on tire valve caps. Yes, seems easy enough until you attach an aluminum bike to your $160 bike with a working stem replacement. I used to put teflon grease on the valve threads to prevent this, but it can still bind and cause problems with the air compressor fittings. So someone guessed and designed these plastic caps. I can't tell them apart from aluminium, they have a silicone washer and the gasket is normal. Also qualitative. Thanks.