2/15/2010, last updated
The design and fabrication of the
Heisler Cylinder Cocks are the subject of this
Model Pipe Threads: This is the first plumbing
done on the Heisler for several years. In the interval
from doing the Shay plumbing and the earlier plumbing on the
Heisler I've changed the threads I'm using on the scale pipes.
I became disillusioned with the MTP thread for the following
The taps and dies are very expensive
Considerable force is required on the dies
Even with the correct taps and dies the
fit is often not good.
In many cases even very tight fittings
leak and require a sealant.
I now use 40 TPI straight threads for all the fittings
--- 1/8", 3/16". 1/4" and 5/16". Straight thread taps and
dies are readily available from Enco for half or 1/4 the price
of MTP taps and dies. (Standard 5-40 taps and dies
are used for the 1/8"-40 threads.) These straight thread dies
require much less force than the taper thread dies and work fine
on brass or hard copper tube. I use a sealer
on all joints --- Loctite 567 in most cases and Loctite 620 for
joints I'd normally want to solder such as the ends of union
fittings. The 620 softens when heated to about 600
PM Research has sold straight thread scale fittings for years.
There is an interesting note about the various threads on their
website. They now also sell MTP fittings as well as fittings
with a taper thread that
matches the thread profile of their straight thread fittings.
I don't hesitate buying the MTP fittings and just run a straight
thread tap or die over the threads to make everything straight
One exception to using the 40 TPI is fittings from Clippard which
use 10-32 threads. For example, the Clippard
regulators use the 10-32 thread. Once I leave those
fittings and go to tubing with scale fittings I transition to
the 40 TPI threads.
One final point ----- unions are required on most pipe runs.
The scale unions look great but are frequently difficult to take
apart. Standard brass tube compression fittings are
much easier to use and about 1/3 the cost. However, the
compression fittings are 2 or 3 times scale size. My
compromise is to use scale unions in very visible places and
compression fittings in the less visible places.
The photo above shows the cylinder cocks on MRSR 91 which are very
similar or identical to those on Cass 6. Note that there are
three cocks per cylinder, one on each end of the cylinder and a third in
the bottom of the exhaust area. The 6 cocks are linked together
Shortly after this page was posted Dave Fontes emailed with the
comment that the third valve
is tied into the bottom of the steam chest.
I went back and took a closer look at Cass 6 and found that
indeed the third valve is in the bottom of the steam chest, see
photos were taken in 2003 near the end of the restoration which
included the new boiler. The cocks are out of the left
cylinder but the holes indicating where they go are visible.
Note that on the right side only two of the cocks have the
mechanical linkage and I assume that is the case on the left
side. The use of the globe valve for the third
cock might indicate that it is not used as often as the cocks at
the ends of the cylinders.
the previous photo of the MRSR91 cocks with a photo that shows
the entire cylinders and piston valve areas. Looking
back at that photo, those third cocks there seem to be between
the main cylinder and the valve cylinder in the area of the
passage between the the valve cylinder and the lower end of the
main cylinder. This is likely not the exhaust area
as originally thought. However, closer inspection of the
MRSR91 photo revealed what appears to be plugs at the lower end
of the valve cylinders. These may be drain plugs out of
the exhaust area, the lowest part of the valve cylinder and is
analogous to the drain cock in the lower part of the steam chest
on Cass 6.
I don't know the reason for the drain
mechanisms out of the steam chest/valve cylinders.
One possibility is to drain any water to prevent freezing
problems in winter months.
Dave Fontes for straightening me out on this. All comments
and input are welcome.
The cylinder cocks on the shay have worked very well (see
Cocks). They are are very close to scale and would work fine here.
However, the mechanical linkage would be more difficult.
It is my understanding that cylinder cocks are needed to remove
condensate from cold cylinders so that excess pressure doesn't build
during the compression phase (water is essentially non compressible).
I don't think that is a concern with small slide valve engines since
excess pressure will cause the valve to lift and excess pressure to
escape into the steam chest. That is not the case with a
piston valve engine like this Heisler.
After thinking about it for a while I decided to use steam
cylinder cocks. The plumbing of the steam control tube appeared to
be easier than the linkages required for mechanical cocks. A
very attractive bonus with the steam controlled cocks is that excess
pressure in the cylinder will force the cocks open to relieve the
pressure even if the control valve is set to close the cocks. The
challenge is to make the cocks as small as possible so that they don't
look too much out of scale. This task is somewhat easier for
the Heisler than the Shay since the Heisler cylinders are much larger.
|The photo above shows the cock components
and the photo on the right shows the assembled cock.
The stud out the side is threaded 10-32 and screws into the
cylinder. (The holes in the cylinders were threaded
10-32 in anticipation of using Clippard fittings. If that
were not the case I would have used 3/16"-40 threads.) The hole in the side is the exhaust port.
The control signal is via the 1/8" OD tube.
The graphic above shows more detail about the components and the
operation. The O-Rings are all #007 (5/32" ID - 9/32" OD) Viton.
The O-Ring under the end cap serves as a seal. The Plug and
End Cap are both threaded 5/16-40. The tube is 1/8" OD hard
copper and threaded 5-40 as in the inside of the Sleeve. The
Plug, Body & End Cap are all made from 3/8" brass hex bar stock.
The piston is 1/4" diameter brass and the sleeve is made from
3/16" brass hex bar.
|Cylinder Cock Body: The
drawing on the right shows the details of the cylinder cock
body. The body is 3/8" hex. The 3/16"
diameter input port is made separately and then silver soldered
to the hex part. As noted earlier, the outside of
one end is threaded 5/16"-40 and the inside of the other end is
tapped 5/16"-40. Both these threads are to a depth of
|The first step in making the body was to drill a 5/32" hole the
entire length. Next, the end was turned to 5/16" and then
threaded using a 5/16-40 die as shown in the photo on the right.
The threading is done by hand but held in the lathe to keep
|The piece was then parted off to the correct
length. The 1/16" diameter output port was then drilled in
one side. A 3/16" diameter hole for the input port
drilled to a depth of 1/16" for the input port. The input
port is a 1/4" length of 3/16" brass rod threaded 10-32 on the
outside and drilled 3/32" on the inside.
|This photo shows silver soldering the
input ports to the bodies. Note that a couple extra parts
were made in case some were damaged later.
|The parts were cleaned up by bead blasting
after the soldering and then the body was mounted in the lathe
again. The inside of the cylinder was drilled 17/64"
to the correct depth and then the end was tapped 5/16-40 to a
depth of 1/8"; the photo shows this tapping. The last step
was to ream the inside using a 9/32" reamer.
|Plug: The plug shown
in the drawing on the right is essentially the same as the end
of the body with the male thread except that there is no hole in
the middle. They were fabricated using the same technique as
used for the body.
|Cap: The cap shown in
the drawing on the right is essentially the same as the end of
the body with the female thread. They were fabricated using
the same technique as used for the body.
|Piston: The pistons
turned on the end of a 1/4" diameter brass rod using a 0.070"
wide cutoff tool.
|Sleeve: The sleeves wwere made
from 3/16" hex brass bar stock and washers cut off the end of a 1/4" diameter brass rod after a
5/32" hole was drilled in the middle. The ends of the hex
rods were drilled for a 5-40 tap and the outside turned to 5/32"
|Next, the washers were silver soldered on the end
of the rods. The photo shows
soldering the washer on 5 sleeves. After the
soldering the sleeves were remounted in the lathe and any excess
solder turned off the 5/32" diameter end and the sleeves were
cut to length. The last step was to mount the sleeves back
in the lathe and tap the center 5-40.
|Control Valve: (Updated 1/20/2016 to
correct dimension errors in text and drawing) Photo at right shows the
control valve. The control valve is similar to the cocks and
uses the same type O Rings in a 9/32" diameter cylinder.
| The bottom port is the steam input.
It is screwed into the
steam manifold at the top rear of the boiler. The other two ports use the same type caps and
sleeves as the cocks. The top port goes to the cocks
and the rear port is the exhaust. When the piston is
to the rear as shown in the diagram steam flows from the input
to the top port and on to the cocks to hold them closed.
If the piston is moved to the left position, the input steam
is cut off and the pipe to the cocks is allowed to vent via the
exhaust permitting the cocks to be pushed open by cylinder
|The Plumbing: The photo
above shows the control valve mounted on the steam manifold.
(Much of the other plumbing remains to be done.)
the right shows the routing of the two 1/8" tubes from the
control valve. The tube from the back of the valve
is the exhaust and vents at the bottom of the boiler. The
tube from the top of the valve is the control for the cocks.
This tube runs down the side of the boiler and then bends 90
degrees and goes between the frame and the boiler to a tee near
the right side cocks.
The plumbing of the right side cocks is shown above. That is a
3/16" tee with a 1/8" tube union on the tube from the control valve and
the another tube union on the tube to the right side cocks. The tee is hard connected to the
tube to the left side cocks.
This photo shows the plumbing of the left side cocks and the pipe under
the boiler to the right side. The 1/8" globe valve serves as a
drain cock for the control tube. This tube run provided a use for
a pair of 45 degree elbows that have been in my parts box for nearly ten
|Tube Unions: This is the first time
I've used the tube unions shown on the right. These are
for 1/8" tube which is soldered inside the cone with the right
side threaded 3/16-40 to match the 3/16" scale fitting.
I secured the tube with Loctite 620 instead of soldering.
These unions were purchased from PM Research.
They sell the nuts and cones separately. If I were doing
it over, I'd consider machining the ends of the cocks to work
with these nuts and cones and use them instead for the end caps
and sleeves. These nuts are 5/16" hex --- smaller than the
3/8" hex body and cap which would permit the cock body to be
screwed down against the cylinder and still have clearance to
remove the nut.
Caution: The cocks have been tested on air and
work great. However, they haven't been tested on steam yet.
I should have steam up in a couple months. I'll update this note
at that time to either confirm that they work fine on steam or will
explain what I had to do to make them work.