Fully printable one cylinder engine. Use 1.75mm filament as a piston pin, it should be friction fit on the piston and lose on the connection rod.

It should work just fine after you have assembled everything, but if the crank keeps coming off the pin, I have modeled a little lip what you can melt to be a little wider so it locks the crank in place. You can do the same thing to lock the rods to the crank pin but I have found out that it's not needed for this version due to the thicker rod.
Just don't melt the parts together!!

For example, heat the tip of a flathead screwdriver and use that to widen the lip, high enough temp that it makes the plastic malleable but not melt it so you don't fuse the parts together. I can do a video about this if this pickups peoples interest.

0.2mm layer height and 0.4mm nozzle are recommended but I have gotten away with using 0.25mm layer height and 0.5mm nozzle, piston pin was just little tougher to get in and had to do a little sanding to the side of the piston. Parts in the picture are printed with this not recommended way.

Check out also my V-Twin Engine what uses the same pistons and crank as this!
https://www.thingiverse.com/thing:3353257

Check out also my Boxer Engine what uses the same pistons as this!
https://www.thingiverse.com/thing:3667783

Check out also my Radial Engine what uses the same pistons as this!
https://www.thingiverse.com/thing:3671312