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Radial engine build


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A long, long time ago, in a thread far, far away, I mentioned that I was planning to build a radial engine. The engine is not destined for a Paramotor, but for anyone who may be interested:

To date, I have modelled the whole engine in Catia and manufactured the oil pumps (one for feed, the other for scavenge).

The engine uses Russian Dnepr heads, barrels and pistons. These were chosen for being the right size and very cheap. Dnepr engines are not known for their reliability, so I have reduced the compression ratio and valve lift to reduce mechanical and thermal stress.



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  • 1 month later...

That's a great project mate!


Details of the machine? is it a manual or a CNC? (I ask as you have a rendered CAD drawing but no doubt the ability to do this manually as well!!)


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Cheers Simon.

All the machining is manual. I have a 1926 Drummond round bed lathe, a Sieg X3 mill and a pillar drill.

The mill is of Chinese origin. When I received it, I stripped it down and rebuilt it, carefully setting everything square and straight.

With a little care, it is fairly straightforward to produce perfectly good parts on a very affordable machine.

I have CAD software, which I have used to design the engine.

Each part is designed such that I can manufacture it with my own machinery, so, for example, the crankcase is an assembly of 9 parts bolted together.

Some of the set-ups required to produce large-ish parts on the small machines need to be 'imaginative'.



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Hi Tom,

Before becoming a design engineer, I ran CNC mills.

I am building this engine as a hobby, so the longer it takes, the more effective it is as a hobby. That rod took about 14 hours. Much of that time was spent making fixtures, which will also be used for the slave rods.

For me, the value is in having something I made myself. I could indeed fit a CNC kit to my mill, but that puts another level of separation between my hands and the parts I make.

I also use these parts to show to the CNC machinists who complain about some of the parts I design at work. If I can create a feature on a manual at home, they can certainly do it on a 5 axis mill. :)

I have to admit I have DROs on the mill. Perhaps in years to come, if I get arthritis, a CNC conversion might become more attractive.

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Amazing work.

I find this type of work absolutely amazing.

Coming from the technical world we really never get to create anything (at least I don’t feel that way).

I’ve being involved in connecting continents together, on ships dropping fiber out a sea etc and for me it’s just another boring day.

But I could sit and watch people make things from metal for hours…..

Great project, can’t wait to see a video of it running.



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  • 2 weeks later...

Nice piece of machining. I enjoyed the time spent on lathes and mills during my year out. Not sure I cold remember much of it now though.

The aluminium is an interesting choice of material for the master rod. Does it not get a bit warm for the ally?

Also intrigued how the rod is connected to the crank? My experience is with fixed cylinder engines so never really looked at the mechanics of assembling a radial. How do you fit the bearings, crank etc. ?

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2014 Alloy is fairly stable at temperature. Some of the higher strength alloys, such as 7075 lose their properties quickly at temperatures above 100°C.

The Aluminium part of the rod has a pair of steel side plates (shown temporarily assembled). The side plates extend into the bore of the alloy part and rolling bearings will be pressed into them. The six slave rods will run on pins located in the bores around the side plates.

This isn't a rotary engine (like the Gnome). The cylinders and crankcase are fixed.


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  • 2 weeks later...

More parts!

I am now the proud father of a complete family of master and slave rods.

The smaller end of each slave rod runs on a pin, located between the steel side plates of the master rod.

There are good reasons why the 'little end' bearings are larger than the 'big end' bearings of the slave rods. I won't go into detail unless anyone is specifically interested.

From the billets I started with, approx. 85% of the aluminium was taken to the scap dealers in a bucket. This will be true of most parts of the engine.

I am now looking at the daunting prospect of turning a 30kg billet of EN24T steel into a 3.5kg crankshaft.


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So are the bearings different sizes to stop people fitting them the wrong way round (just the sort of thing I'd manage to do!)

That's a fair bit of machining. I reckon the crank shaft should be fun. Is this all machined using CAD and CNC, or manually?

I can't wait to see this running. You said it's not for Paramotor usage, but didn't say what it was to be used for, I'm intrigued!

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Bearing sizes:

The gudgeon pin (little end) is a Dnepr motorcycle part which matches the piston. It is 21mm diameter.

I calculated the peak cylinder pressure and piston acceleration to derive the peak load through the slave rods. This calculation, with substantial safety factors, gave a 16mm bearing diameter.

After much enquiry, I discovered two reasons why the gudgeon pin is larger than load calculation alone would suggest.

1) The gudgeon pin bush is poorly lubricated and runs at relatively high temperature due to heat conducted from the piston. The bearing is oversize to reduce it's loading and improve it's durability under these conditions.

2) Under combution pressure, pistons distort (this is why the skirt is slightly oval). Gudgeon pins are closely fitted to the piston, so they also distort. A larger diameter pin is considerably stiffer (double the diameter gives 8 times the bending stiffness). Distortion (bending) of the pin means it only touches the bush at two point rather than across the whole width.

The 16mm pins which attach the slave rods to the master rod are supported right up against the sides of the rods, while the 21mm gudgeon pins have several mm of 'overhang' between the sides of the rod and the bosses in the piston. This results in greater bending load on the gudgeon pins.

The result of all the above factors is that the load cases are significantly different between the two ends. The 16mm end is defined by the overall strength requirement and adequate bearing area in a well lubricated location. The 21mm end is defined by the stiffness requirement and bearing area in a poorly lubricated and hot location.

All the machining is being carried out manually. The crank will be roughed out on the mill, making something broadly crank shaped but of square section. This will then be turned on the lathe using fixtures to offset the mainshaft to turn the crankpin.

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Initially, I am building the engine just for my own entertainment.

If the finished engine turns out the expected power and is durable, I would like to build a three wheeler, similar to the Morgan, with the radial engine exposed at the front.

Not wanting to do anything the 'normal' way ('cos that's the kind of guy I am :) ) the three wheeler would be a tilting design which leans into corners and drives like a bike, with a tilt servo system to maintain stability at low speeds and in traction limiting conditions.

Imagine, if you will, a Fokker Eindekker with no wings, 18" wire wheels and an idiot.

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IF the three wheeler ever gets built, it will have accomodation for paramotor and wing and will be taken to events such as Simon's fly-in.

I can't reasonably expect to build something like this without the prospect of like minded individuals wanting to play. :D

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Simon:- the trike would have two front wheels, therefore conventional car type steering arrangement, but with long wishbones attached to the chassis close to the centreline, allowing sufficient travel for up to 45° tilt.

A productive Sunday morning in the workshop has yielded a trial assembly of the master and slave rods.

I have started nibbling away at a large billet of EN24T steel. It's quite simple, I just need to cut off all the bits that don't look like a crankshaft.



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