This is an awesome piece. The ridges are smooth and comfortable. In fact this piece it is larger than the 7/16 = .4375in = 11.1125mm as advertized. The item I received was .4650in = 11.8mm in diameter. I buffed it down to an exact 7/16 for a perfect fit. Actually the design allows you to insert a diameter larger than the opening by sliding one side over the ridge and then the other. The thinner the disk the easier this is to do. The end disk is 11.9mm and easily slips in using this technique. But if you do you may have to retrieve it with a forceps or long nose pliers. Of course you will need to use the same technique to remove it because it won’t come straight out easily. Again, this is an excellent product.