The DeHavilland Cirrus 1/6th Scale - Rebuild

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This is unfortunately the final installment of the dehavilland Cirrus rebuild... I sadly experienced camera and media failure, and roughly 20 photos were lost into the ether. For this you have my apologies. The entire sequence of photos will stop abruptly, and the Cirrus engine will magically "reappear" in its completed form!

These three Cirrus pages were never intended to be anything more than filler while I wait for the materials and castings of my new project, which will be every bit as challenging as the Hodgson-9 or the turboprop.

Continuing the rebuild...

The bottom of the crankcase was freed from the rest of the engine, exposing the inner workings of the Cirrus. The crankshaft is fixed in place by the front and rear crankcase covers. In the full-sized engine, these are actually part of the upper and lower crankcase shells, but in this miniature, the castings were created to generate the illusion of two pieces. Note the cast-in "split" on this front cover.

Next in the teardown process, shown here, is the removal of the camshaft. Since the timing gear cover has already been removed, the camshaft is now free to be lightly tapped to the rear, from the front, with a punch. Fortunately, there was sufficient lubrication and minimal gum, and the camshaft popped free without any problems.

The camshaft and gear partially freed. Note the timing mark punch on the face of the brass gear, right next to the lightening hole at the 2:00 position. A similar punch mark was set into the crankshaft gear, so that upon reassembly, valve timing would be more or less automatic.

The camshaft has three bearing surfaces, one at each end, and a third in the middle of the shaft. Of course, the diameter of these bearing surfaces is greater than that of the lobe peaks, so the camshaft has no problem when removed, i.e. no interference is generated by the cam lobes on the female bearing surfaces in the crankcase.

The camshaft journals bear on two pressed bronze bushes at the ends, while the middle journal rides on aluminum, which was line-bored directly into the crankcase in a lathe operation.

These gears were cut by me in the shop with a homemade hob, as outlined by Pat Loop in an Strictly IC article. The gear hobbing operation was a lot of fun and very rewarding to do... highly recommended, requiring a number of skills, none of which is insurmountable. Probably the most difficult was the creation of an accurate gear hob using O1 carbon tool steel, machined, hardened, tempered, and then sharpened by hand carefully with a hard Arkansas stone.

The cam had some castor gum but no corrosion, despite the mottled appearance. The center journal is obvious, as are the end journals.

Shown here is the rear of the camshaft with the timing gear removed. The camshaft diameter which is inserted into the gear is, I believe, 0.187" or roughly 4.75mm, quite small. I had no keyway broach, or key, capable of fixing the gear to the camshaft radially.

To remedy this, the timing gear was slotted in the lathe using the carriage as a shaper, and making use of a custom ground shaper-styled tool. A radial hole was drilled into the camshaft, and a pin inserted as shown. This setup is more than adequate to keep the gear from spinning on the end of the camshaft.

The cam lobe closest to the bottom of the picture exhibits only the slightest wear, as evidenced bu the shiny cam peak. In general, the cams were in great shape, and this was after some serious bench running of many, many hours duration.

With the camshaft removed, I had access to the tappets. Like the camshaft, these were turned from O1 steel. A better choice for the tappets is A2 air-hardening steel, which is quickly becoming my favorite tool steel, as parts created with A2 may be wrapped in stainless foil for heat treatment, completely eliminating the need for a borosilicate glass coating to avoid scale.

Scale generated during heat treatment in a home shop is very irritating, and hard to avoid, without either using keep-bryte or similar borosilicate, or better yet, the wrapped A2 tool steel.

I have also begun using argon gas when sealing the parts in the foil. To use argon in this fashion, the foil envelope is closed completely except for a tiny hole in one end. A hollow needle, similar to a fine blowing attachment, is connected to the argon bottle (TIG welding supply), and the air within the envelope is displaced with one minute of low-flow argon. The needle is removed and the end of the foil envelope is quickly folded and pressed in a mill vise.

Stainless foil is not like kitchen aluminum foil... it is significantly thicker, and quite stiff. When an envelope end is folded several times and ultimately compressed, the seal is very effective. When the heat treatment cycle is complete, the entire envelope is removed from the furnace with tongs, and placed on any convenient chunk of cast iron, like a drill-press table. The iron draws the heat quickly from the envelope, creating the necessary hardness. That is what air hardening steel is all about - no liquid (oil or water) quench is required! A2 also exhibits much higher hardness at elevated temperatures compared to O1. Tempering A2 requires temperatures on the order of 400 C or more.

The tappets were all removed and cleaned, along with the tappet guides of bronze.

The front crankcase cover was loosened, and gently tapped forward to free. Note the taper of the crankshaft nose, which engages the propellor hub.

The front and rear crankcase covers have been removed. The crankshaft, and all four rods and pistons, are still retained by the crankshaft center journal, which, like the rods, is bushed with a split bushing made of bronze shim stock of 0.006" thickness.

The larg hole at 2:00 is the camshaft boring.


And just like magic, the engine is restored to its pristine state! Sorry for the camera failure, guys. I love my Sony CD-Mavica, which uses mini CD-ROM disks to store obscene amounts of pictures and MPEG video, but which, once in a while, fails to correctly finalize.

Briefly, I'll mention that the engine was torn down entirely. The pistons were freed, the connecting rods, everything, including the head and the cylinders. The fins of the cylinders were wire-brushed, as were the fins of the head, as there were some deep deposits of castor varnish at the bottoms of the valleys.

One sad tale of woe (as usual), I broke one of the cast-iron piston rings manipulating the pistons. As I type this right now, I'm still not sure what possessed me to remove a ring, but in doing so, I snapped one of them into two pieces. Normally this wouldn't have been a big deal, as I traditionally maintain spares of all small parts for these creations. In digging through my spares box, I was unable to locate a single piston ring! Argh!

Each piston has two rings, one for compression, the other for oil control. I was sorely tempted to reassemble the engine knowing full-well that one piston was missing a ring. No one would have known except me, but, well, it bugged me knowing a ring was missing. Ultimately, I turned a batch of new rings, split, heat treated, and gapped them. Those that have produced piston rings of cast iron know that it is not a trivial process, and it took the better part of a day to make one lousy ring. Of course, rather than make just one, I made over a dozen, so now I have the spares for the future.

I do not forsee running this engine again for quite some time, so the interior was liberally lubricated with one of my favorite oils, Vactra Way oil, medium. This stuff is great because it is powerfully anti-corrosive, being designed to inhabit machine slides subject to water-based coolants and other nasty chemicals. I have confidence that the precious crankshaft and camshaft are both well protected.