A 5 1/2-hour version of 'Faust' that manages to be engrossing

Ever ambitious, the CSC Repertory (the initials stand for Classic Stage Company) looks for a theatrical Everest to conquer every season. Recent years have brought the full ''Peer Gynt'' of Ibsen - about five hours' worth - and the ''Oedipus'' cycle of Sophocles, in productions that were strong if spare.

Topping these, the troupe has now tackled Goethe's monumental ''Faust,'' presenting the American premiere of the complete work. Its adaptation is substantially complete, though it has tightened the drama of Part 1 and curtailed the meandering of Part 2, which ranges through the world with a wanderlust that suits the printed page but has discouraged all but a handful of actual productions of that part since it was published in 1833.

The result is still unwieldy, but a towering experience for all that. Undaunted by the sheer size of the show, which runs about 51/2 hours, Christopher Martin has concocted a production that's downright grandiose by the troupe's usual standard, complementing Goethe's poetic text with dramatic lighting, versatile settings, and an inventive array of effects and gestures. The performers live up to their challenges with admirable energy, racing through a long roster of roles ranging from witches and apes to a ''chorus mysticus'' and the devil himself.

Not every scene is equally successful. Momentum fades as Part 2 wends its way through every cranny of human experience; and even the romantic storytelling of Part 1 has lusterless and long-winded moments. Yet the show has a sweep and scope that rarely fail, drawing the spectator into a web of intellect and emotion that remains enticing even when narrative droops. And when the story threatens to bog down altogether, Martin usually comes up with a visual trick to light a fresh spark under it.

Of the ''Faust'' versions I've read in English, Wayne's is probably the best for staging. It is complete, unlike Walter Kaufmann's - which has more wit, though - and it has a more contemporary tang than Bayard Taylor's. In their adaptation, the CSC company has varied the meter of the verse, lengthening some lines and truncating others, seeking a sense of verbal surprise that never clashes with Goethe's healthy enthusiasm for doggerel. Both the sense and spirit of the enormously diverse poem are well served, in lyrical as well as dramatic moods.

Repeating a past practice of CSC when faced with such a huge work, a group of players (three in this case) shares the leading role. Martin plays Faust in the first and last sections, with Gary Sloan and Tom Spackman ably replacing him in the middle portions. This method works better here than in ''Peer Gynt,'' since Faust undergoes more radical character changes as the action progresses. The performer with a real marathon to undergo is Noble Shropshire, whose Mephistopheles is as sharp, cynical, and insinuating in the epilogue as in the prologue. The supporting cast acquits itself well throughout.

In dealing so well with this massive play, the CSC troupe has also served its own sense of continuity. For one thing, it has used insights gained from such past productions as ''Peer Gynt,'' which Ibsen modeled on ''Faust,'' and ''Oedipus at Colonus,'' another ''tragedy'' with a happy ending. And once again it has illustrated the contemporary relevance of a venerable play, which happens to deal with such perennial problems as inflation, warmongering, and environmental issues.

Meanwhile, it's refreshing to see this able company tend away from its accustomed austerity toward a style that's visually as well as verbally exciting. And it's enlightening to discover that all of ''Faust,'' including the intimidating ''universality'' of Part 2, can indeed be staged effectively. For this demonstration, CSC deserves thanks from theatergoers - and emulation from theater companies everywhere. The show runs through New Year's Eve, in repertory with the solid CSC production of Strindberg's ''Ghost Sonata.'' The season will then continue with ''Wild Oats,'' by John O'Keeffe; ''Balloon,'' a new work by CSC member Karen Sunde; and ''Camino Real,'' by Tennessee Williams.