Marine Compound Engine

February 2011.

John R. Bentley 2011.

        Constructing the Fittings (page 2)
Drain Cocks
-  for the Stuart Compound Launch Engine  -

Top and bottom drain cocks are a convenient feature when starting and shutting down

My Stuart 10V was shipped with drain cocks like these, so I decided to copy the general form except using a spring, washers and an adjusting/retaining nut. I have made this type before and they seem successful and I believe they are less trouble prone.

I wished to use two different types of brass for the plugs and bodies to prevent galling and the resultant destructive seizure. I chose standard "hobby shop" brass for the plugs and handles and Naval Brass for the bodies. The latter is a cross between brass and bronze in our continent's terminology, containing both tin and zinc alloyed with the copper.

Putting a flat on an old Naval Brass propeller shaft

Some varieties of this material are harder to machine than steel
(cutting it in the bandsaw is particularly slow)

There is a side story here - keep your eye on that fine horizontal line on the cut face of the right hand piece...

Brass lumber in the making

"Rigged for Red" in the drill press to view laser lines safely around shiny metal

OK - here's how it looks to the Canon's unprotected lens (I don't have red goggles for my camera)

The point is that I am drilling center holes in the block to rotate it in the lathe with tailstock support

Remember that horizontal line I mentioned earlier? I see more than one!

I didn't notice these when I put the piece in the lathe (the camera shows them up much better than the eye)

I am not going to drag this story out, but essentially here's what happened: I was merrily turning the stock to make a round bar for the cock bodies when it looked like the stock flew out of the lathe - but it didn't - some of it was supported by the tailstock center and another bit was swinging around, supported only at one end by a single chuck jaw. Needless to say I hit the Off switch!

I have used a lot of old fishing boat shafts for stock in past and have been very pleased with the results. This is the first time I encountered this situation. It appears that this shaft must have been over stressed in the boat and fractured. The hairline cracks are almost invisible without magnification or better still - a digital camera. Those digicams respond to light differently and show up flaws seldom noticeable otherwise. In any case I have learned a lesson - an extra close inspection of any suspect material will precede mounting any of it in my machines.

This was so interesting that I decided to investigate that piece of stock further

I used a hammer and chisel to begin to separate the metal at the cracks, then pulled it apart by hand

Look at this - it is just like a rotten tree trunk!

Next time you put a bar in your lathe - have a good look first

I looked at a remaining piece of the stock to see the indications
- here is what to be looking for:

Clearly the wiggly crack IS visible...the rest of the metal in the bar is fine.

Now back to the job at hand...

This is a cylinder drain cock from my 1987 Stuart 10V engine

Note the internal plug is retained by a long nut, half of which is hex and the rest is round. A washer containing a square hole is located between the nut and the cock body. The hole engages a square section on the threaded shaft protruding from the bottom of the plug. While this system works well, regular maintenance is required - disassembling, cleaning and regreasing. If it seizes there is little that can be done. Alternatively by using a spring-loaded cock, the conical plug can be pulled straight upward to relieve friction completely, usually allowing the handle to be turned without damage.

In every other way I like these little valves - they are beautifully proportioned and as straight-through cocks they allow passing a hypodermic needle directing into the cylinder to add oil during storage periods. For these reasons I decided to continue making my own modified versions.

Here the Naval Brass stock is being turned under the microscope in preparation for threading

For those not in the UK, "M.E." stands for the model engineering series of threads, in this case 5/32" x 40 T.P.I.

Working under the microscope has taught me a lot about what actually happens when turning metal in a lathe.

Drilling the cock body 1/16" for the passage

To play it safe, I only drilled in the length of the threaded section
- I wanted to be sure the hole came out on-center at the outer end where it shows!

To form the body I cut two light grooves to show where to carve the first recess

The main plunge will begin in the left groove, the light groove on the right is the limit of the flange thickness

The globe will be formed to the left of this recess

The globe roughed out

Note that the design dictates the left flange (actually the nose) will be a smaller diameter

Spinning at high speed after some polishing

Stopped to check the progress

Ready for parting off

Searching through my things from past projects, I found this handy packet

It contains a 5/32" x 40 fixture and a properly-tapered reamer that I made previously from hardened drill rod

The body was turned around and screwed into the brass hex fixture to complete drilling of the steam passage

Here is a lineup of tools in their ER-16 collets that I used for making the tapered holes in the body

First a 1/4" endmill puts a flat on top of the globe

I center-drilled, then drilled a 1/16" pilot hole

Final drilling to the minimum size of the taper

Here is the taper reamer I made previously from hardened drill rod

After removing the reamer from the newly-tapered hole

Using the endmill to clean up and create a small flat to increase the durability of the edge

Same deal on the bottom side

Trying an original Stuart plug in the new body

Holding the prototype against the embryo plug/handle for reference under the microscope

Cutting the taper freehand - it isn't difficult and not worth setting up the compound

Fitting the taper - blackening with a marker

Trying on the body to look for high spots as evidenced by light areas in the marking ink

Appropriate adjustments have been made to the taper

Cutting the 8BA thread

The plug's handle section taper has now been turned as well (freehand)

I don't have a picture of bending the handles, but I will explain below!

I clamped the plugs in my vise between two sheets of aluminum for protection and heated the handle red hot while I pushed it over with flat face of a small hammer... I didn't say I hammered it - I simply pushed.

This method is not very good - I got away with it but I don't recommend it. I risked deforming the plug taper and getting the heat to the narrow neck of the handle was all but impossible due to it being conducted away by the aluminum sheets. It took a lot of heat from a swirl torch and a good deal of gas. Swear words were needed but not actually employed.

I think a much better method would be to make a matching conical hole in a small bit of stainless steel and hold that in the vise protruding above the jaws about an inch. Heat conduction in stainless is next to nil comparatively speaking and this little fixture should simplify the job. The time in making the fixture should be less than I spent bending the handles otherwise.

Drilling brass hex stock to make the plug/handle retainer nut

I use the tool for support for the drill when starting a hole in very small pieces

8BA (British Association) tap about to enter the hole

Size comparison - the hex is .0625" (1/16") across the flats

If the parting tool is ground the wrong way and laziness prevails, here is a solution:

A standard set of cuticle cutters works as side cutters to remove the thin walled tube formed on the nut

I cut the springs from longer units and I had small washers on hand
(washers are easy to cut from small rod)

The four drain cocks not quite finished sitting beside a standard 5X eye loupe

A trial in the threaded holes on the cylinder block

Here is the Lomo MBC-10 stereo microscope set up to view a cock body in the lathe ready for drilling

Drilling through the existing passage to make the hole through the plug
(obviously the handle must be properly oriented for this job!)

It seems to be a nice clean hole considering the amount of magnification in this image

This is the grease showing in the last picture - I will add powdered graphite

One of the completed valves in my hand for size reference

The four finished cylinder drains, awaiting installation with custom spacer washers
(to provide proper orientation - upper handles on top, lower handles at the bottom)

Next: Fittings: Exchange Pipe, Flanges and Glands


Castings, Materials and Fastenings


Cylinder Block

Top Covers

Bottom Cylinder Covers

Steam Chests

Crosshead Guides and Bracket




Connecting Rods and Crossheads

Main Bearings


Fittings: Oil Cups

                 Fittings: Drain Cocks (this page)

Fittings: Exchange Pipe, Flanges and Glands

Stephenson Link Reversing Gear (5 pages)

Completing and Erecting the Compound Launch Engine


Compound Launch main page

Main website home page

(c) John R. Bentley 2011.