Rita Ryack spent several weeks tangling with Tom Cruise’s leather pants.

The costume designer for the upcoming 1980s-set musical “Rock of Ages” (opening Friday) was instrumental in Cruise’s conversion into the fictional rock icon Stacee Jaxx, a self-involved guitar-playing idol in the vein of Guns N’ Roses frontman Axl Rose.

The coyote-fur jacket, the jewel-encrusted codpiece and the custom-made cowboy hat did wonders in transforming the normally strait-laced Cruise into a drug-addled performer with more in common with Mick Jagger than with Ethan Hunt.

But it was the hand-stitched pants that proved the most trying component of the outre costume. Every time Cruise performed a rendition of Def Leppard’s “Pour Some Sugar on Me,” the pants would stretch out and lose their rocker slickness.

“We had a distresser standing by who would hand him his next pair, take the ones Cruise wore, dry them, shrink them up again and re-age them,” Ryack said. “We had lots of pairs.”

Beyond the Cruise transformation, Ryack was charged with turning Alec Baldwin into an aging hippie, Paul Giamatti into a cheesy music manager and Mary J. Blige into a jumpsuit-wearing ’80s diva without making it too costumey, per the instructions of director Adam Shankman (“Hairspray”).

“In our first meeting, Adam said very clearly that because the ’80s is such an inherently funny costume time, he didn’t want the joke of the movie to be the clothes.

“It’s still nuts, but we wanted to do hard rock ’n’ roll, not costume rock ’n’ roll like the White Stripes and Motley Crue. We stuck to a Guns N’ Roses look.”

Except when it came to Diego Boneta, the aspiring rocker pressured into joining a boy band at the be- hest of his manager (Giamatti).

Leather was turned primary for Boneta’s ridiculous look as a member of the boy band Z-guyeez.

“Those were my favorite costumes,” Ryack said. “The color-block stuff was so funny and so in opposition to real rock clothes.”