There's plenty of sag in Willie Stargell. The years and gravity are doing their worst. His muscles ache and the corset he plays in doesn't help much. When he pulls his Pittsburgh Pirate jersey over his head, it is with an audible sign, an exhalation of deep relief.

"Some days, I feel a little bit older than 38. But when you get to the park, you realize there's a cause," Stargell said in his quiet, deep, almost hypnotically relaxed voice just minutes after he had helped crush the Montreal Expos with two titanic pennant-race home runs.

"I'd be lyin' if I didn't admit that I had goose bumps the size of golf balls tonight," Stargell after his Pirates had moved half a game ahead of the Expos into first place in the National League East with a 10-4 clobbering that the old man instigated.

Slowly, Stargell went down the list of all the Pirates who had played key roles tonight: Ed Ott with his three RBI, Jim Rooker with a shaky victory and Enrique Romo -- the relief aid that Pittsburgh calls Romo Seltzer -- who fanned seven Expos in four shutout innings for a save.

"I just happened to be the big brute who got a couple out of the yard," mumbled Stargell, who has now gotten 470 out of the yard in 17 Pirate seasons.

"This is Tuesday night," warned the clubhouse soul philosopher. "It has nothing to do with Wednesday."

Wednesday will, in fact, be extremely important to these Bucs and Expos -- a game that may live long in their baseball memories. If Montreal wins, it will have the four-game series split it came here seeking and leave this rugged town with a slim half-game lead, a game in the loss column on its side and its fate in its own hands.

If the Pirates win, with Bruce Kison pitching against Montreal's Steve Rogers, Pittsburgh will have a 1 1/2 game lead with five to play and Montreal will head to Atlanta for its Thursday doubleheader with all manner of dark thoughts.

If the finale is clearly the highlight of the four-game set, then tonight's lopsided Pirate victory -- insured by seven Buc runs in the fifth and sixth innings when 17 Pirates batted -- was predominantly a stage on which Stargell could project himself.

When the 6-foot-3, 230-pound first baseman struck out in the ninth inning, the throng of 39,180 (31,948 paid) in raucous Three Rivers cheered every one of his creaky steps back to the dugout.

When it no longer mattered, when the victory was salted away, Stargell looked in need of a crutch to get to the plate.

But when it mattered, in the early innings when a season may have been on the line with the Bucs dragging after a brutal a.m. defeat in the nightcap of a doubleheader begun the evening before, Stargell was present and accounted for -- to the tune of about 800 feet worth of home runs.

"We needed something badly," said Pirate reliever Kent Tekulve, the stringbean who now looks like a wire after 91 relief appearances. "Willie came right out the chute in the first inning and hit a two-run moon shot.

"Stargell is the heart and soul of this team. Everything we do and mean comes out of him. We're just followers, even the best of us. He is the Bucos."

The Pirates are a well-known hurricane -- a team of bluster, loud music, insults and swaggering Piratical cockiness. Stargell is the placid, stoic eye of that storm.

"We can be crazy because he's sane," Bill Madlock said. "This team has plenty of hard noses and bad guys. Or that's what they called people like Tim Foli and me on other teams. Here, we're just regular guys.

"You can play aggressive and tough, you can let your feelings show, you can blow your top if you need to, like me, and the team doesn't come apart," said Madlock. "That's because Stargell is the center of the team. It all revolves around him.

"He puts together the parties. He's always got a card game going. When we're in trouble, he's talking to the pitcher on the mound. On this team, he's the one steady one."

Stargell has reached a point where he is an almost embarrassingly loved player -- an uninterrupted institution in Pittsburgh that is unmatched within baseball circles, even by Boston's Carl Yastrzemski.

No duty is too much trouble for Stargell, no question dumb enough to annoy him, no hour too early to come to the park.

These are his last days, and he is relishing them. From 1973 to 1977 Stargell's homers dropped every year from 44 to 13. But under Manager Chuck Tanner's loose reign, Stargell has set his own schedule, consulted his weary bones daily, and hit 28 homers in '78 and now 31 in '79 (in just 402 at-bats).

"He's what I'd like to make myself," said Tekulve.

"As a player, he's totally consistent. He rises to the occasion. Last week we played Montreal until 2 a.m. Who hit the homer in the 13th inning after 6 1/2 hours? Willie, of course.

"As a person, he's twice as good.He seems to care about everybody. He is so warm. If you didn't know, you couldn't tell if he's the No. 1 man on the club, which he is, or if he were the 25th."

In repose, deep lines of strain etch Stargell's face. However, as the Pirates play cards between games of a doubleheader, Stargell seems almost comic with his hat on cockeyed and his rubbery mug naturally sliding into goofy expressions.

When the grounds crew presented Romo a portrait -- of the worst scarfaced, cigar-chewing, one-eyed Latin American bandit imaginable with the inscription "No. 15" -- Stargell was among those who insisted on giving it a place of honor. It hangs in the center of the Pirate clubhouse.

That is Stargell's style -- to set the mood, dictate tone. Tonight, his first blast off Scott Sanderson produced a 2-0 lead. When Montreal caught up, 2-2, there was Stargell to hit the wall far behind the right-field fence with a solo homer.

Stargell had seen two pitches, hit both over the wall, and set a mood which, events proved, was irreversable.

"Pops, I guess you'll take tomorrow off," said Phil Garner to Stargell.

"Nah," replied the man who calls himself by his real name -- Wilver. "I think I'll show up again."