Mills .75cc model diesel engine - Page 7 - RC Groups
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May 19, 2012, 08:17 AM
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earlwb's Avatar
Way back in 1977 I had bought a couple of Mills .075 diesel engines from Hobby Lobby (they used to actually sell internal combustion engines then). Anyway, both were bad right out of the box. One engine didn't even have the intake or transfer ports drilled in the cylinder sleeve. The other had a connecting rod go bad on me while trying to start the thing. I wound up selling or trading off the one engine without the ports. The other languished around in my various engine parts boxes for many years. I suspected they were making the engines for collectors and didn't expect anyone to actually try and run them.

Using two little holes for the intake and transfer ports is OK, as that allows for the wrist pin on the piston/rod to not catch in the port holes. But they drilled the intake port holes quite sloppily.

I decided to give it another go and after several attempts I managed to make a new connecting rod for it. Something tends to usually happen, like you get it almost done and a mistake or error happens and you screw it up somehow. But I was able to get the last rod made and fitted OK.

Here are some pics of the engine as I was refurbishing it. I was surprised to find that the bronze bushing for the crankshaft actually came out of the engine. When you look at the pics you can see how they got sloppy drilling the two little holes for the intake port. So I don't know if the thing is going to run or not still, even after making a new rod for it.

Here is the box and the engine before I did anything. I had just stuck it back in the box in disgust and let it sit like that since 1977.

Here is how they drilled or machined in the ports on the cylinder sleeve. I don't mind the little round holes, but at least they could have gotten the two intake ports a little more straight and not so uneven. The originals used little holes too. But they tended to drill them a bit better.

New rod alongside the old rod.

Here is the engine disassembled.

Inside the crankcase showing the bronze bushing.

Yeah the bushing actually came out too.I never tried to remove the bushing on other Mills engines, so maybe they are all like that.

After cleaning and reassembling the engine:

Last edited by earlwb; May 19, 2012 at 09:13 AM.
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May 19, 2012, 09:28 AM
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earlwb's Avatar
Oh Yeah, here is the instructions that came with the engine. You would have thought that they would suggest some propeller sizes to use on the engine. But they didn't of course. I suppose all the diesel folks would have known what to use, so why mention it.

May 19, 2012, 12:29 PM
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DeeBee1's Avatar
Thanks for the pictures Earl. Sorry you had a hard time with the engine.

Your engine is one of the Aurora Indian made versions which, so I'm told, didn't always live up to the standard of the original English made versions (although I've owned three of the Indian ones and haven't had any problems).

If you're intested, there's a good history of these engines here:
May 19, 2012, 01:06 PM
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Gluehand's Avatar

Watch this piston !

Seeing your Aurora Mills doesn't surprise me at all....

I've owned one too, of about the same was dead stuck right off the box....not by old castor though....look at this...!
May 19, 2012, 03:09 PM
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earlwb's Avatar
Thanks for the link, I remember reading it before. But it was the first time I noticed that the bronze bushing for the crankshaft also comes out of the original engine too. So that was the way they made it.

Yes, Gluehand, I suspect lots of people wound up buying these as collector engines not intended for use.

It sort of looks like the cylinder sleeve rotated when they assembled it and the rod jammed up against the cylinder sleeve. They cut a groove in the bottom of the sleeve to clear the rod, so if the sleeve rotates it can jam up on you.

I do have a couple/three later model Mills Replica engines, made by Aurora and they work though. So Aurora did improve on their quality control from the early days.
May 19, 2012, 11:06 PM
Balsadustus Producerus
earlwb--I have a CS version Mills with a stuck contrapiston. At first, the contrapiston would work as it should, but got more and more difficult to move until it stuck. I've taken the engine apart and found the CP is some sort of iron or steel, and it needs a refit to the cylinder. What would you recommend for resizing or relapping the CP without overdoing it?
May 20, 2012, 12:14 AM
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earlwb's Avatar
Originally Posted by Balsabird
earlwb--I have a CS version Mills with a stuck contrapiston. At first, the contrapiston would work as it should, but got more and more difficult to move until it stuck. I've taken the engine apart and found the CP is some sort of iron or steel, and it needs a refit to the cylinder. What would you recommend for resizing or relapping the CP without overdoing it?
I haven't tried that yet. But I would likely try some method like they discussed in Model engine news here:

You can usually heat the cylinder sleeve using a hot air gun like a heat Shrink Tubing gun, and the cylinder sleeve should expand before the contra piston does allowing you to move it.
May 20, 2012, 12:49 AM
Sticks, Tissue & old Diesels
brokenenglish's Avatar

Fitting a contra-piston

I've had this problem on quite a few "repro" Mills.
Proceed as follows:
Remove the cylinder from the engine.
Take a piece of hardwood dowel (the kind used with wing fixing bands) that's slightly smaller in diameter than the contra-piston.
The cylinder bore will (or should!) have a very slight taper, so place the cylinder on a hard flat surface and, using the dowel and a small hammer, knock the contra-piston down the bore (from the top, downwards).
This has always worked for me but, if the CP is really stuck very hard (i.e. you break the dowel!), you may have to use heat (go easy!).
When you've tapped the CP out through the bottom of the cylinder, put a good film of lubricating oil (3in1 or similar) on the CP and the inside of the cylinder, and use the same dowel to tap the CP back up the cylinder.
When the CP gets difficult to move again, towards the top of the cylinder, tap it back down again, and repeat this process, with more oil, until you can tap the CP up to the top, level with the top of the cylinder. Don't knock it completely out of the cylinder at the top.
If you repeat this process a few times (usually between 2 or 3 and about 10), you'll end up with a properly fitted contra-piston.

Then, when you reassemble the engine, cut a small piece of plywood (say 1/16" sheet) to the width of the exhaust ports. Insert the cylinder into the crankcase (make sure you know which are the induction port holes and which are the transfer - the transfer will be slightly higher and face forwards). Then insert the piece of plywood right through the exhaust ports (crankcase and cylinder, in one side and out the other), so that you can tighten the cylinder head down without the cylinder rotating (this is a problem on all the repro Mills engines I've seen).
One last word... If, in the future, you have "inexplicable" problems to start the engine, check that the cylinder hasn't rotated, i.e. look through the exhaust ports to see that the ports in both the cylinder and crankcase are properly aligned.
In the past, I've had around half a dozen Indian Mills, and all mine were good runners, at least as good as any other of the repros.
May 20, 2012, 02:33 AM
Registered User
I never owned or ran an Aurora Mills from that period. Mine were all from late 80's and all did well. Apart from the ones I owned, I did start and fly quite a few during my Air Wing NCC days (akin to ATC scheme in UK). I even took some apart and put them back without having so much as a worry. And guess what all still worked. Beginner's luck??
May 20, 2012, 08:12 PM
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earlwb's Avatar
yeah I suspect Aurora was making the engines rather poorly way back in 1977. Maybe it was the first time they tried making them in quantity too.

Anyway I was successful at getting the old junky Aurora 1977 Mills engine to run. It is still running on the weak side. But as it runs in and gets more time on it, the RPMs keep getting better and better. So maybe after a few more tanks of fuel, it might almost be up to how my other Mills engines are doing with performance.

At first it was barely able to run at about 3.600 RPMs and I was considering giving up on it. But after a few tanks of fuel, it started running around 4,600 rpms. After a while longer it was up to 5,500 or so RPMs. On the last tank of fuel for the day, it was just doing about 6,200 RPMs. So maybe after some more run time, it might be up around 7,000 RPMs which is where my other Mills engines are running. I was using a 8x3 Master Airscrew prop and DDD diesel fuel in the engine.

so it is sort of a cheap thrill when one can make a homemade connecting rod and get a engine to run with it too.

May 21, 2012, 04:27 AM
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fiery's Avatar

Mills .75

Excellent work Earl. Never say die.


My Mills .75 (shown in post no. 72 of this thread) is now all tidied up.

My thanks to David Owen for his help in sorting it, and supplying the new tank bowl.

It has serial no. 926 stamped on the right hand lug and 76 tamped on the left hand lug. I would be grateful for an explanation of what these numbers mean, and in particular would like to know what year the engine was made.

Running report to follow.
May 21, 2012, 04:52 AM
Registered User
I'm with Earl. My Aurora was so bad that the casting lines were mis-aligned, shaft and rod unbelieveably sloppy and totally unusable for anything other than a fishing sinker. Over the years I've had original, Doonside, Irvine and now a Boddo (purchased from David prior to his demise) and to be honest I think the Doonside was the best.

Other than the Boddo version and an as new ED Bee Mk1 have been fully committed (almost) to electric for well over twenty years so my whole lot got sold off at various times.
May 21, 2012, 08:40 AM
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DeeBee1's Avatar
Nice work Earl, now you've just got to decide which classic free flighter to put it in (maybe with some 'radio assist' )

Fiery, well done also for getting your engine back in shape. The short number plus the long number would have matched the serial number printed on the original certificate that would have come with the engine when it was new (76926 in the case of your engine). Sorry I can't help with dates - all I know is they were manufactured between 1948 and 1964 and yours is a P75 (P for 'popular' i.e. with no cut-out arm).

David, I've often heard that the Doonsides were the best (never owned one though). Currently got an original, a couple from Aurora and one from CS. The Boddo Mills are still manufactured by CS, who David originally commissioned to make them, although the engines were more recently called CS Mills and are now called CS Navos.
Last edited by DeeBee1; May 21, 2012 at 08:57 AM. Reason: Added dates.
May 21, 2012, 06:17 PM
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earlwb's Avatar
Actually I have the plans for a Tomboy. So I am planning on building a Tomboy for the Mills Engines to fly them on. I think nowadays, I could put in a small receiver, small battery pack, and a couple of micro servos to be able to keep the plane around close instead of chasing it across the countryside. It shouldn't affect the plane too much for the added weight. On a good calm day I probably wouldn't even need to touch the TX joysticks.
May 21, 2012, 07:16 PM
Registered User
The Tomboy Boys,
Taken at the Hobart (Tasmania) flying field where Tomboys and Mills abound even though there are a couple of additions (A Cardinal and a small Simplex).

All flown on RC and great fun!

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