Okay, you big tough Pittsburgh Pirates, tell us how loose you are now. Turn that disco music up loud and laugh about the pennant race. Pressure? What's pressure? You guys don't feel it.

Not much.

To their credit, the Bucs finally stopped pretending today, stopped acting like these months of strain were a picnic. The boogie still blasts in the Pirate locker room, but no one feels like dancing anymore. Not after today.

Not after the fifth-place Chicago Cubs played them toe-to-toe for 4 hours 13 minutes, then beat the Pirates, 7-6, in the 13th inning to cut their league lead over Montreal to one game.

Not after the winning Cub run scored on an awful, and unnecessary, wild throw by Buc captain, hero and main man Willie Stargell in that final inning.

Not after the last out of this marathon day was made by the same symbolic Stargell, striking out with the tying run on second base and the winning run on first.

Not after winning Chicago pitcher Bill Caudill, who hurled 3 1/3 shutout innings, was a kid with an 0-7 record who had never won a big league game in his life -- until today. And only throws one pitch: a fast ball.

Not after the Pirates squandered a three-run lead. Not after the final score of Montreal's 3-2, bottom-of-the ninth win over Philadelphia had stared them in the face tauntingly for more than 90 minutes before this classic of attrition ended.

No, above all, not after their edge over the Expos in the loss column had been eradicated, reduced to zero.

The Pirates, now 97-64, still are in the driver's seat. Their final game of the regular season is here Sunday against the Cubs. If they win, giving them the highest Buc victory total since 1909, they will clinch a tie -- at worst.

But the Expos are right on the Bucs' black-and-gold back bumper, playing a game of high-speed chicken. True, the Expos still have three scheduled games -- against the Phils on Sunday, and a makeup doubleheader on Monday in Atlanta, if necessary. But, after today, the Montrealers have their baseball fate back in their own hands. If they win all three games, they can clinch a tie, and a playoff on Tuesday in Montreal -- at worst.

The Pirates have enough on their minds to produce many a sleepless hour. In a way, Stargell, the day's goat, has a simple task of forgetting.

After receiving a routine throw from third baseman Bill Madlock for the second out of the top of the 13th, Stargell realized that the Cubs' Mick Kelleher, who had been on second base and held his ground on the hopper to Madlock, suddenly had hightailed it for third.

Stargell's heave, which had no chance of nailing the speedy Kelleher, sailed over Madlock, over the backup man, and off the box-seat railing as the go-ahead run scored easily.

"Don't make excuses for me," said Stargell, totally composed and pleasant, sitting at the Buc card table, munching chicken.

"It was a horsefeathers throw. I looked like a monkey trying to wrestle a football."

Perhaps Stargell with his 470 homers, could forgive himself. But many another Pirate felt that inner gnawing.

"We had a 3-0 lead after one inning and we wouldn't put 'em away," said Phil Garner. "They tried to hand us the game and we just wouldn't put 'em away. We sat right on it."

Interim Cub Manager Joe Amalfitano gave his club a pregame pep talk about pride and the spoiler's role. So, the Cubs made an error on the first play of the game, and saw a Stargell ground smash with the bases loaded go directly between Kelleher's legs for a two-run "hit".

But the Pirates fritzed around, stranding runners in the first seven innings, leaving 13 for the game, until the Cubs awoke -- aided by the red light of national TV.

"We've had a losing attitude for weeks," said Cub first baseman Bill Buckner, who made a brilliant diving stop on Rennie Stennett's two-on, two-out grounder over the bag in the 10th inning to save the game. "This is the first time we've had our spirit back."

While the Cubs were aroused -- pitcher Bill Caudill saying, "I'm still shaking . . . when Stargell came up in the 13th, I had to step off the mound and forget who he was" -- the Pirates' energy was draining.

The Pirates could have been ecstatically exhausted today. After their starter, Bert Blyleven, was knocked out in a five-run, Cub sixth that put Chicago ahead, 6-3, the Pirates rallied against the Bruins' brilliant bullpen of Dick Tidrow and Bruce Sutter to tie the game, 6-6, after seven.

Then came Caudill with his 4.91 ERA. "He threw hard, but not nearly hard enough to flat shut us down for four innings," said Garner. "I can't remember when a guy with just one pitch could stop us." But September is different. The bats get heavy.

For the Bucs, who wheeled in six pitchers, including hard-luck loser Don Robinson, there was grist for second-guessing Manager Chuck Tanner. "In the 11th," said Madlock, "how could we bunt Omar Moreno from first to second? Just let the man steal the base, then bunt him to third with none out.

"We played it like Bruce Kimm was Johnny Bench. Moreno's got 77 steals (two today). He owns that base. We've been aggressive all year. Now, all of a sudden, we get conservative."

"Wait 'til you see 'em tomorrow," crowed the Cubs' Dennis Lamp, doing a comic walk immitating a player with an extremely tense posterior.

The Bucs have clung together well, despite three divisive loses this week. "We win together. We lose together. That's the name of that," said red-hot Dave Parker whose five singles today were overshadowed. "We're entitled to a bad day."

But the penultimate day of the season is a brutal time for the most psychologically corrosive loss of a season.

Fun and high jinks are the Pirate style -- the camaraderie and mutual support that make a six-month grind bearable.

At sundown today, the lone card game in the subdued Buc clubhouse was a fraud. At the round table where six to a dozen Pirates always sit playing cards, gobbling meat and gravy, and kibitzing, there were only two today -- Stargell and an ancient club house man.

Stargell, the leader, even in his worst hour, knew the importance of one man saving face, one man tapping his foot to the beat, even if he didn't feel like dancing.

The game that Stargell and the old man were playing is the Pirates' favorite. It is called Pluck.

And that is exactly what the Pirates need desperately now.