AMERICAN WEDDING []

Stale third helping of “Pie.” Running time: 95 minutes. Rated R (sex, raunchy humor, profanity). At the Orpheum, the Union Square, the Chelsea, oth ers.

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‘AMERICAN Wed ding,” this week’s entry in Hollywood’s summer of endless sequels, has some gut-busting moments, but for the most part the thrill is gone from what the ads describe as the “thrilling climax” of this gross-out franchise.

Much of the original cast, minus Chris Klein and Tara Reid, is back for a third go-round, which ostensibly revolves around the impending nuptials of pastry lover Jim (Jason Biggs) and his flute-playing cutie Michelle (Alyson Hannigan).

What it’s actually about is the nuptial havoc wreaked by Jim’s horny buddy, Stifler (Seann William Scott), who wangles an invite to the wedding so he can pursue Michelle’s stunning sister Cadence (January Jones) – much to the chagrin of Jim’s brainy pal Finch (Eddie Kaye Thomas).

The screenplay by Adam Hertz, who wrote the first two installments, is mostly a thin excuse for a series of gross-out moments.

The funniest by far is the opening sequence – excerpted in a toned-down version in the trailers – in which Jim’s dad (the irreplaceable Eugene Levy) arrives expecting to witness his son proposing marriage, but actually stumbles into Michelle down on her knees under a restaurant table, uh, servicing Jim.

There is also the Stifler-organized bachelor party, to which an unwitting and mortified Jim shows up in the company of his fiancée’s parents (the dad is played by the brilliant Fred Willard, who is given criminally little to do).

They are greeted by the sight of Jim’s pals bound and gagged and wearing chocolate syrup; two erotic performers pretending to be a maid and a police officer; and a gay bar owner (the very funny Eric Allen Kramer) posing as a butler named “Mr. Belvedere.”

The focus is mostly on Stifler, an unrepressed id on legs who burns up the dance floor in a gay bar with a medley of ’80s oldies; has an inadvertent romantic encounter with Jim’s girlfriend; and tries to coax Jim’s accidentally consumed wedding ring out of a dog in a sequence that makes “Pink Flamingos” look tasteful.

There’s also a memorable cameo appearance by Jennifer Coolidge as Stifler’s sexually irrepressible mother, and a running gag involving pubic hair and wedding cake.

Jesse Dylan (Bob’s son, late of “How High”) moves things along briskly, but this latest installment lacks the light touch and heart that made the first two more entertaining.

Let’s hope the producers make good on their promise to let well enough alone – and not inflict “American Baby” and “American Divorce” on us.