Product Idea

28 Cylinder Radial Engine

Pratt & Whitney R-4360 Wasp Major “Corn Cob”

Functioning 28 cylinder radial engine.


Somewhere in the middle of the last century, radial engine development was at it’s pinnacle with the development of very large engines. One of those, the Pratt & Whitney R-4360 engine had 28 cylinders, arranged in four rows of 7 cylinders each. I wanted to make a working model of that engine but needed a way to arrange 7 cylinders equally spaced in a full circle.

I found out that using the 3x5 liftarm pieces in a certain way, angles of 50.3 degrees can be made. That is very close to 51.4 needed to make a perfect heptagon. So, because the angle is just slightly less than 51.4 degrees, I could arrange 7 cylinders in a row on such heptagon, making a 7 cylinder radial. Another difficulty was making a master rod to which another 6 slave rods could be attached, but it can be done. I have a short movie, showing it running but I don't know how to add that to this project page. I've added a couple of photos instead.

Making a 14 or even 28 cylinder engine now is just a matter of adding rows and extending the crankshaft. I don’t have enough pieces to build the 28 cylinder P&W R-4360 Wasp Major engine but made a CAD model instead. The CAD models shown here are largely made using files created by DK. You can find examples of his beautiful work on

Deleting 2 or 3 rows of cylinders would make a 14 or 7 cylinder engine like the R-1830 Twin Wasp or R-1340 Wasp. With some patience I think you can also convert it into a rotary 7-cylinder engine, as used in numerous biplanes. I’ve encluded some screenshots of these smaller engines but this project is about the 28 cylinder engine. If you compare the different engines, you’ll see how the difference in staggering of the different rows can be achieved.

Details of the actual R-4360 engine can easily be found on the internet. I'm showing some pictures how the master rod evolved from a 6 cylinder master rod – which is a very odd number of cylinders in a single row – via a master rod for a 9, 18 or 36 cylinder engine into the master rod for the 7, 14 and 28 cylinder engine.

I’ve called this model the R-4360 but many more engines can be built using the principles of this model. Below more pictures of the engine. It’s mostly a mechanical model but I hope you like this model anyway. offers lots of technical background information. I’ll show the before mentioned 18 cylinder engine in another project.

I think this engine on a stand would make a great item to display. Or one could use the engine to make a large model of an aircraft in which such 28 cylinder radial engine was used.