Wooden double flare Although it's amateur measurements, it's 20mm when measured normally. Since there's a flared part for the double flare, the inner diameter is 19mm. I received this from a friend several years ago as something they no longer needed, and I kept it without using it. There are stains and scratches on the star part that could not be removed. Please check the photos. It will be disinfected before packing, but please disinfect it yourself before use. We cannot guarantee any trouble after purchase.