Fans of the legendary British rock outfit Spinal Tap were given a rare treat Saturday night as the band’s core trio reunited for a rare acoustic performance following the Tribeca Film Festival’s 35th anniversary screening of the landmark documentary “This Is Spinal Tap.”

David St. Hubbins, Nigel Tufnel and Derek Smalls delivered a hit-packed set complete with a special guest appearance by Elvis Costello on a rousing rendition of the “Gimme Some Money” — a hit from the band’s pre-Tap mop-top Thamesman period — that had the capacity crowd at New York’s Beacon Theatre singing along with pure glee. Marty DiBergi, the director who captured Spinal Tap in all its early 1980s glory, was also on hand for the post-screening, pre-performance Q&A.

The all-acoustic performance magnified the amazing musicianship of the Tap troika that was so often overshadowed by pyrotechnics and over-the-top props on stage. A band with a canon the caliber of the Tap has plenty of raw material to dazzle the crowd. Saturday’s performance proved they simply don’t need to draw attention away from the music with oversized demon heads, or the mishap-prone pods that so often marred their performance of “Rock ’n’ Roll Creation” (which sadly wasn’t on the set list), or the miniature Stonehenge that sparked the row that made longtime Tap manager Ian Faith quit as DiBergi’s cameras rolled for 1984’s “This Is Spinal Tap.”

St. Hubbins, Tufnel and Smalls have mellowed with age while their musical talents have only gotten better. They’ve wisely traded their Spandex for sport coats and slacks. Fans were cheered by the fact that the trio are clearly getting along well at the moment. Moreover, the acoustic format Smalls’ rock-steady rhythm chops on bass left no doubt that Tap in 2019 doesn’t even need a drummer. Why tempt fate?

During the Q&A, DiBergi shared a few behind-the-scenes stories from the making of the documentary, or “rockumentary” if you will. His favorite line in the beloved movie is a sentence started by St. Hubbins and completed by Tufnel — an illustration of that pair’s Lennon-McCartney-esque creative partnership that has endured for more than half a century. “There’s a fine line…” St. Hubbins observes, “between stupid and clever,” Tufnel finishes.

DiBergi has long since put away the “USS Doral Sea” cap that was permanently attached to his head during the lensing of “This Is Spinal Tap.” Although his output has been spotty of late, DiBergi declared that he hadn’t seen the movie in 20 years. “It’s still pretty good,” he said, to the delight of the audience.

The crowd demonstrated its “Rocky Horror Picture Show”-level of devotion to the movie by laughing well before all of the film’s big moments that have become part of rock ‘n’ roll lore: Nigel showing Marty the amp that “goes to 11,” St. Hubbins’ former girlfriend/spiritual guru Jeanine flubbing “Dolby” as “Dubly” and many more. More than a few attendees brought aluminum foil-wrapped cucumbers in solidarity with Smalls and the LOL scene where he is fairly humiliated by an over-zealous airport security agent. (Smalls mentioned as an aside that it was actually a zucchini.)

DiBergi reinforced how low Tap’s fortunes had fallen at the time the movie was released by noting a comment scrawled on a card at an early test screening in Dallas. “Why make a movie about a band no one ever heard of,” he recalled, incredulously. Today, of course, “This Is Spinal Tap” has been added to the Library of Congress’ National Film Registry, among many other honors. DiBergi confirmed that every frame of the film is completely candid, or what he called “improvised,” with one notable exception: the speech given by the late Polymer Records leader Sir Denis Eton-Hogg at the pre-tour kickoff party. DiBergi also tantilized fans by noting that his first cut of “Spinal Tap,” all those years ago, ran “about seven hours.”

The nine-song set opened on a strong note with “Celtic Blues,” allowing St. Hubbins and Tufnel to show off their finger-picking skills. “Hell Hole” followed, with Tufnel mouthing what would otherwise have been splendidly cheesy electric guitar effects. The mood turned psychedelic for “Listen to the Flower People,” featuring a soulful vocal by St. Hubbins, with Tufnel providing reverb via his throat.

A deep cut for the deepest Tap fans followed — “Rainy Day Sun,” the B-side of “Flower People.” St. Hubbins gamely referenced the band’s long-settled legal skirmish over the tune by quipping “We have to pay Ray Davies every time we play this.” Tufnel then trotted out a another lesser-known tune, “Clam Caravan,” which he reminded the crowd was a typo by the record company that screwed up his vision of a “Calm Caravan.”

When St. Hubbins asked “What’s next, boys?,” everybody knew it was time for “All the Way Home,” famously the skiffle-flavored first tune that St. Hubbins and Tufnel ever wrote together.

Tap’s versatility and endless reworking of their material was on display as they turned to a jazzy, finger-snapping rendition of “Big Bottom,” a nod to Smalls’ love of jazz fusion.

Costello, just one of the many artists influenced by Tap over the years, recalled making his Carnegie Hall debut with the band nearly 20 years ago after he joined them on stage for “Gimme Some Money.” Costello brought some grit to the song but wisely left the spotlight to his elders. “Go Nigel go,” St. Hubbins exclaimed as Tufnel delivered a perfectly mid-60s short-and-sweet guitar solo.

The most unusual arrangement of the night was saved for last with “Sex Farm Woman.” Typically a raunchy rocker, St. Hubbins gave the tune a country flavor that ended with a jarring bit of what might be described as almost-rap.

In all, the show proved that Tap still has the goods to delight and surprise its legion of fans, which stretch from Japan to Alabama and most points in between.

“We’ll see you in another 35 years,” St. Hubbins promised, to thunderous applause.

(Pictured: Christopher Guest as Nigel Tufnel, Harry Shearer as Derek Smalls, Michael McKean as David St. Hubbins and Rob Reiner as Marty DiBergi)