MUSIC REVIEW : Spinal Tap’s Joke Wears Very Thin : Parody: The heavy-metal spoof loses much in the transition from the screen.

One could argue that “heavy-metal parody” is a redundancy. There would seem to be little room for comedic interpretation in a genre whose practitioners torture their hair into leonine manes; indulge in aggro-macho posturing that alloys the power fantasies of adolescent boys, televangelists and pro wrestlers.

However, in playing a trio of British hard-rock schlemiels in the cultish 1984 mockumentary film “This Is Spinal Tap,” American comedian-actors Harry Shearer (as bassist Derek Smalls), Christopher Guest (as guitarist Nigel Tufnel), and Michael McKean (as guitarist David St. Hubbins) disclosed a successful game plan for metal mockery. Their idea was to play it as straight as possible, thus enabling the idiom to exhibit its inherent silliness like some defective plumage.

The gag worked because the three are competent musicians, as well as actors with a gift for subtle, frequently improvised comedic shadings. But they vowed not to milk the success of their spoof-metal formula, and over dinner at a La Jolla restaurant three years ago Guest insisted to a reporter that there would never be a Spinal Tap “reunion.”

Nevertheless, the quasi-group reunited last year for a concert at a music-industry trade show in Anaheim and, more recently, released its second album, “Break Like the Wind.” Spinal Tap’s first-ever tour brought them to Copley Symphony Hall on Wednesday night, where they played before an enthusiastic crowd of 1,255. And played. And played.

For tickets costing $17.50 to $20, fans got $40 worth of music, if you can put a dollar value on time. With no opening act, Spinal Tap emerged at 9 p.m. to chants of “Tap!” “Tap!” “Tap!” For the next 75 minutes, they played such cuts from the new album as “Bitch School” (apparently, MTV didn’t understand that this is parody--they wouldn’t air the video), the power ballad “Just Begin Again,” and “The Sun Never Sweats.” They also did such songs from the 1984 soundtrack as “Sex Farm,” “(Listen to the) Flower People,” “Hellhole,” “Heavy Duty,” and “Stonehenge.”

After a 15-minute intermission, Spinal Tap played for another hour. To say the show was generous is being kind; this was one running gag that suffered by being turned into a 10K.

Spinal Tap as a road act lacks the key ingredient that made “This is Spinal Tap” devilishly funny: context. As bands go, the trio (augmented in concert by a drummer and keyboardist) is no better and no worse than a thousand struggling hard-rock bands in a thousand cities. Their average-ness, in fact, was an important subtext of the film, with which the writer-actors and director Rob Reiner provided a deliciously seriocomic backdrop of tour drudgery, management dickering, record company hassles, and other music-biz nonsense.

If you subtract that backdrop, you are left only with the subtext of so-so musicianship, and two-plus hours of that can leave you squirming. Assuming that those in attendance Wednesday night “got” the original Spinal Tap joke, I’m sure that an hour-long show would have satisfied their need to have it re-told.

Thankfully, there were a few attempts at visual and verbal diversion, some of them hysterical. As in the film, the three opened their concert by emerging, with much fanfare, from huge, egg-shaped esophagi. The stage props, which included a huge, demonic skull suspended above a towering, totem-like, metal spinal column, and Guest/Tufnel’s ridiculously over-sized Marshall amplifier, were chuckle-worthy but otherwise inconsequential.

For a while, the between-song patter, especially that of front-man McKean/St. Hubbins, saved the day, because it recalled the inane dialogues that were the blood of the film. But, eventually, an overhead video screen bore funnier fruit and served to bring the concert closer to its filmic mother ship.

“This next song was written by the gentleman to my left (Tufnel) the last time we toured the States,” St. Hubbins deadpanned at one point in a faux- Cockney accent. “It’s about his impressions of this great country of yours. It has an anthemic quality, so, you know, if you’re looking for a new anthem, we, uh, get at least partial royalties.”

As the band played 1984’s “America,” video images of various American scenes--interspersed with goofy visual non sequiturs featuring the musicians--contributed an appropriate dash of absurdity.

Musically, the highlight of the second set was an acoustic segment in which the three demonstrated their “sensitivity” on the ersatz-’60s pop of “Rainy Day Sun” and “Cry All the Way Home.” But, again, it was the marriage of sight and sound that lifted the show to its apex.

In a play on those interminable drum solos that are de rigueur at heavy metal concerts--for which the other musicians usually vacate the area--Shearer/Smalls and St. Hubbins left the stage to drummer Ric Shrimpton and Tufnel. As the two traded solo spots, the overhead video showed Smalls and St. Hubbins parting ways backstage. Alternating clips then showed Smalls dining at an Italian restaurant and St. Hubbins undergoing a beauty treatment--complete with mud-pack facial, pedicure, and leg waxing.

Tearing one’s eyes from the screen, you caught Tufnel at the dramatic climax of his solo, tossing horseshoes at his guitar, which was upright on its stand. The combination of this amplified clanging and the overhead scenes of passive self-indulgence provided the most inspired satire of the evening. If only there had been more of them.