My initial thoughts for the connecting rod for this single valve poppet engine were to make it from mild steel as I could make it thin to make it light, but an aluminium conrod would have a bigger cross-section for a given weight.
Measuring the distance between the top surface of the cylinder liner and the top of the main crank pin. There are a few other data points I need to calculate the length of the conrod:
- top of piston to gudgeon pin centre
- diameter of the crank pin
- compression ratio
The longer the connecting rod, the more sinusoidal the relationship between the rotation and movement of the piston – ie smoother.
I ended up with 43.7mm between holes giving me a rough starting compression ratio of 5:1.
Initially both holes were drilled 3mm diameter and in the next steps you will see that this was done to make it easy to provide some fixtures for the next machining steps.
The final part was to machine phosphor bronze bearings for each end of the conrod. These were machined, drilled under-size and then pushed into a hot conrod. The holes were then hand reamed to size: 3/16″ for the big end and 1/8″ for the gudgeon pin end.