The engine at Michigan was valve train-related while the issue with the Charlotte engine was a piston problem. "It started with a valve spring – just one of those valve spring problems," Hendrick said of Junior's engine which expired after he led 34 laps at Michigan.
"It's the only one we had, but it's kind of the Achilles' heel in most of your engine problems. You break a valve spring and then you drop the valve. There's nothing you do about it. You try to get the best parts and pieces, the best batch numbers but that track is so hard on equipment because you're on the throttle so much and have little time for the engine to breathe. Fox Sports