Mills 2.4

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Name Mills 2.4 Designer Mills Bros.
Bore 0.500" Stroke 0.750"
Type Compression Ignition Capacity 2.4cc
(0.147 cuin)
Production run 1500-3000 Country of Origin England
Photo by Ron C Year of manufacture 1948-51



The Mills 2.4 was the third engine designed by Mills Brothers (Model Engineers) Ltd to enter production. Their first two engines, the Mills 1.3 and the Mills .75 were absolutely brilliant, so we have to say, in the time honoured way, two outa' three ain't bad. Not that the 2.4cc rear rotary valve "diesel" did not share the same high manufacturing standards, flexibility, and ease of operation of its older but smaller brothers; it did! What it lacked was the ability to compete with other engines in its class in terms of power to weight ratio, size, and overall performance.

Perversely, the general perception of "...a nice enough engine, but..." that doomed Mills Bros last model engine to relative obscurity makes it a prize to engine collectors of today. You don't see a lot of them, but when you do, they are generally in remarkably good condition—supporting the view that they did not get a lot of use during the active period of their lives!

The engine was introduced quietly in late 1948. It was reviewed by Sparey for the Aeromodeller in May 1949 [1] where it was headlined as the Mills Mk III. Although the title and another (highlighted) entry on the page call the engine the "Mk III", no company advertising I've been able to uncover uses this designation. This error is corrected in the edited reprint of the review that appears in the Aeromodeller Annual for 1949 where the engine is called the "MILLS DIESEL 2.4cc" [1a]. Reading the May '49 review, one gets the strong impression that Mr Sparey could not find a lot to praise in the engine. So faced with two pages to fill, he fell back to saying nothing and spent almost the entire review discussing the lack of collaboration between engine reviewers and the modifications made recently to his "apparatus" following some correspondence. His figures and comments show that he was unable to get more than a shade under 11,000 rpm from the test sample.

A review in Model Aeronautics circa July 1949 (see footnote) [2] gave no performance data at all, but was accompanied by this very nice American style air-brush cut-away illustration. The factory instruction leaflet quotes 8,500 rpm on a 10x5 prop as producing 0.18 BHP and 32oz of thrust. This was the recommended free-flight prop. For control-line, props in the range 9x8 to 8x12 (!!) were suggested.

The last Mills promotion mentioning the 2.4 appeared in the Aeromodeller dated June, 1951. Estimates of the total production for the 2.4 vary. You can take your choice of about 1,500 [3], or less than 3,000 units [4]. Determination is complicated by the fact that engine serial numbers are not contiguous. Reference [4] states that the first "few hundred" engines carried no serial numbers at all. Regardless, production was definitely way below the tens and tens of thousands for both of the other Mills Brothers engines.


The 2.4 shows a strong family resemblance to the two smaller engines and apart from the rear-rotary induction, shares the same basic construction. The crankcase is pressure cast magnesium. The counter-balanced crankshaft runs in a phosphor-bronze bearing and carries a steel prop drive washer fitted on a taper. The hardened and ground steel piston runs in a hardened and ground steel liner which is retained by four 6BA screws and threaded for attachment of Mills' distinctive "bee-hive" shaped head turned from aluminium bar stock. The head came fitted with a compression-stop pin which most users promptly removed. Like the other Mills engines, this pin was intended to limit compression screw movement to a bit less than 360° after the running position had been established. While this was factory set, on the 2.4 the owner could reset it to suit a different fuel mix by unscrewing the "tommy-bar" and refitting it at 90° to the previous orientation. Altogether a bit of a pain, hence all those Mills engines with missing pins.

The drawing shown here is an original factory print for the cylinder. Sadly, it does not mention the designer, but does credit the drawing to one T Wooderson. Of more interest is the dimensions and tolerances specified. For example, the bore was to be 0.497 ±0.0005" before nitriding (hardening) and grinding, after which it was to be 0.500" ±0.0001". The skirt, which needs to be a close fit in the crankcase to prevent leakage between transfer and exhaust, was to be ground to 0.5993", plus 0.0005", minus 0.0000"! These are fine tolerances for a mass produced model engine and a credit to Mills quality.

This view shows the venturi assembly and the boss it screws into on the cast magnesium backplate. The boss extends to the center in order to carry the screw that retains the rotary valve. The needle is of the "gramophone needle" type used on the other Mills engines. And in common with the other engines, the needle seats on a screwed-in fuel nipple which is fitted with a cut-off arm spring-biased to the "stop" position. This indicates that the designers saw the engine being fitted to larger free-flight models. In fact, a Mills Bros advertisement suggests the Keil Kraft Junior 60 as a suitable model. However the market for engines of this size in England at the time was moving towards control-line team-racing where a large, heavy 2.4 of rather ordinary performance was at a distinct disadvantage, so it quietly faded away.

The Patented Mills Transfer System

All the Mills Bros engines utilized the same, patented transfer design. This was described in a Mills "Infomercial" in this way:

In all Mills Diesels, the Air-Fuel mixture enters the combustion chamber in two separate streams. These streams impinge on one another and strike sharply against a flat wall machined in the piston crown. Any heavy droplets of fuel or oil are smashed up in the process and dispersed in the turbulence created. At the same time the charge is deflected upwards for good scavenging of the combustion chamber. [5]

In my review of the Mills MkI 1.3, I expressed some scepticism of this description and noted that this was one patent I'd like to read carefully. Well, it's said that you should always be careful of what you wish for. Late in 2006, I received an email from Mr Michael Harding, son of Mr Arnold Louis Harginge of Mills Bros Ltd and arguably, the real and actual designer of the Mills Mk I. Over the ensuing email exchange, Mike kindly offered to send his copy of the original patent to Model Engine News for safe keeping. This is possibly the last copy of the patent left on the planet and is now carefully preserved. As you will be able to read from the scans below, it echos the description printed above. I'd even go so far as to say that the accompanying diagram on page three, while somewhat stylized, is recognizably a Mills Mk I.

Mills Transfer Patent 612,224

Mike also shared his memory of growing up in and around the Mills Bros factory. With his permission, Model Engine News is pleased to provide this unedited account of A.L. Hardinge and Mills Bros (Model Engineers) Ltd. As you will read, Mike's account runs counter to some "accepted wisdom". I will leave it to you, Dear Reader, to form your own conclusions. For myself, the fact that Mike was able and willing to pass on to me the patent, photographs, and Mills Bros advertising material provides compelling weight to his account.


The big Mills may not have been a big success during its short life, but it is still a handsome engine and sure looks like it should perform. But its long stroke to bore of 1.5:1 could not compete with the emerging high performance engines like the ED Hunter with S/B of 1.05:1 where the shorter stroke results in lower friction and inertia losses. Like Sparey, I've stretched to find good things to say about it, but I'm certainly not sorry to have one in the collection.


If this reads like a footnote to a reference, it is! Ref [2] is hard to date. This, sadly, is a frequent problem with British publications from the first half of the twentieth century. It's almost as if including any publication date was against the Official Secrets Act or something. Maybe it was intended to frustrate German invaders. It sure frustrates me.

By reading through it, I'd previously placed the issue supplying reference [2] as 1950, but in the process of researching the Mills "Mk III" for this review, I stumbled across an advertisement for the publication Model Aeronautics in the Aeromodeller of July, 1949 [6]. Even then I may have passed it by were it not for the fact that the blue and black cover depicted in the ad was staring me in the face on my desk! So I'm now dating this publication from the advert. Given lead times and marketing optimism, this may not be 100% correct, but I doubt we can do better.


[1] Sparey, LH: Engine Review #13: Mills Mk III, Aeromodeller, Volume XIV, No 160, May 1949, Model Aeronautical Press Ltd, p312.
[1a] Russell, DA (ed.): Engine Analysis, Aeromodeller Annual ~ 1949, Volume XIV, No 160, May 1949, Model Aeronautical Press Ltd, p149.
[2] annon: Mills 2.4 and .75, Model Aeronautics, July 1949 (? see [6]), Ian Allen Ltd, London, p36.
[3] Schroder, RJ: Engine-uity: Mills Brothers Engines Part 4, Engine Collectors' Journal, Number 100, May 1992, p7.
[4] Noaks, M: The Mills Brothers Story ~ Part 4, Model Engine World, Volume 2, Issue 8, Number 21, January 1996, p7.
[5] (Mills Bros Advertisement), Aeromodeller, Volume XIV, No 156, January 1949, Model Aeronautical Press Ltd, p81.
[6] (Ian Allen Ltd Advertisement): Aeromodeller, Volume XIV, No 162, July 1949, Model Aeronautical Press Ltd, p414.




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