Using a standard brass valve core in an aluminum valve stem cap will result in galvanic corrosion, making 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.