MRSR 91 Engine 

Nelson Riedel, Nelson@NelsonsLocomotive.com
10/1/2004, last updated 04/08/2006

The Mt. Rainier Scenic Railroad # 91 Heisler locomotive was examined on September 24 and 25, 2004. The engine appears to use the same parts as Cass 6 except for: 

• The cylinder castings

• The valve (piston vs. slide)

• The eccentric straps & pivoting eccentric rods.        

These parts that are different are described below.  Also, one of the connecting  rod bearings was off so I was able to learn more about the bearing and strap.  The details of the bearing and strap are probably identical to Cass 6 but are described here.  

Cylinder Top: These next four photos show the cylinders.   This first photo shows the right cylinder.   The cylinder is to the rear of the casting and the valve is toward the front.   Note that the edge of the head is about even with the cylinder side and the head cover diameter is larger than the cylinder casting and hangs out about 3/4" beyond the casting side.  The casting is 23" high  and 21" across.   The piston and valve centerlines are 24" apart.
The valve and piston centerlines are 17" apart on this, the left cylinder.  I think there is just enough room so that the valve and cylinder heads are full circles with no overlap.  The photo shows that the left side cap is a single piece covering both the valve and cylinder.   The two cylinder cocks are linked together and also linked to the cocks on the other cylinder. The third cock between the valve and the cylinder probably drains the steam input area of the valve.  This cock is also linked to the cylinder cocks.
Steam Entry Port:  This shows the steam entry port on the front side of the cylinder, in this case the left cylinder.  The small tube feeding into the elbow is the cylinder oil supply.   The pipe diameter is about 6" OD and the flange diameter is about 11"
Exhaust Port: That is the exhaust chamber and exhaust port on the inside of the left cylinder.  The chamber runs the full length of the valve area with the port at the bottom of the chamber.  The exhaust pipe is about 6.5" OD.
Cylinder Top: The next set of photos show the right cylinder of a smaller (70 ton) late Heisler.   The boiler and heads had been removed exposing the boiler side of the cylinders as well as the inside of the cylinders.
Inside Cylinder: This is looking down into the cylinder.  The nut on the end of the piston rod has been removed.  The rod -piston joint is a tapered fit.   I suspect it was at this point in the disassembly that they discovered a special tool is required to remove the rod from the piston.
Cylinder Steam Port: This shows the steam port into the cylinder to be about 1" high.  The cylinder is 17" diameter. The port appears to be about 20" long giving an area of 20 sq in.  That gives a cylinder to port area ratio of about 11, appropriate for a slow speed locomotive.
Valve Bushing: This shows the valve cylinder with bushing (sleeve).    The upper holes (slots) lead to the exhaust chamber.   The next set of slots connect to the upper cylinder port.  The wide slots in the middle connect to the valve steam entry port.   Further down is the slots that connect to the bottom cylinder port.  The slots at the bottom connect to the exhaust chamber as do the upper most slots.     That is a spider web spanning the top of the cylinder.
Exhaust Chamber: This shows the outside of the exhaust chamber that runs the length of the cylinder casting on the inside beside the valve. The chamber connects to the exhaust holes at each end of the valve bushing. The bulge at the bottom just below the frame bar connects to the exhaust pipe.
Exhaust Pipe: This shows the exhaust chamber port and the S fitting that routes the exhaust below and inside of the upper frame bar.   Note that the fitting is held in place by a single bolt.
Steam Input Port: This is a view of the steam input port.  One of the valve bushing ribs is visible at the through the port hole.   The exhaust pipe is on the right side of the photo.
Cylinder Drawing: This drawing shows the MRSR91 left cylinder.  The overall dimensions are accurate.  The dimensions of the ports, the exhaust chamber and the mounting pad are based on rough measurements and may be off  by up to 10 or 15%.  Because of the possible inaccuracies, those dimensions are not show on this drawing but may be picked off the file in the Drawings section.   The right cylinder casting is identical to the left cylinder casting except that the  section between the valve and cylinder is wider so that the valve and cylinder center separation is increased from 17" to 24".

Eccentric Straps:  Since MRSR 91 uses the piston valves which require much less force to operate than slide valves, the components that drive the valves were inspected to see if any changes were made from the earlier design.   The only change detected is the eccentric strap and eccentric rods.   The earlier design used on Cass 6 and the later design on MRSR 91 are both shown below,

Cass 6 Eccentric Strap & Arms:  The early design eccentric strap has one of the rods rigidly attached (left arm in photo) and the other rod attached via a pivot.  This pivoting rod has a fixed length.  The rigid rod is attached by three bolts through elongated holes in the rod.  The rigid rod length can be adjusted by loosening the bolts and sliding the rod.  Mr. Artie Barkley, the Cass shop foreman told me the procedure to adjust the valves is to set to full gear with the piston at the extreme and the pivoting rod lifting the reversing link.  Then adjust the valve stem for correct lead.  The reverse lever is then set full gear in the opposite direction and the rigid rod length is then adjusted for the correct lead.
This shows just the eccentric strap of the type used on Cass 6.  There is a bearing strip attached to the inside of the strap.
MRSR 91 Eccentric Strap & Arm: The left rod in this photo appears to be  similar to the rigid rod on the earlier design.  The right rod appears to be identical to the left arm but is attached to a band that can rotate within the strap.  The fixed rod length in the early design was adjusting via slotted holes in the rods for the attachment bolts. In this later design the bolts (or maybe studs) are fixed in the rods so the attachment holes in the strap and  band are probably slotted.  The band is positioned between the strap and the bronze colored bearing.  The band is split with one side of the split visible just above the joint  between the two halves of the strap.
This photo was taken sometime after the previous photo and after the engine had been moved.    Note that the band has rotated from the position in the previous photo; the joint between the two halves of the band are nearly aligned with the joint between the two halves of the strap.
MRSR91 Eccentric Strap & Rod:  This rough drawing shows the principle dimensions of the strap.  The rod is the same as the fixed rod on the earlier design used on Cass 6.  I didn't see the inside of the strap, the bearing or outer band, so I can't speculate as to the inner configuration.   For the model I'll probably combine the inner band and the bearing into a single bronze part.

Connecting Rod Strap & Bearing:  This is one of the MRSR91 connecting rod straps.  Note that there is a wedge in the bottom of the strap that can be moved from side to side by adjusting bolts that go through the strap sides into the wedge.
The lower bearing half also has a wedge shape that mates with the movable wedge in the previous photo.  Slack between the bearing halves can be adjusted by moving the wedge.   This is similar to the wedge used on the Shay rod except on the Shay, the wedge is above the bearing and the wedge mates with a wedge shape on the rod end.
This is a grease fitting that fits in the bottom of the strap.  The tube goes through a slotted hole in the wedge and then through a hole in the lower bearing half.
This is the assembled strap and lower bearing ready to reinstall.   That's the upper bearing half setting behind the strap.  I suspect that the Cass 6 is identical to this configuration. I didn't make a drawing of the connecting rod  internal parts but will likely try to work something similar into the model design.

The MRSR91 cylinder and eccentric strap shown earlier are combined into a single file that can be downloaded from the Drawings page.