When someone damages another's property by neglience (unreasonable carelessness) or intentionally, he or she is responsible for the cost to repair or replace that property--which would include the cost to "tear the engine apart to fix it." While the mechanic could have been absolved of liability by a waiver, in the absence of such a waiver-which you would have had to agree to--the mechanic should be resonsibilty for paying for the repairs.