Using a standard brass valve core in an aluminum valve stem cap will cause galvanic corrosion which will make the cap impossible to remove. Installed these caps on my new VW Jetta about 6 months ago. Since the car has a tire pressure monitoring system, I didn't usually remove the caps to check tire pressure. When I tried this a week ago, the caps wouldn't come off. I had to cut them off with a dremel and replace all the valve stems.