Here is a simple oscillating steam engine, built to the design of Steve Bodiley.
This engine is a great first build.
I would like to thank Steve for drawing the plans and writing it all up.
The excellent plans and comprehensive build instructions can be found at Steve's must see site  steves-workshop.co.uk
↓ Here is the engine fully assembled.
I closely followed Steve's plans and suggestions, with a few deviations (as you do). He points out some of the pitfalls and I managed to take care of these during the build, and that helped a lot.
Everything was made very square and accurate, and much to my delight it ran first time straight out of the box without any tweaking at all.
I do not have an air compressor or boiler so I nearly killed myself blowing down the inlet to test it's action. It then dawned on me to try a single barrel car tyre foot pump. I connected the pump up and pumped away by hand. After giving the flywheel a push, the engine ran at a pleasing steady slow pace, surely and continuously, as long as I kept pumping. Boy, what a great delight.
Here are pictures of the components (30 including screws and washers).
I can see this engine wearing pretty quickly, so I changed a few things to allow the components to be easily disassembled and/or replaced.
I made a round base, instead of the rectangular one, with a pocket underneath for the 2 screws that secure it to the frame.
↓ CYLINDER CAP.
↓ CYLINDER PIVOT.
I used a locking collar to adjust the spring tension, rather than a screw.
I also made some caps to contain the ends of the spring.
↓ MAIN BEARING.
I made this from brass as I don't have any bronze.
↓ CRANK AND AXLE.
The crank wheel was drilled and reamed to 6mm for the axle, then turned between centres to give a nice even run.
The crank pin is secured to the crank wheel using silver solder.
I secured the crank wheel to the axle using a 3mm steel pin. The parallel pin is a press fit in that hole in the wheel periphery.
The flywheel blank was drilled and reamed to 6mm for the axle, then turned between centres to give a nice even run.
The flywheel is secured to the axle by a 6mm grub screw.
↓ PISTON AND CON ROD.
I made the con rod shaft diameter 6mm for rigidity, instead of 4mm or 5mm.