Conrod – Single Valve Poppet Engine

My initial thoughts for the conrod 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.

The conrod, abbreviation of “connecting rod”, connects the piston to the crankshaft. This is important as it transfers the combustion forces applied to the top of the piston into the crankpin. It also transfers the forces from the crankshaft to move the piston and draw air/fuel in and then to compress it. This simple part has to carry some substantial compressive forces and some quite high tension.

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 stage was to machine phosphor bronze bearings for each end of the conrod. These were then 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.

This concludes my discussion around the conrod single valve engine, but you can follow this engine build from the main Single Valve Engine page.

the first few rotations with the crank in place

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