April 2014 marks the 50th Anniversary of a sad moment in model engine manufacturing.

Peter Chinn's somewhat belated report (from the November 1964 edition of "Model Aircraft" magazine) appears below.

Doonside Mills Mk1 #214. Beautiful engine.
http://www.rcuniverse.com/forum/atta...mentid=1999582

I have a question about the tommy bar , how to adjust it when its so hot , I had trouble today ?

An appropriate length of narrow bore tygon (or similar) fuel tubing slid over the tommy bar does the business. Insulates the heat, and gives the fingertips cushion.

It must be a very tight fit on the tommy bar.Typically you will know it is tight enough if to slide on it requires lubricant. I use a drop of air tool oil. If the tubing is a loose fit, it will slide off at a most inopportune time when you adjust compression.

Mills 2.4 restoration by Jon Fletcher (aeroland).

Super workmanship. Before and after shots below.

Jon's comments:

"Just test ran it ... will need to run it in. Gets up to temp. then slows slightly and head is quite hot, all the signs of needing to be run in. Holds it's compression hot yet free enough for the following blade to rap the back of the finger when turned slowly over TDC. Needle 3 ¼ turns open, 10 x 4 prop. and 32/32/32/3 fuel mix. Air bleed cut out works well although it takes a few seconds for the engine to stop. I made the cut out arm boss bore a precise fit on the venturi lower boss as it has to be airtight. Venturi assy. quite challenging to make especially getting the curved sides to the body. Take care with the cut out arm lever which, just like the original is made from mild steel and can be bent easily. Should that happen just straighten with fingers. Spinner nut uses a 3/16” music wire tommy bar.

The starting and running characteristics of the Mills 2.4 are quite different to the Mills 1.3 and .75 sideport induction engines. Exhaust note has quite a crack to it. Engine is now back to the specification it was when it left the Mills factory."

Engine had been 'customised' with non standard parts. Replacement exact replica parts fabricated by Jon include the following:

* Spinner Nut
* Spinner Washer
* Prop Driver (set for .010" end float)
* Venturi, lock nut, needle, spray bar and springs with cut-out arm

Plus

* sort slightly over tight contra piston and re-instal with sulphur anti-sieze compound
* straighten cooling fins and instal comp. lever stop pin
* tidy up of fasteners and re-blacking ancillaries where required.

Owner will be pleased. 'Tickled Pink" as my grandfather used to say.

An interview with Ivor F., co-creator of the "Doonside Mills .75", where he discusses his collection.

https://www.youtube.com/watch?v=TgqUgFlLb9s

Some years ago while looking for Irvine spare parts I came across a listing for a crankcase for an "Irvine Mills .75 Mk. II". At the time I had never heard of such an engine, despite being familiar with the Irvine copy of the P.75 Mills.

The Mk. II crankcases are still available from the UK; along with a limited range of other spares:

http://www.hobbystores.co.uk/default...0-160,b:Irvine

I wondered whether the Mk. II case, which is reminiscent of the original Mills .75 Mk I (shown below missing its tank bowl), could be used with the internals of the Irvine Mills .75 Mk I.

Recently a well known engineer who assists aero modellers with engine repairs and renovation advised that it was in fact possible. Others have taken that path. Parts install straight in. You need to supply a 5 x 13 x 4 metric ball bearing for the Mk II's main race though. A ball raced Mills? Definitely a departure from the 'original'!

I took the plunge. However, I did not want to just exchange parts. I wanted my Mk. II to look as close to a Mills Mk. I as possible. Why? The Mills Mk. I is as rare as hens teeth. The chance of finding an unrun example I could afford is slim and none ... and 'slim' just left town.

So, why not go for it? The result is shown below alongside an original Mills .75 Mk I and a standard Irvine Mills .75 Mk. I.

In the words of the person who brought this engine to reality, Jon Fletcher:

"Creation of the Irvine Mills .75 Mk II came about following your request. I have seen one pic. of the actual engine Irvine released and by memory it appears to be simply the Mk I parts fitted to the Mk II crankcase.

So in the spirit of creating an engine reminiscent of the original English Mills Mk I it firstly occurred to me to make a replica of the parallel sided cylinder head. Secondly it then occurred to me to machine a dummy cylinder flange and secure it to the crankcase to look just like the original Mk I. Some careful calculations were performed to ensure the screw heads would just miss the crankcase. The cylinder head was based on the Irvine Mills Mk 1 design but shortened to clear screw heads and flange and the head fortuitously ended up with exactly the same number of head fins of the English original. ... bore [of donor engine] was lapped back true and new pistons made in spheroidal graphite cast iron lapped to fit. Big end was sloppy so new conrod made in 2024-T3 high tensile al. alloy. I wasn't happy with fit of the original comp. screw so replica made with thread adjusted to a snug fit. Finally you suggested to black anodise the back plate, a process I can now do to complete the look.

This particular engine is one of the most powerful Mills .75's I have built or tested, in contrast with the original Mills .75 Mk I's which are not as powerful as the Mk II's."

"Under test very easy to start and I tach'd it at 7,000 rpm on a Taipan white nylon 8 x 4. Tested on a Taipan black nylon 7 x 4 it does 7,900 rpm, making it one of the fastest Mills .75's I have ever run."

Andrew Heath, the well known Sydney F2A flyer has produced a number of twin ball race Mills .75.

He converts the crankcases to suit his own screw in front TBR assembly.

Can't remember which type he converts but someone else here may.

He was out at the Richmond F/F field on Sunday flying his TBR Mills in his flying carpet in the scramble (he may have won it, not sure?)
Didn't take a picture of it, I got side tracked.
I think he said he used the Russian Mills of Ivor's. The first one had the front housing screwed on with a LH thread but all the others he did were RH threads using loctite and not one has come loose.
Lovely little engines, Andrew does very nice work.

Customising is a good way of salvaging damaged engines.

The example below takes it to a whole new level. Pass me the brandy!

An interesting thread from a UK forum concerning the history of the 'Irvine Mills' replica's. There may be just a few 2.4's, which never made it to production.

http://controlline.org.uk/phpBB2/vie...ighlight=mills

Hi got a 1.3 mills mk2 and it seems original , what prop shoud i use for it and is there any way i can get how old it is

laurence

Mine likes a 9x5, but they are very flexible and will tolerate anything from an 8x6 to a 10x4.

Mk II's were produced from 1948 to the early/mid 60's.
Very early Mk II's had a different position intake.
Modified fairly early to give higher performance.

Hi Laurence,
Best prop, for bench running and FF or RC flying, would be a light 10x4, 10x3 or 9x4. Don't go smaller than that. For control line flying, you could try an 8x6.
Use an old technology prop (wood or plastic) that you can comfortably flick. Carbon fibre props are very hard on the fingers and, if you crash a plane, they're likely to be stronger than the crankshaft. That applies to all old engines really.
The 1.3 Mk II was made from 1948 until the early sixties.

First confirm that it IS an original English Mills-if it is it will have a 2 digit batch number stamped on the left mounting lug and a 3-digit serial number stamped on the right mounting lug (in both cases-on the top surface of the lug)-if it doesn't have these then its not an original-but most likely an Indian made one (which were made in India from the mid 70s onwards using the original English tooling jigs and fixtures until only a few years ago)-the early Indian ones had plain aluminium cases, later ones were painted black and closer in appearance to the english originals. There are a few other minor detail differences that distinguish them as well.
If yours has a set of numbers as described then its undoubtedly a Mk2-but there was a series 1 version and a series 2 of this mark-if the top of the intake tube exterior is level with the bottom of the exhaust port opening, then its a Mk2 series 2, if its somewhat higher than this-in relation to the exhaust port-then it's a Mk2 series 1. The change occurred around the end of 1949-so if yours is a Mk2 series 2 it could have been manufactured anywhere from very late 1949 until 1964. There is no way to tell how old yours is, if an original-unless you happen to have the original box and a sales receipt-and even that will only tell you when it was sold-you can safely assume the engine is older than the receipt!
The serial numbers are not a great deal of help in 'dating' a Mills-other than the self evident one of a higher batch number (the 2 digit number) being made later than a lower batch number! The batch numbers and serials are only sequential with a particular model range-in theory at least-it should be possible to find Mills 75s and Mills 1.3 engines with the same serial numbers (far less so with the bigger 2.4 as only about 2000 of these were produced-and only for a short time ~1948-51

I'd agree wholeheartedly with the prop size suggestions already made-nothing less than a 9x4 as a load-(and that's underpropping it slightly)-and the engine is most pleasant on a 10x3-10x4. My personal preferred flying prop (and I have half a dozen 1.3s!) is a Topflite 10x3-1/2 nylon-which is very flexible, not too brutal on the fingers if it clouts you, has plenty of blade area-moving a lot of air-and is still reasonably plentiful on Ebay. You can also find them in wood on Ebay but the latter are less common. It would pay-if you get your hands on one-to give it a gentle simmer in water for about 10-15 minutes, (I use a frying pan on the stove) allowing it to cool back to ambient temerature still in the water. This annealing process usually removes any age brittleness which may have crept in over the years.

ChrisM
'ffkiwi'

Hi,
I have an original Mills 1.3 mark1 that is missing its fuel tank. Does anyone supply them or has anyone got one they would be willing to sell?

I doubt that you'll find any spares after 67 years !!! Someone with a lathe can no doubt turn one up for you though-I generally use delrin for such items. Perspex (acrylic) works-but requires a lot of polishing to get back to transparency after machining-and is still prone to crazing...
What is your key requirement-usability? or restoring to as original appearance? Polycarbonate might be another alternative clear plastic but i've never tried machining it...given your location, have a chat to Gerald Nally-he's done Mills bits for various people over the years...

ChrisM
'ffkiwi'

Very true Chris, however you never can be sure.

A few weeks ago a gentleman from Victoria in Australia kindly sent me a genuine Mills 2.4 gasket set.I was amazed such items still existed; and very appreciative.

fiery-I'd suggest than any original Mills 1.3 Mk1 series 1 or 2 tank (they are different BTW) even if an usused spare is likely to be unusable by now through either cracking or distortion or both -based on the behaviour of other plastic tanks from the same era.The only option for mrmoosy is to either get one made or get hold of one of the replica ones (and I'm not sure such exist-the main source for the replica tanks was Roger Whittingham in the UK, and he stopped producing some time back. Its possible that David Owen or Barton Model Products still have stocks-though I have no idea whether a Mills 1.3 Mk1 tank style was among the range once offered-I know 75 tanks were, Comp Spl, ED Baby and Mk1 and Mk2 Bees were...

ChrisM
'ffkiwi'

Again very true. The tank on my 1.3 Mk.2 S.2 is very yellowy now, and it has not seen a lot of fuel over its life.

Another option may be to see if Ed Carlson in the US still has some Aurora 1.3 spares ... including the tanks. I am assuming they will fit or could be easily modified to fit.

Inline 4 by Tom Crompton

Anyone know how they made the tanks for the engines? It had me curious if they were doing something other than turning one out on a lathe.

They would have been molded-initially from perspex or a similar plastic, later from nylon. Very few if any of the integral tanks were machined apart from some of the metal ones [and most of these were either pressed or spun...] Some of the replica ones are machined, since its easier for one or few-offs-but not for commercial production when you want hundreds if not thousands of copies. [of course with modern CNC machining centres it IS possible-but we are talking the late 1940s and 1950s-so at best it would have been capstan lathes for repetitive work. Ever sat down and worked out exactly how many separate machining operations it takes to produce a complete (spraybar type) needle valve assembly?
The photos show a replica Mills 75 tank I made a couple of weeks ago (machined from solid Delrin rod) for a recently acquired original Mills 75 that had its original cracked beyond repair. There was about 2 hours machine work to make that one-off tank on an Emco 5-a molded one would have taken about 5 minutes (after you'd made the tooling!) plus the cool down time-and you could if you chose-produce multiple copies in a single draw....I imagine Mills would have used 1/8" Perspex sheet as the starting material for the 75 tank...

http://www.rcuniverse.com/forum/atta...mentid=2072083http://www.rcuniverse.com/forum/atta...mentid=2072084

ChrisM
'ffkiwi'

Are they vacuum forming the tanks then? Or are they pouring or injecting the plastic into the molds? I guess one could use a plug to insert into the mold with the plastic heated up and softened in between too.

I've always understood that it was a hot molding process-much the same as molding a canopy for a model from acetate, and used a male and a female form-a pretty common technique in plastic molding for all sorts of domestic, commercial and industrial plastic components. The problem with machining items like tanks is you have to polish out the machining marks afterwards if you want a transparent tank-so in the case of the Mills 75, (and earlier 1.3s) you'd need to do this to the inside and the outside of the tank-so that's two more manufacturing steps on what was a very simple non essential engine component.....bad production economy when you could do it in a one step molding process (OK there would have been a hearing step, and a flash removal step afterwards....) Some of the later tanks-such as ME, and AM and DC were almost certainly injection molded-as you can see a parting line on some of them...

I don't know if Mills produced the tanks themselves (it is known that much of their work was outsourced) but it is not difficult to envisage a setup where they might have molded anything up to 100 tanks in a single operation think of an egg tray mould-in a square layout of say 10 x10 male forms and a matching female half-and a sheet of perspex say 15" square, gripped in a moulding frame.....no one seems to know with any certainty how big the batch number for production was-but we know it cannot have been more than 1000 in a batch, since the serial numbers only go to 3 digits..