I made this inline 6 engine a couple of years ago, and I still fidget with it. It's very mesmerizing to watch the rotating assembly spin, especially when you add the flywheel onto it. Without it, it doesn't really carry enough momentum to keep itself rotating. Mine is not pictured with the flywheel since I dropped it, and it snapped off right with the crank. The flywheel also has a drill attachment for those who want to see it at high speeds (it's awesome). Something I was always curious about, but never got to, was seeing whether the rotating assembly could be balanced to get a better spin, if anyone tries it, let me know!

This print requires no extra parts and is fully 3d printable, even the rod pins, all you need is a nice gel superglue and a touch of coconut oil in the cylinders to get it running smooth. You may want to use a little sandpaper in places, but I honestly don't remember if I needed to or not. Enjoy!