A [Expletive] Surprise : The raunch of his radio show is slightly toned down as Howard Stern displays a natural presence and gift for self-deprecating charm in ‘Private Parts.’

The opening chapter of radio deejay Howard Stern’s best-selling autobiographical riff “Private Parts” recalls the supposedly true story of a Long Island businessman who became so aroused by an on-air interview between Stern and a lesbian that he had to pull over to the side of the expressway during rush hour and relieve himself.

That chapter is titled “My Philosophy,” and it’s no joke. While some suspended adolescent males dream of hitting the winning home run in Game 7 of the World Series, or becoming a famous rock star, or dating a contortionist, Stern seems to fantasize a crowning moment when the world will pull off the road to listen, moan and swear allegiance to the King of All Media.

In the meantime, Stern himself is moving to the middle of the road. In Betty Thomas’ tamed yet very funny film version of “Private Parts,” the 6-foot-5, long-haired shock-jock plays himself as an overcompensating Alvy Singer, Woody Allen on growth drugs. He’s a once-timid Jewish boy from Long Island who was driven into a shell of fantasies by a father who called him a moron and never allowed him to speak.

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Once Stern broke out of that shell, sometime in the early days of his radio career, according to “Private Parts,” a testosterone-charged id came storming out, guided only by wit and ambition, and the warning went forth: Women and children, get away from the radio!

That Howard Stern, the one who first caught the Federal Communication Commission’s attention by having an on-air guest play the piano with his penis (no, his selection was not the Rach 3), is hardly present in the movie. The radio Stern--part-scripted, part-improvised--vacillates wildly between comic brilliance and sophomoric glibness, between locker room taunting and outright cruelty. In contrast, the movie’s Stern is an earnest cuddle bear, a bit outrageous, perhaps, but at worst a class clown, and at best . . . well, a loyal friend, devoted husband and the most honest personality in America.

In the film, which might have been called “The People vs. Howard Stern,” the radio star assumes the voice of reason and becomes a champion of free speech, standing his ground against the FCC and hypocritical station bosses who love his ratings but hate how he’s getting them.

After being warned against using obscene words by his New York program manager (played with eyeball-bulging gusto by Paul Giamatti), Stern orchestrates a game show bit where co-hosts Robin Quivers, Fred Norris and Jackie Martling (all playing themselves) are asked to fill in the missing parts of the words “(blank)willow” and “(blank)-a-doodle-doo.” When Stern’s show is cut off in the middle of a massage being administered to him by a naked lady, he charges into the manager’s office and succeeds in confronting him on the air.

Stern’s acting is often awkward, particularly in the sentimental moments with his spectacularly tolerant wife, Alison (Mary McCormack). In fact, he appears so ill at ease kissing his co-star, you can believe his frequent claim of 22 years of marital fidelity.

But there’s no denying Stern’s natural presence or his gift for self-deprecating charm. Of all the celebrities we’ve seen playing themselves in adoring biographies, he may be the one most up to the task--and certainly the one most anxious. “Private Parts” is the promo-op of a lifetime.

Thomas, with a script by Len Blum (“Stripes,” “Meatballs”) and Michael Kalesniko, has turned Stern’s life into a sequence of “best of” anecdotes. Some of them are plenty raunchy, but there is little of the mean-spiritedness that often creeps into the radio show. And the raunch itself is within the range of mainstream sexual comedy.

Wisely, Thomas chose not to depict Stern’s male listeners in the throes of side-of-the-road ecstasy--that would have been as revolting as the freak show extremes on the radio show. Instead, we watch a spectacular blond, in the privacy of her living room, following Stern’s instructions to straddle the woofer on her stereo speaker, crank up the bass and turn up the volume.

Nor does the film go into the seamy particulars of the FCC fines leveled against the deejay’s stations. He’s just a humorist showing his listeners a good time, while the regulators are cynical, mirthless spoil-sports.

His polished persona will be hard for Stern’s detractors to swallow, in the event any show up. And it may even be too sanitized for some of his most ardent followers. But setting expectations aside, “Private Parts” is a supremely crafty, smartly written, and--given the number of “himselfs” and “herselfs” on the cast list--surprisingly well-acted piece of pop kitsch.

* MPAA rating: R, for strong language, nudity and crude sexual humor. Times guidelines: Nudity and humor are mostly for the boys.

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‘Private Parts’

Howard Stern: Himself

Robin Quivers: Herself

Mary McCormack: Alison Stern

Fred Norris: Himself

Gary Dell-Abate: Himself

Jackie Martling: Himself

Paul Giamatti: Program manager

An Ivan Reitman production, released by Paramount Pictures. Director Betty Thomas. Producer Reitman. Screenplay Len Blum, Michael Kalesniko, based on the book by Howard Stern. Cinematography Walt Lloyd. Production design Charles Rosen. Editor Peter Teschner. Music Peter Afterman. Costumes Joseph G. Aulisi. Art director Rick Butler. Set decorator Beth Kushnick. Running time: 1 hour, 49 minutes.

* In general release.