The threat of a transit-worker strike is putting a damper on Howard Stern's elaborate plans for his final day on free radio.

For weeks, Stern has been hyping plans to hold a rally tomorrow on West 56th Street — likely to be attended by thousands of fans — that will feature his family and a slew of wacky friends from his long-running radio show. But the event will be canceled by cops if New York City's transit workers go on strike.

"If there's a strike, they [city officials] are anticipating more vehicular traffic, and they won't be able to close the street down," Tom Chiusano, the general manager at K-Rock (WXRK), the station where Stern works, told The Post yesterday.

K-Rock staff, security and workers, who have been hired to build the stage where Stern will conduct the rally, will have to wait until after midnight tonight — when the transit union announces its decision — to begin converting the street for the event.

Crowds are expected to gather around 6 a.m., with the outdoor portion of the final broadcast slated to begin around 8 a.m. Police will be patrolling the area tonight to keep it clear for traffic.

"It's impossible to know what's going to happen until the last minute," Chiusano said. "We're working really hard, we've got a lot of staff, we're spending a lot of time, we're spending a lot energy, and we're going to spend a fairly significant amount of money for this event, and I hope there is no strike."

Although Chiusano has frequently been at odds with Stern over the years, he wished the departing radio star well. "This is the end of what's been a glorious 20-year run with Howard — I've got a world of respect for him, and I'm sorry to see him go."