The stoppers are used on my horizontal Hummingbird feeder. The original end caps became infused with a black fungus/mold that just could not be removed. I tried your Cork Stoppers, but they had a tendency to leak. The also coated the interior end with a slimy black fungus/mold. The silicone stoppers fit perfect and seal the sugar water perfectly. Not only was the silicone much cheaper, but appears to be doing a perfect job for my purpose.