meant the rest of the '80s would belong to the Dodgers.

Some who don't bleed Dodger blue suspect that the L.A. organization's self-serving tradition of one-way loyalty will finally blow up in its face; this rolling-in-cash club that wouldn't pay market value to keep Tommy John or Don Sutton has gone too far, they think, in keeping the vault locked when it came to re-signing Mr. Dodger and The Penguin.

If the tall, strapping Marshall, 23, and Brock, 25, find that it's a long way from Albuquerque to L.A., then Garvey, 34, and Cey, 35, may start looking young and handsome again.

This is the year when baseball's closest-to-the-vest club takes its biggest gamble. Over the winter, the Dodgers, in effect, gave away Garvey (free agent) and Cey (traded for two minor leaguers). The dumpy duo combined for 40 homers and 165 RBI in '82; pretty good, but not good enough.

Now, the future belongs to rookie Brock, who'll play first base, and Marshall, who will start in right while last year's right fielder, Pedro Guerrero, moves to third base.

On one hand, these kids look as though they can't miss. After all, the Dodgers have produced the NL's last four rookies of the year: Rick Sutcliffe, Steve Howe, Fernando Valenzuela and Steve Sax. These guys can judge talent.

Marshall, a 6-foot-5, 220-pound right-handed hitter with fair agility, won the '81 Pacific Coast League triple crown with a .373 average, 37 homers and 137 RBI in 128 games; then, in '82, he hit .388 until L.A. called him up; technically, he is no longer a rookie.

Brock, a 6-3, 200-pound lefty hitter, merely used Marshall's 1981 PCL stats as a guideline; in '82, he batted .310 with 44 homers and 138 RBI in 135 games.

Of all baseball statistics, the most misleading are those in the PCL, with its short fences and thin air. If you don't hit .330 with 100 RBI at Albuquerque, the Dodgers send you back to Lodi to lift weights. This is the league in which Dickie Thon hit .394, Mickey Hatcher .371, Jeff Leonard .365, Billy Sample .352, Danny Heep .340 with 129 RBI.

So, how good are Marshall and Brock?

Not even the Dodgers know. Marshall hit .233 in 120 at bats in the majors. He has holes and chases breaking balls. Brock, who has excellent opposite-field power, was too for 17 (.118) late last season.

Actually, Marshall and Brock's performances closely parellel two former Dodgers -- Garvey and Cey. What is of interest is that, after hitting .373 at Albuquerque with more than an RBI a game, Garvey needed five more years before he ever hit 10 homers in a big league season; he batted .227 as a rookie. Cey also dismantled the PCL (32, 123, 328), but hit only .245 as a Dodger rookie.

Nonetheless, the Dodgers are creating huge expectations. The press guide touts Brock as "the best Dodger power-hitting prospect since Duke Snider."

After their notices, Marshall and Brock seem a little smaller than life, despite their size. Marshall, a converted first baseman, is battling to become a mediocre outfielder. Skiptics are waiting to see how many errors shortstop Bill Russell and novice third baseman Guerrero make without Garvey's Gold Glove at first to scoop up their mistakes.

"We're putting Marshall and Guerrero through a four-year course at Harvard in one month at their new positions," said Manager Tommy Lasorda. "They're taking thousands of balls off the bat. Guerrero's got a condo at the beach, but he tells me he hasn't seen the ocean yet 'cause it's always dark when he gets home. I've told my coaches that every practice this spring will be a long practice. The only way they're going to get to play golf is if they wear miners' helmets."

"Garv kind of took Steve (Sax) and I under his wing last year," said Marshall of Garvey. "He probably saw that we'd be in his shoes someday and wanted to help us, show us how to handle outselves. He's a class guy . . . not just ome of the time, but every day. He never had a bad day with people."

Marshall has an easy, natural confidence, saying things like, "I guess the number (goal) is .300," and, "Last season was fun, like getting to start against Steve Carlton." Carlton, fun? The taciturn Brock is not to loose.

"The fans in L.A. are behind me, I think," said Brock, "but a lot of people would like to see me not do too well. I've already heard it: "You'll never be as good as Garvey." I've gotten fan letters that say I'm horsefeathers for replacing Garvey. You'll always get that."

Few newcomers will ever get more of "that" than Marshall and Brock this year. Their stats and their bearing say that, in time, they will probably be as good as Garvey and Cey. But that's not the issue at the moment. If Marshall and Brock have the same tepid rookie years as Garvey and Cey, they'll endure pressure and criticism the like of which their predecessors never dreamed.

Also, the Dodgers will get a lot of lectures about their lack of loyalty to Garvey and Cey, two stars who, after a total of 30 years in Dodger Blue, couldn't collect a slice of the huge fiscal pie that they helped bake.