In my experience, car guys make poor bicycle mechanics. They figure that since they know cars, bicycles will be a piece of cake. My bicycle shop was right next to an automobile repair shop and the mechanics would constantly look down their noses as us, yet there was a fairly steady stream of family bicycles that they embarrassedly brought over because they couldn't fix them and had, in fact, screwed up even worse. If they had a tail, it would have been between their legs. It was so tempting to charge them their repair rate (which was triple ours) plus a 10% premium because, let's face it, they couldn't fix it. In your case, it was likely even worse because he probably wasn't really a mechanic but just an assembler, which requires limited, easily trainable skills. Regardless, almost anybody who works with cars seems to develop a god complex when it comes to mechanical products and looks down on bicycle mechanics.