Engines has given an excellent summary of the tech bits. What has not been mentioned throughout, is the diminution of the AAES technical knowledge across the plumbers' trade. That is partly the result of a more centralised, and/or contractorised, maintenance programme Does RAF Scampton have a seat bay (?), I believe not and the opportunities to gain such knowledge are few nowadays. Also, IIRC, on older design seats, the shackle bolt was shouldered and could not be over-tightened as such.