A 5cc version of the 1.3cc Mills Diesel.

[KEITH BEAUMONTParticipant @keithbeaumont45476]

In my youth,many years ago,as an avid Aeromodeller,my favourite engine was the Mills 1.3cc Diesel. At one time I had four of them. They were popular due to their reliability and easy starting from hot or cold.The design was clearly about right for the style of modelling at the time,being free flight and control line in reasonably modest airframes.      I have often wondered why the design  was not taken further and a larger version produced beyond the 2.4cc that did appear.   Recently visiting the excellent site hosted by Adrian Duncan (AdriansModelelAeroEngines.com) for historical informationof “Classic” model engines, I found a reference to a prototype 5cc Mills that they did not continue with. Its external design was not exactly attractive, trying to get the weight down etc, but I then formed the idea of making a 5cc version as my next project. Several yeras ago I made the 1.3 cc version, from Ron Chernichs “Motor Boys” drawings, so a straight enlargement from theses was decided upon.

Most of the engines I have made have been from bar stock,with a few from castings. A rough sketch of the size of the block of aluminium for the crankcase made me decide it would make too much swarf, so decided to make a pattern and have a casting made in L25 Ali.   The design of the cylinder porting and timing has been copied exactlyfrom the 1.3cc design.  The only changes I have made to the internals is to add a ball race to the rear of crankshaft, alter the prop driver to collet fixing rather than taper the shaft and drill the crankshaft for lubrication of the front bronze bearing.  With the crankshaft, I decided not to machine this from solid, but to build it from separate parts. I have tried this once before on another engine a few years ago and it failed due to me having a thread to a shoulder for fixing the disc to the shaft, then silver soldering. It failed on that shoulder after a backfire.  This time I have kept the shaft parallel and the hole tthrough the disc a sliding fit.The front side of the drilling was very slightly chamfered, to act as a reservoir for the brazing material.  It all went together  very well and I have clean brass witness rings each side.

The aluminium casting did not machine well and has a lot of micro porosity that can be seen on the machined surfaces. This has prevented me having the crankcase bead blasted, as I normally do. So. I decided to have the crankcase finished Black. After testing several high temperature paints for resistance to Ether/Castor oil, I gave up and arranged to have it Powder Coated with a local company,using a Polyester based powder coat that proved impervious to Diesel fuel.   The first powder coating was not a success visually, as gas expansion  from the porosity  when heated to 200 C, blew bubbles in the paint, which then collapsed, leaving craters on the surface finish.  Afterrubbing down, filliing thecraters with JB Weld, followed byanother careful rub down the second attempt at coating was much better, but still left a coulple of marks.  Now the engine is finished and assembled, I am quite pleased with overall appearance and I think it resembles the Mills smaller brothers.   An accurate measurement of the finished engine gives a swepped volume of 5.07cc.

On the test bed, I had no trouble in finding the point of ignition. Some really vicious backfires once or twice that nearly knocked the “chicken stick” out of my hand, but then the magic sound of it bursting into life for the first time.   So far I have had a total run time of 30 minutes, in a series of around 4 minute rich runs, with the occasional burst to full 2 stroking. With cool down between each run. Fitted with a 12″x 7″ APC propeller, using Model Technics 1000 fuel. Max RPM is 6200 to date.   It has all the typical Mills characteristics of superb starting. Hot or cold, it starts first flick after a prime.  A very satisfying project.

[PropPeteParticipant @proppete]

Very impressive, Keith! Are you going to put it on your test rig to see what thrust you can get from it?

[KEITH BEAUMONTParticipant @keithbeaumont45476]

Hi Pete,

Will do that after I have given it some more running in. Watch this space

Keith.

[JasonBModerator @jasonb]

That has turned out well, shame about the casting though I quite like the black which is a bit different to the usual bare alloy.

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