The DeHavilland Cirrus 1/6th Scale - Rebuild

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This entire web site was begun some years ago when I was deep into the construction of the Hodgson-9 radial engine. Prior to this, I had completed a few other projects. One of them was the DeHavilland Cirrus glow engine in 1/6th scale, which was published in a serial format by Strictly I.C. magazine. This was my first internal combustion engine. Construction was relatively uneventful, and it ran like a dream after I became used to it's unusual induction system, and solved the head to cylinder sealing issue.

The engine started life as a set of aluminum sand castings, mostly of the crankcase and front and timing covers. The castings, to the best of my knowledge, are no longer available. This is a shame, as it is a very nice engine and would be perfect for the scale R/C enthusiast. The power output is roughly equivalent to a modern .60 CI 4-stroke engine, but with a sound and appearance surpassing anything available.

After extensive bench running, I loaded the internals of the engine with some light oils, a combination of WD-40 and 10wt 3-1 type oil. It then resided with pride on my desktop. As the years passed, it gathered dust. I also noticed that the rockers, pushrods, and tappets were all gummed with castor oil, as the engine is glow ignition. It was crying out to me in its neglect, and I had to respond!

I decided to tear it down, oil and clean where necessary, relubricate for longer term storage.

The left side clearly shows the current, model induction system. On a full-sized Cirrus, the carburetor is mounted on the intake manifold pipe in an updraft configuration, which in this photo would place it on the far side, not visible. Since this engine uses glow fuel mixed with oil, the path that the induction must take begins at the rear, through the gears, and into the crankcase. It travels fully forward through a sealed channel, lubricates the front bearing, and is then directed back through the little ends of the connecting rods, ultimately proceeding up to the intake manifold. The path is long. Priming the engine takes some serious turns of the prop, and the engine is slightly laggy with the throttle response.

The first step was removal of the rocker arms and support posts. My fingers give away the scale a bit. These rockers are A2 steel and show little wear. The pushrod seats are 2-56 cap screws, with the screw heads cleaned into a hemisphere internally with a dental burr. The bottoms of the cap screws were slotted for a jeweler's screwdriver, and the nut locks the seat in place for correct valve clearance.

There was just a bit of corrosion which cleaned up with a fine wire wheel in the drill press.

A pair of rockers is mounted onto a silver-brazed post. The post is then inserted into the head. The green color is a trick of lighting... the actual color is a light grey. The rockers are hardened and tempered, while the post is left mild.

A scale post bolt would be a nice touch here, but I went with cap screws for now.

Next off was the exhaust. It is collected via 4 stub headers into a single manifold. Between the visible flanges here and the heavier manifold are the stub pipes.

This was a tricky assembly of all stainless. The four flanges were first silver-brazed to the stubs, resulting in four stub/flange assemblies. To silver braze round piping onto a flange, the silver braze wire is first wound into a tight coil of the correct diameter so as to fit around the pipe. The pipe is inserted into the flane, and a ringlet of braze is added. The entire assembly is fluxed heavily, then brought to dull red with a propane torch. The braze will flow instantly, and it will form a perfect filet if this technique is used.

The four flanges were mounted to the head, and the stubs were inserted into the drilled exhaust pipe. The torch was applied to the pipe, with the heat flowing to the stub, thus executing the final braze but keeping the head relatively cool.

The exhaust flange seats. Yes these were angled and quite tricky, executed with a 5/16" end mill, swept. Like many machining operations, the setup consumed 95% of the time. Once the compound angle was set, the cut itself was a snap.

More burned and dried castor oil is evident.

I like this view... note the gaps in the finning created by the swept end mill. For scale, the two flange holes are tapped 2-56.

There's a lot of items which access the combustion chamber on a small, complex head. As this head progressed, each setup became very nerve-wracking, as the slightest goof could potentially scrap a lot of work.

No scrapping required; the head came through in one attempt.

On the intake side. Eight valves with springs and keepers...10 long hold-down bolts... and some deep fin work, executed with a slotting saw.
The cam followers (tappets) are visible, with their bronze inserts. The intake manifold is brazed copper and brass piping, with the "T" executed from solid.

Like the exhaust manifold, the flanges were silver-brazed using the ringlet + flux method.

The manifold has been removed, greasy and gummed.