Cass 6 Engine Part IV 
Rocker Arms & Valve Stem Heads
Nelson Riedel, Nelson@NelsonsLocomotive.com
6/26/2004, last updated 10/22/2005

The valves on the Heisler are very compact and use fewer parts than other locomotive designs.   The photo above shows the upper part of the the left side valve system with some of the parts labeled.  The objective of the valve system is to move the valve stem up and down at the correct time. The valve stem head is attached to the lower end of the valve stem.  The valve stem head is also attached to the rocker arm.   The rocker arm is hinged at the rocker arm base so the valve stem head actually moves along an arc rather than exactly along the axis of the valve stem.  The arc radius is relatively large and the angle traveled is very small so the deviation from a straight path is small.   I've been told that the valve stem bends slightly to accommodate the deviation.    

The rocker arm and valve stem are driven by the reversing link via the link block.    The reversing link is positioned by the link hanger via the link saddle.   The photo shows the link block near one end of the reversing link which corresponds to the engine running in one direction  ---- the reversing lever in the cab is near one extreme.   If I traced the linkage correctly, this is the link position for forward.   If the reversing lever in the cab is moved to the rear, the link hanger will push the reversing link out on the link block to a position where the link block is near the other extreme of the reversing link.  The engine will run in the reverse direction at that setting.         

This photo shows the lower portion of the valve gear.   There are two eccentrics on the crankshaft which serve both cylinders.   Each eccentric has an eccentric strap.  Each eccentric strap strap has two eccentric rods, one fixed and one pivoting.   One rod from each strap goes to each reversing link.   In the photo, the fixed rod on the rear eccentric strap connects to the left side (right side of photo) reversing link.  This connection is right under the valve stem so it is this rod and the rear eccentric that is controlling the left cylinder.  The pivoting rod on the rear eccentric strap goes to the other reversing link and will be directly under that side valve stem and be driving that valve.   So, in the forward direction, the rear eccentric and rear eccentric strap drives both valve stems.   The direction is controlled by the tumbling arm.  If the tumbling arm is rotated clockwise as far as it will go, the reversing links will also move to the right  and position the front eccentric strap rods under the valve stems to drive the valves.          

Rocker Arm Base: The rocker arms as well as the tumbling arm and associated lever all mount on the rocker arm base.   The photo at right is taken from the right side of the locomotive and shows some of the many surfaces of the base casting.   The other valve components  obstruct the view of the base so it is impossible to get a photo --- or a view of the complete base.   It is possible to see --- and measure parts of the casting.  This photo gives a good view of the upper front.
This photo taken from the left rear shows how the base straddles the top seam between the two haves of the crankcase.
This view of the left side shows 2 of the 4 bolts which attach the base to the top of the crankcase.
This shows the right lower front of the base where the shaft connecting the tumbling arm and the tumbling lever  passes.
This shows 2 of the 4 bolts that attach the base to the front of the crankcase.

The drawing above is the product of several visits to Cass, examination of about 20 photos and a dozen or so drawing changes.   This base dictates the position of most the other components so it must be exact for everything to fit together correctly.  As we'll see later, everything does in fact fit together.  

Rocker Arm:   They have a spare rocker arm at Cass that I was able to measure it in detail.    Working outside in the bright sun  was sure a switch from crawling under the greasy locomotive inside.  I should have been more careful of my shadow.
This drawing  shows the rocker arm overall dimensions.   It sure would have been easier if they would have made the shape more regular with at least one straight side.   The hole is the center is for an oil line to the front main bearing.

Valve Stem:  The long rod with the loop at the end pictured above is the valve stem.  The loop fits in the steam chest where it connects to the sliding valves.   The steam chest contains a set of springs that are involved with the stem-valve connection.   I was not able to observe or photograph the inside of the steam chest and the sketches I have are of so poor quality that it is impossible to interrupt them.  Will update this section if I find better data.  The photo on the right shows the threaded end of the valve stem to be 1.25" diameter.

Valve Stem Head: This casting connects the valve stem to the rocker arm.  There are adjustment nuts on the threaded end of the valve stem that allow the position of the head on the stem to be adjusted.   This photo is of the right valve stem head.
This is a photo of the left valve stem head.  I have several photos of each side that I used to make a drawing.   After redoing the drawing several times and not being able to get it to match the photos I finally realized that the two sides are different.    This left side seems a bit heavier and stronger so I assume it's a later improved design --- possibly  a replacement for a broken part.
In the end I drew the left side casting because it is stronger and more importantly it is easier to draw and should be easier to model.

This is a good point to break;  the valves are finished in Part V.