It was just after 3 p.m. on a drab, wintry day when the door to the Kronk Gym swung open and Leon Spinks strode in, flashing his famous gap-toothed smile, his top teeth so far apart they reminded you more of a 7-10 split in bowling.

Inside this small room, about the size of a school classroom, that has become famous as the home of champions Thomas Hearns, Hilmer Kenty and Milton McCrory, it was sweltering--in the 90s, so hot the door handles hurt your hand.

Dave Collins, an aging middleweight who now trains Spinks, the ex-Olympic gold medalist and heavyweight champion, glanced at his watch, smirked and nodded to himself as Spinks entered. Spinks, who has been known for having an unusual body clock, to say the least, was almost a half-hour early for his training session for his April 9 fight in Miami.

That`s not what is new about the kid who went from being a weak, asthmatic youngster in one of the worst ghettos in the nation to beating Muhammad Ali for the world heavyweight championship to an existence that became a national joke.

As Spinks stripped off his blue sweatshirt and dungaree coveralls, a gold figurine with a raised image of Jesus dangled from a chain around his neck. Spinks grabbed at it subconsciously before starting his work on the heavy bag. Spinks, whose fourth-round knockout of Lupe Guerra Feb. 21 was his first fight in almost two years, has found more than a new clock. The man who for years couldn`t find his driver`s license has also found God.

''I`ve finally got good people behind me, and I`ve got God on my side,''

Spinks, 31, said during a break in his workout. ''Everything I do now is for Him.

''My goal now is to try to make it back so I can correct all the mistakes I made.'' He referred loosely to his parade of arrests, from drug possession to enough traffic violations to make him more famous than A.J. Foyt.

''The problem is, you get out there in the fast lane and you forget God,'' said Spinks, one of seven children raised by a deeply religious mother who always carried a Bible with her.

Leon, meanwhile, just carried on. ''I started living a wild life . . . women, drugs, all that stuff. I did a lot of things wrong. It`s like I had no responsibility at all.

''But see what happened? God put me on my back. He made me think about Him again. He got me there when I was on top, and the only way to get back was to follow His word. I know now I`m here for a reason. And that reason is to serve God, to help kids and old people, to spread His word. I can show people where I was and that I`ve come back because God blessed me.''

Spinks is silent for a moment as the buzzer drones to signal the end of another round of sparring.

''I want to lead a different life,'' Spinks finally said. ''You know, I`ve got three boys (he is divorced but considering remarrying soon). I want to make a life for them. I want my kids to be proud of me.''

That was not a consideration when Leon was growing up in the notorious Pruitt-Igoe low-income housing project in north St. Louis. Most similar complexes are slums, but Pruitt-Igoe was so bad the city finally dynamited it in 1972.

His father had deserted the family when Spinks was 2, and he struggled through a youth marked by frequent beatings from neighborhood toughs. Though tall, he was skinny and anemic. The other kids called him ''Goofy,'' not an inappropriate appellation for his later years, either, and he was always getting robbed.

So Leon, known to still suck his thumb as a 13-year-old, and his brother Michael, who also became an Olympic gold medalist and is now light-heavyweight champion, went to the Capri Recreation Center two blocks from Pruitt-Igoe to learn to defend themselves against the thugs and bullies who were their neighbors.

Yet, for the introverted Leon, there was no indication he might pull that amazing upset over Ali some day, only that his antics would be considered amusing. He dropped out of high school in the 10th grade and took a job as a welder. But he almost killed himself several times falling asleep on the job. It seems the heat of the torch made him sleepy.

He then went into the Marines, but making Spinks a man was more than even that proud corps could achieve. It came closer than most, though. It was in the Marines that Spinks racked up most of his victories in a 178-7 amateur record. But he was mostly unknown in the boxing world until his fast hands and aggressive, charging style helped him win the light-heavyweight gold in the 1976 Olympics in Montreal.

Then life started moving fast for Spinks, too fast, he says. ''I was going through life so fast that I never thought what it was about, what I wanted to do with my life.''

Seven fights and 13 months after turning pro in January, 1977, the 24-year-old was in Las Vegas preparing to meet 36-year-old Muhammad Ali for the heavyweight title. He was such a big underdog that bookies weren`t taking bets on Ali.

Even though he was outweighed by some 25 pounds--as he often was because of his smallish size for a heavyweight--Spinks won a 15-round decision over Ali, and the legend of Leon Spinks was born.

Spinks held the title only until September, 1978, when he lost a 15-round decision to Ali. As for the crown of national buffoon, no one would take that away.

A month after he had won the title, Spinks was arrested in St. Louis for driving the wrong way on a one-way street without a license. In June of `78, he was arrested in North Carolina for driving without a license and 26 hours later for speeding. In October, he was involved in an accident in his 2-day-old Corvette in Ohio, and a week later he ran it through a fence. In November, he was arrested for driving without his license after an accident in Detroit, and in December his car was found smashed into a tree.

Also that year, he was arrested in St. Louis for marijuana and cocaine possession. Again, instead of feeling sorry for him, people laughed. The street value of the stuff was estimated at $1.50.

Although he now had fame and fortune ($3.75 million for his second fight with Ali), Spinks was still a child, still uncomfortable with people who spoke the English language clearly and dressed neatly. His taste in women and bars was on the tacky side, which led to his being mugged outside a Detroit bar in 1981. He woke up later naked in a nearby motel room, missing $45,000 worth of clothing, jewelry and his solid-gold dental plate. His wife had now ballooned to 275 pounds, and he wanted a divorce.