The new Us Weekly (on newsstands now) has a sneak peek at Dustin Diamond's shocking tell-all, Behind the Bell, which claims drug use and sex were rampant on the set of the hit NBC Saturday-morning sitcom Saved By the Bell.

"I could smell a certain 'smoke,' wafting from from the crack" underneath his castmates' dressing rooms, says Diamond, 32, who played the dork Screech.

He also alleges costar Mark Paul Gosselaar  — who played popular Zack Morris — used steroids before production started on 1994's short-lived Saved by the Bell: The College Years.

"He suddenly exploded with manliness, loading 25 pounds of muscle on his once-scrawny frame in, oh, about a month," says Diamond, who is not in touch with any of his costars. (Gosselaar, 35, has dismissed his costar's so-called revelations to Us: "We weren't in rehab and Mr. Belding wasn't my crack dealer.” His rep had no comment.)

Diamond — who filed for bankruptcy in 2001 and had his own sex tape released in 2006 — goes on to say his costars hooked up off-camera.

"If Kelly [Tiffani Thiessen] was interested in Slater [Mario Lopez] one week, then backstage there was a lot going between them in Mario [Lopez]'s room," he tells the new Us Weekly. "Then, if Jessie [Elizabeth Berkley] kisses Zack, then you know Elizabeth Berkley is going in Mark-Paul's room."

For more on Diamond's shocking claims — including how he says Thiessen used guys to further her career; how he says Thiessen struggled with her weight and how he says Mario Lopez was a serial womanizer who "hit on every costar" — pick up the new Us Weekly today!