Friction disks to thick?

Also TrailBeast was instrumental in helping me rebuild my 904. A great friendly resource. Also I can't remember so I made a point about a thinner retaining ring? I believe as remember some of the videos that this was more the answer then thinner disk? Also I hear them Machining out the bottom of those so they can receive an extra disc also? Food for thought LOL or just something to make you more confused...