From the Rio Grande Southern Technical pages:
(http://users.viawest.net/~bdwhite/)
"...To further increase tractive effort, boiler pressure was to be 180-lbs. psi, 30 lbs. over what was considered "standard" at that time. In order to set the steam chests containing the engine's flat, or slide, valves under the large 60" diameter smokebox, the chests were canted outboard. This peculiar Brooks design had been applied to some of Brook's recently built standard gauge engines. Unfortunately, increased steam pressure made possible by new boiler technology and size caused an unbalanced load on the old-fashioned slide valves. This in turn caused damage and excessive wear to the traditional Stephenson valve gear, which was "standard" at that time. Also, the sloping nature of the steam chests caused major lubrication problems.
Soon after construction of engine 2951 (C&N No.30) was underway, but before sister engines 2969 and 2970 (C&N No.'s 31 and 32) were begun, Brooks decided to abandon the dubious canted-chest slide valve design, and created inboard piston valves for the other two. This was now to be considered "standard" for engines with higher boiler pressure. Traditionally, slide valves had been standard design for controlling the flow of steam to the pistons, had "worn in" as the engine was used over time as constant steam pressure was applied to only one side, and required little maintenance or replacement over time. Piston valves, which allowed steam to flow on both sides of the valve, tended to "wear out" over time, and required replacement more frequently, but were easier to handle at higher steam pressure.
All three engines were fitted with inside frame Stephenson valve gear (the mechanical apparatus that connects the valves to the wheels). When delivered, engine No.30 differed slightly from her sisters. These differences, in addition to No.30's sloping steam chests, included lower, frame-mounted running boards, and the application a New York Duplex #2 air compressor on the engine's left, or fireman's side. Engines 31 and 32 were equipped with boiler mounted running boards and each sported a Westinghouse 8 or 9 ½" pump on the engineer's side. All were fitted with extended piston rods, the covers of which protruded from the cylinder head covers near the pilot wheels. Thinking at the time was that by having the piston rod guided through the forward end of the cylinder and the rear of the cylinder, the connection of the piston to the rod would be strong enough to handle the increased steam pressure at 180 psi. Brooks engineers felt this was a necessary precaution at the time.
...
The old problems with high pressure and uneven wear had not improved over time. The C&S had begun a campaign to "modernize" its fleet of standard gauge 2-8-0's in 1921. Among improvements to many of the engines was the addition of Walschaerts valve gear, which had corrected similar problems to those of No.74.
In 1926, George Lundberg of the mechanical department designed a distinctive, customized Walschaerts valve gear for No.74. This addition would help fix the problem of the Johnson bar being so difficult to manage and also correct the excessive wear to the existing Stephenson valve gear. The most major and significant rebuild the engine would ever experience was soon underway. In order to install the new valve gear assembly, the engine had to be completely dismantled and rebuilt. Because of the placement of new hangers supporting the rocker arms, the air tanks had to be raised on both sides to allow clearance. This caused the running boards to be "stepped" up, or raised, which resulted in a much different appearance for the 74. Sister engines 75 and 76 retained their original Stephenson valve gear, and thus, retained their straight running boards. The 74 was the only narrow gauge engine the C&S equipped with Walschaerts valve gear. The redesign was successful and corrected the problems of wear and control, which had caused problems since the engine was built. Had this project not corrected these problems, new piston valves would have been fabricated at considerable expense. This modification did not address the lubrication problems associated with the sloping valve chests, and those continued to plague the engine for the rest of it's operational life."
So there ya go.
Chris
