Heisler Cylinder Cocks
Nelson Riedel, Nelson@NelsonsLocomotive.com
2/15/2010, last updated 01/20/2016

The design and fabrication of the Heisler Cylinder Cocks are the subject of this page.

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 reasons:

  • 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 degrees.

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 thread.

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 mechanically.

Correction:  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 photo below.

These 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.   

I replaced 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.   

Thanks to 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 Shay Cylinder 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 controlled 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 1/8'. 
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 everything straight.
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 was 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 were 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 pressure.    

The Plumbing: The photo above shows the control valve mounted on the steam manifold.   (Much of the other plumbing remains to be done.)

Photo on 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 years.  

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.


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