NFL owners bowed to pressure from coaches yesterday and scrapped the experimental 40-second clock, reinstating the old 30-second clock for the regular season and playoffs.

In a vote taken by computerized teletype, 13 teams voted to retain the clock, 15 voted against it. Under NFL bylaws, a new rule must be approved by 21 of the 28 teams.

Under the experiment, approved by team owners last spring, the 40-second clock was started as soon as the whistle blew to end the previous play. After timeouts, teams had 25 seconds to put the ball into play.

Under the old rule, which will be back in use for the final week of the exhibition season, teams had 30 seconds to snap the ball after the referee signaled it ready for play.

According to NFL figures, the 44 exhibition games played with the 40-second clock this season averaged 3 hours 5 minutes 42 seconds, compared to 3:09.05 for the first 44 exhibition contests last year. Regular season games last year averaged 3:11.40.

But proponents concurred there was little chance for permanent approval because of opposition by coaches.

"I like it, but the coaches are running the league when it comes to things like that," said Cleveland Browns owner Art Modell.

Washington Redskins Coach Joe Gibbs was one of the harshest critics of the 40-second clock, saying it was too restrictive . . .

Cornelius Bennett, the No. 2 selection in this year's NFL draft, must sign with the Indianapolis Colts quickly or not at all, owner Robert Irsay said.

"It's now or never," said Irsay. "He has missed all of training camp and he wouldn't play many games this year. If it takes more than a day {to sign him}, the chances {he will sign} won't be too good."

Irsay met yesterday with Richard Woods, agent for the all-America linebacker from Alabama. They were near agreement in June before Bennett demanded a deal equal to the eventual 10-year, $11 million pact rookie linebacker Brian Bosworth signed with the Seattle Seahawks.

"I don't want to hear anything about Bosworth," Irsay said. "I don't even know Bosworth. I have not met the man." . . .

Houston Oilers owner Bud Adams said he has asked officials in Jacksonville, Fla., for guaranteed profits of $115.2 million over 10 years to move the team.

Jacksonville Mayor Tommy Hazouri met with business leaders to urge them to draw up a letter of credit, and believes the city can meet the team's profits request through government financing and private fund raising.

"We'd be crazy if we didn't {move} if he can come up with that kind of money," Adams said. "But there is more to it than that."

Adams said the Oilers also seek a $5 million practice facility, additional skyboxes at the 82,000-seat Gator Bowl and revenues from advertising, parking and food sales. He said Jacksonville has approved that request in its bid to lure the Oilers after the team's 10-year Astrodome lease expires at the end of this season . . .

Atlanta Falcons owner Rankin Smith said he wants to show his appreciation for retired running back William Andrews' accomplishments by making "financial adjustments" to Andrews' contract.

"He's been with me a long time," Smith said of Andrews, the Falcons' all-time leading rusher. "He's been very supportive of the team ever since he's been here. He's done everything we've asked him to do."

Smith declined to discuss the arrangement, saying, "What I did was between me and him." Andrews reportedly was to be paid $535,000 for 1987, with a contractual arrangement for deferred payments through age 70 . . .

Billy Cannon, the 1959 Heisman Trophy winner and a convicted counterfeiter, has reopened his dental practice in Baton Rouge, La., his hometown and site of his glory years at Louisiana State.

Cannon, who went on to star for the American Football League's Houston Oilers and Oakland Raiders, was arrested in 1983 for heading a group of counterfeiters that printed $6 million in $100 bills. He agreed to testify against four suspects in the case as part of a plea agreement and was sentenced to five years in prison and a $10,000 fine.

Cannon placed an advertisement in the Baton Rouge Morning Advocate that said he was practicing orthodontics. He referred all calls to his attorney, Robert Kleinpeter.