Kansas City, Mo. ? By winning 81 games, the Royals have guaranteed Mike Sweeney cannot opt out of his contract, which runs through 2007.

“He’s locked up,” manager Tony Pena said Monday. “I took the key out of the lock and threw it away.”

When Sweeney signed a $55 million, five-year contract extension in April 2002, he insisted on an escape clause: If the Royals didn’t produce a .500 season by 2004, he could void the deal and become a free agent.

The Royals’ 10-4 win Sunday over the Chicago White Sox gave Kansas City an 81-74 record with seven games to go.

“I never really have thought about the clause when I’m on the baseball field, but sometimes when I’m off the field I’ve wondered, ‘Am I going to be here for one more year or four more years?'” Sweeney said before Monday night’s game against Detroit. “Now that I know it’s going to be four more years, it’s a great feeling.”

Had the Royals been competitive but not finished at or above .500 during the target period, Sweeney said he might not have exercised the escape option.

“If we were close this year and closer next year, then I probably would have stayed,” he said. “But now it’s signed, sealed and delivered.”

Last year, when the Royals lost a franchise-record 100 games, Sweeney criticized owner David Glass for cutting payroll and questioned whether the front office was doing all it could to make the Royals competitive.

“I never had any desire to leave,” he said. “I had a desire to know what was going on when I heard things … that seemed totally contradictory to what I’d been told. This offseason, I caught some flak from media and people across town because I spoke out — but I felt I had a right to.”

The contract language caused a stir when it was announced.

“We put that clause in there for a purpose, because we believed it (a .500 or better season) could happen,” general manager Allard Baird said. “When we came up with this — or when Michael came up with it for us to go along with — it put the pressure on him as much as it put pressure on the organization. It’s one thing to say you want to stay, and another thing to do it. Mike Sweeney did it.”

The Royals accomplished this year’s turnaround with Sweeney, a four-time All-Star first baseman, at less than full strength. He was on the disabled list from June 26 to Aug. 8 because of a sore upper back and has not played in the field since his return.

Going into Monday, Sweeney was hitting .298 with 15 homers and 81 RBIs.

Sweeney said he would undergo an MRI exam this week, but said it was “90 percent likely” that he would not need surgery.

“This offseason, he needs to rest and get back to where he was health-wise,” Baird said, “and I think we’ll see one of the better right-handed bats in major-league baseball back next season.”