By the early 1980s the boilers of Mikhail Rodnivacek's 423E 4-8-4Ts were feeling (and showing) their age. Ketterik clearly felt that these engines had not yet had their day and decided to bring them into line with current practice. The most drastic aspect of the rebuild was the conversion to 3-cylinder compound drive. The outside cylinders retained their quartering and received low-pressure steam and a new, high-pressure inside cylinder was installed, its crank being set at 135° to each of the outside ones. All three cylinders drive the second coupled axle. The leading coupled axle is cranked to clear the inside connecting rod. Steam is fed saturated to the high-pressure cylinder and resuperheated for the low-pressure cylinders. In this way, an excessively high temperature of superheat is avoided, simplifying the lubrication of the high-pressure cylinder. In addition, the rebuild involved poppet valves, boxpok wheels and a new, tapered boiler, with a similar firebox to that used earlier on the 323FFs and 533BBs, the difference being the installation of a gas producer combustion system. Vanderbilt tenders were not fitted, since the ability to run tender-first was still highly valued. The existing clipper-sided tenders were fitted with new bogies. All 58 of the engines surviving in 1986 were rebuilt; work proceeded quite slowly, the last rebuilt locomotive emerging from the works in 1991. In the final analysis, not much was left of the original design except the mainframes (suitably modified) and the cabs and tender bodies.