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Kevin Hart and the 2 people in his car that crashed Sept. 1 have all lawyered up, TMZ has learned, and everyone's expecting a big lawsuit.

Sources with direct knowledge tell us, the driver, who sustained serious back and chest injuries has an attorney, as does the passenger in the backseat, who sustained only minor injuries. Kevin has a lawyer as well.

We're told Kevin is in the crosshairs of the other 2 people in the 1970 Plymouth Barracuda because the car did not have safety harnesses or airbags. The argument is ... Kevin should have worried less about keeping the car authentic and more about safety -- and not doing so constitutes negligence.

The driver could also be on the hook for Kevin and the backseat passenger, but it's a little premature since the CHP is still investigating the cause of the crash.

The big issue ... we're told the company that customized the car could be looking at a lawsuit. The argument goes like this ... the company is the expert and even if Kevin wanted a custom job without safety harnesses, the company should have refused the job because it wasn't safe ... especially with a powerful, 720 horsepower engine.

We contacted 10 custom car companies around the country and asked if Kevin's crash has opened their eyes and changed their minds about customizing classic cars without installing safety harnesses. 8 out of the 10 said they would if that's what the customer wanted. Only 2 said they would not.

It's interesting because it opens these companies up to a lawsuit, especially if there are passengers in the car who get injured.