Sorry about the pictures, it keeps turning them 90 degrees to the left and it's frustrating.
This is a 2-cylinder miniature model based off my older 4-cylinder. I wanted to experiment with making it as small as possible and test out telescoping brass tube for the piston and cylinder. It runs really well on compressed air and is very efficient, leaking almost no air. This is mostly what I wanted to improve on from the 4-cylinder because it would leak air around the pistons, as it was a plastic-plastic air seal. It has 1/8th the displacement of the 4-cylinder but makes at least half as much power. It actually accelerated so fast during a test that it sheared the output shaft from the inertia of the flywheel.
It also has high rpm capability, with mine getting up to a recorded 4750 rpm and I know it was revving higher before I got my reader out. However, that is not the limit of the engine. My compressor is old and small, unable to flow a large volume of air and I don't think the engine gets more than 20 psi. I'm sure it can reach 6000 with a higher flowing compressor or a much shorter hose than my restrictive 20' one.
Here's a video of it running on about 30psi: https://youtu.be/DfSkyHttvSg
I'll post what parts/materials are needed soon, I just wanted to get it posted so you can check it out.
Print the piston_no_brass if you just want to sand the piston to fit the engine block.
Please leave a comment if you would like further build instructions, a parts kit, or a fully built engine. I'd be happy to talk about building one specifically for you. Maybe you want a model boxer 4-cylinder or V8 with brass pistons and cylinders, I can build almost any engine format if you'd like!