If a mechanic or other service person damages an engine or other property on which he or she is working, that mechanic should be responsible for all costs or repair or replacement. The problem, as a practical matter, is (1) proving that the mechanic damaged the engine, since the fact that it stopped working after the mechanic worked on it does not definitively prove that the mechanic caused the problem; and (2) if the mechanic simply refuses to pay, you'd have to sue him, which could cost as much or more than you'd recover. Therefore, it's often in a person's best interest to work out some settlement in situations like this which will satisfy both parties.