This is the most involved step of all. Since every design is different, regardless of how much I write on what I did to design the engine it will only be an excersise in boring narrative because it will only really apply to this engine which was already designed. So in order for sanity I will keep this step concise.

In order to design the engine there we're a lot of things that had to be decided upon. And a lot of the decisions I made would influence other aspects of the design for the better or for the worse, so I had to make sure I put a lot of thought into every decision. The very first thing I did was define my design intent. It was to build a small two stroke engine that could be built with the limited experience and tools that I had. This influenced every decision I made because I had to ask myself "can I actually build this and do it accurately?" I couldn't have extremely tight tolerances or I would find myself throwing away a lot of parts that looked good but wouldn't work because they weren't accurate enough. So another goal was to keep the number of tight tolerances to a minimum.

The next thing that I did was figure out exactly what bore I wanted for the cylinder, and what stroke I wanted (and after those were decided, what compression ratio I wanted). Those measurements literally determined the size of every part on the engine. I used a mathcad to go through a few different iterations and finall settled on a bore of 7/16" and a stroke of .38". With a compression ratio of 9.5:1 I was able to determine that I would need ~.025" between the top of the piston and the cylinder head at top dead center.

Now that my design was starting to come together a little bit, I figured out which part designs would influence other part designs and started designing those parts. I decided I wanted to make my cylinder, head, crank case, etc. from 3/4" square stock. I decided on fasteners for the head and made some roughly dimensioned sketches of all of the parts and finally started using one of my favorite tools: Autodesk Inventor. Autodesk Inventor is a 3d design package. It really helps to visualize parts and make sure everything fits together and works properly. It allowed me to create a virtual working model of my engine which allowed me to see that there was adequate clearance around the connection rod etc.

I determined the components of the engine that we're likely to fail and did solids calculations (that is solid mechanics, the study of how materials behave under force: how they deform, break, etc.) to ensure they wouldn't. I checked that the bolts could hold, that I had enough threads engaged so that if something terrible happened and the bolts snapped, they would snap before the threads would tear out, and that the pin on the crankshaft was strong enough and wouldn't shear off.

From my models in Inventor I made drawings of all the parts which you can download below.

Right Click the image below and select "save as" to download the plans for my engine: