These bleed screws are clearly marked as stainless steel in the detailed description. They aren't. They were the wrong colour for stainless, but looked like zinc, so I popped one in vinegar overnight, and the results can be seen in the photo. The black one is ordinary steel that has had the zinc stripped off by the vinegar. It's a classic test. Note that some people use a magnet to test for stainless ... it's not a reliable test as some stainless steels are magnetic. Having said all this ... the bleed screws I received are a decent quality, but are incorrectly described as stainless steel.