Tom Watson, the Masters and British Open champion who is favored in the 59th PGA Championship that begins Thursday at Pebble Beach, unknowingly has been using illegal irons for the past 14 months, it was determined today.

Former PGA champions Gary Player and Ray Floyd also discovered that they have been carrying illegal irons after asking Clyde Mangum, assistant commissioner of the PGA tour, to check their clubs in the wake of the self-disqualification of former University of Maryland golf captain George Burns at the Greater Hartford Open last seek.

During a delay on the tee at Wethersfield, Conn., Jerry Heard, one of Burns' playing partners, rubbed his fingers over one of Burns' clubs and said the grooves appeared unusually wide. Burns, who had completed rounds of 65-72, promptly asked Mangum to check the clubs and then disqualified himself.

"We found that Burns' clubs had grooves wider than the 35/1,000 of an inch permitted," Mangum explained. "That opened the Pandora's Box. Watson, Floyd and Player also asked for checks. All had illegal irons - although it was not their fault. They were all victims of the manufacturer's error in die-casting because other clubs made by the same company (Ram) were found to be perfectly O.K.

"As far as other tournaments won by players with the illegal clubs, nothing could and will be changed. Once a tournament is over, it is closed."

Watson immediately called his home in Kansas City and will have his old set of irons here by the time the PGA starts. Floyd had an extra set of irons. It was discovered that Player had five irons - 5, 6, 8, 9 and the sand wedge - that were illegal. He immediately had the grooves filed down. Burns had all his irons redone in Chicago last week.

Watson changed to his current irons during the U.S. Open in Atlanta last year. He tied for ninth in that tournament. He didn't win an event in 1976, although he did finish in the top 10 in his last seven tournaments. This year, he has won the Masters, the British Open Crosby, San Diego and Western.

"I began to worry about the clubs after the Burns incident," said Wastson , who passed up the Hartford Open last week. "In a way, those clubs have been a blessing and something else."

Those were the same clubs lost enroute to the PGA at Congressional last year and, oddly, the clubs were lost again coming to this tournament. But Watson didn't do too badly with his old clubs (MacGregor) in 1975 when he won the British Open, the Byron Nelson Classic and the World Series of Golf.

Dave Stockton is the defending champion in the field of 141 here but few give him much of a chance. Stockton also won the PGA in 1970. Of interest to Washington fans will be the performances of Lee Elder, who finished tied for 15th in last year's PGA, and Larry Ringer, club pro at the National Academy playing in this tournament for the first time.

Stockton insisted he was playing better this year than he did at Congressional. "I had seven birdies in my practice round Tuesday" he said. "My game must know that I'm going to take next month off so it's coming out of the woodwork. I was second in the Phoenix Open and sixth in the Atlanta Open this year and that was my best."

"I feel fine physically," said the 35-year-old Stockton. "I weigh 175 pounds - the lightest I've been since I joined the tour in 1965. I've played Pebble Beach many times. It was a strong course before; it's weak now. The fairways are not good but they're (the fairways) will affect many shots. This tournament will be won and lost around the greens. Good iron play and good chipping will do it.

"I don't feel overshadowed by Jack Nicklaus and Tom Watson. It's just been their time.My time will come again. If not, I'll know I was there once."

Stockton adopted a new driver last week. "It's pear-shaped with a flat head," he explained. "It has more bulge and roll and it's heavier. It's so heavy that I have had to slow down my swing. I used to short and accurate and I wanted to be a big hitter. But I lost control. This new driver, I think will give me the control I lost."

Nicklaus commented on the heavy rough around the greens. "It's tough to pop out of that grass," he said. "You can't hit left or right or you're penalized. But it's still the same old Pebble Beach. There will be only three or four shots that won't have perfect lies. But the ball isn't rolling as much as I thought it would. I'd say the fairways are cushiony. The ball will run but not too much. Yes, I like my chances. I always do. If I didn't, I wouldn't be here."