Selling defective motor parts

They re-conditioned the top of my engine with new valves, lifters and cam shaft. Two weeks later a valve broke off on the line of the hardener and hit a hole in the piston and damaged the valve seat and top. They tell me the engine gave to mush petrol and over heated the valve and caused metal vitage and damage and not the quality of the valve. All the aluminum parts sush as the top and block was not effected, buckled or cracked. I must pay to get it fixed. So they have to worst client service and think people are stupid and does not understand engines and metals. They think they can tell you any story and you must believe them. Mechanics that have 30+ years experience say this is the first time they have seen this problem and it is impossible for the scenario they selling to work because the engine will melt before valve shows metal vitage.