Drifting Valves or Valve By-Pass

Updated August 2, 2004

What were they?

Wagner cylinder bypass valves, also called "drifting valves," are the smaller-diameter cylindrical chambers above the valve chambers on modern and modernized Santa Fe steam locomotives. Their purpose was to provide lubrication to the cylinders while the locomotive was drifting (coasting) with the throttle closed, as was often done when running downhill on long grades when the engine was equipped with bypass valves. They also relieved compression in the cylinders to protect the cylinder heads, which is what I think Linn Westcott meant by "ramming" in the photo caption in the MR Steam Cyc.

I don't know who Wagner was, but I suspect he may have been a Santa Fe employee because these bypass valves don't seem to have been used by any other railroad. There were other kinds of proprietary drifting or relief valves shown in the steam Cycs, but they weren't used by the ATSF.

The first Santa Fe locomotives to have these valves were the 3765-class 4-8-4s and 5001-class 2-10-4s of 1938. All the later 4-8-4s and 2-10-4s had them also, and they were retrofitted to the 3751-class 4-8-4s, many of the 3800-class 2-10-2s (and the 3829), and several of the 3700-class 4-8-2s.

Andy Sperandeo & Francis Post

See also: http://www.wheelsmuseum.org/stagner.html
http://www.sbrhs.org/faq.html

What do they look like?

3708 has the original cylinders, while 3721 has been modified with drifting valves.
(Photos from Ainsworth & Karam, 3700 Class)

3803

3856

The Broadway Limited 3751 modeled this feature correctly for the modernized engine. The announced 3800 class do not have them.

Corrections and input to Steve Sandifer.