I've just acquired a new MP for my Alto. I applied grease to the cork before sliding it onto the neck. After playing for about two hours I twisted the MP off the neck to put my saxophone away. When the MP separated from the neck it also removed a 1/3rd of the neck cork with it! On closer inspection I discovered that the MP's orifice was 0,2mm smaller in diametre than my 4C and S8O MP's. The interior surface of the MP was rough and dull looking which is why the MP had gripped the cork so firmly. I used fine emery sandpaper wrapped around a 15mm felt-tip pen's shaft to smooth down the surface. This made it polished, dark and shiny like the outside surfaces. Be warned and inspect the inside of a new MP closely, before fitting it to the neck of your sax or the same thing may happen to you.