IT WAS ALMOST 20 years ago today that Billy Gibbons got the band to play. ZZ Top's guitarist had just left a Houston band called Moving Sidewalks and teamed up with American Blues drummer Frank Beard, who invited unattached bassist Joe "Dusty" Hill to join in a casual jam session one afternoon.

"And as Dusty stepped to the stage, he was strapping up and he said 'What key?' " Gibbons recalled from Memphis earlier this week. "I thought, 'This is a novel approach -- the guy doesn't want to know what song, just what key.' So we launched into a shuffle and an hour and a half later we all shook hands and said 'This works.' "

It certainly has. By the mid-'70s, the Lil' Ole Band from Texas had taken its act nationwide, setting concert attendance records for an American band that were not broken until Springsteenmania in the '80s. The '76 tour took a little bit of home on the road (vultures, rattlesnakes and buffalos vied for space on a huge Texas-shaped stage), but folks attending concerts Sunday and Monday at Capital Centre will have to settle for the auto junkyard theme that ties into ZZ Top's "Recycler," their first album in five years.

The Capital Centre shows are part of the group's first official tour in three years, during which their only appearances have been on behalf of the Delta Blues Museum in Clarksdale, Miss. Last weekend, the Texas trio was honored in nearby Memphis, its second home and the city where the band has recorded all of its albums. Before a Saturday concert, the three Tops received keys to the city, citations from the city and state, a proclamation of ZZ Top Weekend in Memphis and the Blues Foundation's first Keys to the Highway Awards, created to honor mainstream artists rooted in the blues.

"We felt that a gesture of repayment to the art form of the blues was in order," Gibbons explains. "Our opportunity arrived rather inauspiciously during a break in the 'Recycler' recording sessions. We had a free weekend to visit the museum {it occupied a room in the town library} and the day we arrived, there was an inspection nearby of Muddy Waters's cabin home."

The band was given a stick of lumber taken from a portion of cabin being hauled off and they had it turned into an electric guitar, donating it to the museum as "the Muddywood guitar" and starting an ongoing association with the museum. While the band was on hiatus, the guitar toured the country's Hard Rock Cafe circuit to raise both funds and awareness of the museum.

"We use the word 'interpreters' of the art form," says Gibbons of the group's blues roots. "I don't think that ZZ Top is easily mistaken as a stone-out blues band but there's a B. B. King lick here, or a Jimmy Reed riff there, and maybe a taste of Howlin' Wolf that creeps through and that's enough to say 'Hey, where are we coming from? Where did we learn from?' And in some small way our involvement with the Delta Blues Museum and referencing the blues as an influence helps keep it alive. It's a changing art form, it's not static, and it keeps showing up in weird places."

It shows up on "Recycler," which Gibbons describes as "Tres Hombres" meets "Eliminator," a reference to the grungy roots sound of the first album (recorded in 1973) and the technogarage nature of the later (from 1983). It was, Gibbons confesses, "accidental -- we arrived two days before our equipment truck showed up and so we were forced to play on some scrounged-up instruments. It was just enough to temper the direction of the sessions themselves. After several months on Beale Street, gone were the synthesizers from their previous prominency, and hello blues! We had some material prepared closer to 'Afterburner,' but there was a quick revamping of the material and it felt good. Once the groove set in, it was straight ahead, bound for bluesland."

And now bound for the concert stage, MTV and even films, where the band has established a memorably hirsute image. In "Back to the Future Part III" they appeared as a desperado band.

"We call that our one acoustic concert for the year. . . . . I wish we could take full credit for some mastermind plan, but it was a rather of-the-moment evolution," says Gibbons. The image so clearly defined in three Tim Newman videos ("Gimme All Your Lovin'," "Sharp Dressed Man" and "Legs") involves pretty girls, cars and a key ring that unlocks the door to some fantasy-bred paradise. Not surprisingly, it has been revived on the new video, "Give It Up," featuring the band's latest four-wheel mascot, Cadzilla.

"We're the band that takes old cars and tries to make them run again," Gibbons says, which partly accounts for the new album's title. ZZ Top also finds new uses for old riffs "and as we get deeper into the issues surrounding this generic term, we find ourselves as a band becoming educated to some of the deeper aspects of the environmental cause. It's been real interesting."

Not that ZZ Top is turning into Billy Bragg. "We're known for weird cars and guitars," Gibbons admits, saying ZZ Top might unveil in concert the futuristic guitars-with-Sony Watchman seen briefly in their "Sleeping Bag" video. "Right now we're suffering under the ridiculous notion that we need to be playing giant cowboy guitars, the antithesis to the pointy guitar era. But they weigh 20 pounds apiece, so we're doing weightlifting and blues shouting at the same time."

ZZ Top will have lots of time to practice since they are but three months into a 16-month world tour.

"It's demanding but we're still those guys in a joint playing music," Gibbons says. At one point in the concert, Gibbons, Hill and Beard (ironically, he's the one without the beard) will squeeze together near the drum kit, recreating the cramped quarters they once worked in.

"These large venues can become somewhat overpowering and we figured why not continue this 'recycler' theme, this return to rootsiness? So there's a portion of the program where we 'shoulder up' and it feels quite good because it is the same and it turns a 20,000 seat arena into an intimate club. ZZ Top is simply three guys that enjoy playing music and that part of it hopefully will never change."

ZZ TOP -- Appearing with Black Crowes Sunday and Monday at Capital Centre. Call 202/432-0200.