A mechanic friend of mine bent my pistons while fixing my car. Do I have a legal case against him. My car no longer runs.

You can file a small claims complaint or Justice Court complaint against him for the damage to your vehicle. Your damages are the cost of repair to the vehicle, less the value of what is referred to as "betterment." Essentially, to get the car running, it might require the installation of entirely new valves and other components that would normally not be changed out absent the damage. Those new parts will extend the life of the car beyond that which it would have been given the original estimated repairs. If that is the case, you get the cost of the total repair, less a credit back to him for you getting a better car than what you should have received back from his repairs. You can also claim as damages anything you paid to him since obviously, his repairs had no "value" to you.