In 1972, the hit Broadway musical “Grease” was ready to go on the road, which meant that a new cast had to be put together.

In Los Angeles, an 18-year-old actor just breaking into the business auditioned for the part of Danny Zucko, clad in a black motorcycle jacket and a white T-shirt. He slicked back his hair and sang Carole King’s “I Feel the Earth Move.”

“His audition was completely unfocused,” recalls Tom Moore, who directed the original Broadway production. “It was all over the place. But he had a terrific voice. He was immensely charming. And he was very attractive. He looked like a French movie star.”

His name was John Travolta. 

Moore cast him in the show, but not as Danny. “He was too young for the part,” Moore says. “So we made him Doody, a goofy character who got kind of thrown away in the movie.”

Travolta toured with the show for two years. “I got to observe three to five Zuckos and got to see exactly what worked and what didn’t,” he recalls in “GREASE: Tell Me More, Tell Me More: Stories from the Broadway Phenomenon That Started It All” (Chicago Review Press), a new oral history of the show. What worked was his performance in the movie, which to date has grossed $400 million.

“Grease” opened in New York 50 years ago this year. The book is edited by Moore, “Maude” star Adrienne Barbeau — who played the first Rizzo on Broadway — and Ken Waissman, who raised $110,000 to produce the musical on the Lower East Side before moving it to Broadway, where it ran eight years.

The inspiration for the musical struck one night in 1969, when Jim Jacobs and Warren Casey, two aspiring theater writers living in Chicago, went on a nostalgia trip. Drunk and stoned, they’d been listening to Led Zeppelin, but then decided to put on a record by Dion and the Belmonts. It took them back to their high school days in the 1950s. Jacobs wondered why no one had ever written a musical with a ’50s style rock ‘n’ roll score.

“Fun idea,” Casey said, “but what the hell would it be about?”

Jacobs thought for a moment and replied: “Maybe it should be about the people I went to high school with.” A moment later he had the title. Since everything was greasy in those days — food, hair, guys working under the hood of a Ford De Luxe — Jacobs said, “It could be called ‘Grease.’”

Jacobs and Casey didn’t know it at the time, but their nostalgia trip would one day become one of the longest-running shows in Broadway history as well as the 1978 movie that had the world doing the hand jive — and made stars of John Travolta and Olivia Newton-John. The original cast would include actors such as Marilu Henner (before her role in “Taxi”), who played Marty in the original Broadway cast, Richard Gere as Sonny and Barry Bostwick (later of “Rocky Horror Picture Show” fame).

Waissman first heard about “Grease” from his college roommate, an orthodontist living in Chicago. Jacobs and Casey had scraped the show together and, with a cast of amateurs (including one Henner), put it on in the basement of an old trolley barn. The dentist, “who never had a good word to say about anything,” Waissman tells the Post, “called me and said, ‘I think I’ve found a show for you.’”

Waissman flew to Chicago, sat on newspapers (there were no seats in the trolley barn) and watched teenagers from Rydell High sing “Beauty School Drop Out,” “We Go Together,” and “Greased Lightnin’.”

“What I saw was my high school yearbook coming to life,” Waissman says. “The show was barely the show it became – it was just some scenes and some good songs — but the characters were authentic. You knew Danny. You knew Sandy. You’d gone to school with them.” 

Waissman believed that, for the show to work in New York, it had to retain its authenticity and ragged edge. The actors would have to look like they really were in high school. An agent urged him to hire Michael Bennett, just a few years away from creating “A Chorus Line,” to stage the show, but he balked. Bennett’s work was slick and depended on trained dancers. It would be too polished for “Grease.” He went instead with choreographer Patricia Birch, who had a reputation for teaching non-dancing actors how to dance.

For his director, he turned to Tom Moore, a recent Yale graduate who had one New York credit to his name: “Welcome to Andromeda,” a two-character play about a quadriplegic who wants to kill himself and a nurse who urges him to go on living. Waissman had seen the play and was struck by how real the characters were. Moore, he thought, would bring the kind of authenticity to the acting that Birch would bring to the dancing.

“I had done a play in which only ONE person moved,” Moore told The Post, “so I wasn’t sure musicals were in my future.” He read the script, which “had severe problems,” and listened to the “impressive” score. But he turned down the job. “It wasn’t something I wanted to,” he says. Then he flipped through the script again and caught what Waissman had seen in Chicago: His high school yearbook coming to life.

“I went to high school in Indiana and there were about three Greasers there,” he recalls. “They were soft-core — they drank, they smoked, they cut class. I wasn’t part of their group at all, but I remembered them and there they were in ‘Grease.’”

In the original draft of “Grease,” the characters were rough-edged. Some were downright unlikeable. Working with Jacobs and Casey, Moore kept them rough, but gave them humanity.

“There are two reasons why ‘Grease’ has been so successful,” Moore says. “Number one: the characters are prototypes for everybody you knew in high school. But they have heart and vulnerability. I think I helped with that. And number two, and this is what Jim and Warren understood, the show is about everybody’s first experience: Your first time in a new school; the first time you become part of a group; your first kiss; your first love.” 

Moore and Birch assembled a cast of young and unknown actors, including Alan Paul, who thought he was auditioning for a musical about “Greece, the country,” he says in the book.

Rehearsals were “delightful,” Moore recalls. And then came the first preview at the Eden Theater on Second Avenue. “Rocky does not begin to describe it,” Moore says. “It was a disaster. It shook us to our core.”

“I wanted to commit suicide,” says Waissman. “There was gloom and doom all over the place. But I said to Tom, ‘How do you clean up a room? You start by picking up the first sock.’”

They began picking up the socks and within three weeks, they had transformed the show from a disaster into something that, they felt, audiences were enjoying. The movie version of “Grease” would usher in a wave of nostalgia for the 1950s, but the ripples began down at the Eden Theater.

Unfortunately, the critic for the New York Times was British — Clive Barnes, who was immune to ‘50s Americana.

“They are starting to be nostalgic about 1959 now, and almost all I can remember about it is that it was a great year for Burgundy,” he wrote. “The show is a thin joke … and once the initial joke has been established, it is bound to wear thin.”

Everybody told Waissman to close the show. His lawyer told him he was $20,000 in debt. But Waissman held on. He couldn’t pay the $20,000 in any case, so he thought he might as well keep the show open and see what happens. Within three weeks, “Grease” was a hit. Word of mouth trumped the critics. Audiences were flocking to the Eden to relive their high school years.

Waissman moved the show to Broadway, where it immediately sold out. Among its many fans were Richard Burton, starring in “Equus” on the same block, and his ex-wife Elizabeth Taylor. Burton and Taylor took the cast to dinner at Sardi’s. Everybody jockeyed for a chance to sit next to Taylor, who polished off a fair amount of Johnny Walker.

Neither Waissman nor Moore had much to do with “Grease,” the movie. But that’s never bothered them. 

“They arranged a private screening for me, and I thought, ‘They’ve turned it into bubble gum,’” says Waissman. 

Adds Moore: “The movie is a success because it’s the ‘50s seen through the eyes of the ‘70s. That would not have been my choice. It wasn’t the nitty-gritty show we did. But when the movie came out, I threw a party, which ‘Grease’ paid for. ‘Grease’ has been very generous to all of us.”