The Mogami class was designed with five triple six-inch turrets that could later be replaced by five twin eight-inch mountings. That planned conversion did not include comparable provision to upgrade the armour. The declared standard displacement of 8,500 tons is contrasted with an actual figure of 11,000 tons, so treaty restraint does not excuse the imbalance in this criticism. Funnel arrangements and aviation-fuel stowage also remained unsatisfactory when the heavier guns were installed. The objection is that a ship intended for a later heavy-cruiser role needed an upgrade plan for the supporting systems and protection as well as its armament.
Chapter · 51:51-55:15
Mogami's gun upgrade did not rebalance the cruiser
Mogami was prepared to exchange six-inch for eight-inch guns without making the rest of the cruiser equally ready for its heavier intended role.