This is a simple Vacuum Engine model that I made in about a week. Every part of the model can be replaced if it breaks. The speed of the engine depends on a couple of factors - Vacuum strength and Lubrication. I have thoroughly tested about a dozen turbine, shaft and housing designs and this model contains only the best and most stable iterations. 

The vacuum adapter's (entry hole for the vacuum) internal diameter is exactly 34.5 mm, while the outer diameter is 38.5 mm.
The engine works well even without the vacuum adapter but I strongly recommend using it if you can.

The shaft's ends are designed to accommodate different peripherals including: gears, saws, etc. They are exactly 4x4 mm. 

I have tested every part in PLA and PETG and both types of filament work well.

! THIS IS NOT A TOY - the engine accelerates very rapidly and can reach very high speeds, causing potential harm if not used properly. BE VERY CAREFUL WHEN USING THE ENGINE!

! For extra safety you can glue the 2 walls together, thus trapping the shaft and turbine in the engine and removing the possibility of disassembly.