The piston is a critical component. The diameter needs to be a close fit in the cylinder and the piston rod needs to be both parallel and concentric with the piston in order for the engine to run with minimum friction.
The piston and piston rod were screwed together and machined between centres as a unit to guarantee concentricity.
First the rod was machined. A centre hole was drilled in each end of the piston rod using the 4 jaw-chuck to get the greatest accuracy.
Then it was mounted between 2 centres and driven by a drive dog as shown, to turn the reduced diameter section. The thread was added using threading die.
Next the piston blank was faced square and turned to length in the 3 jaw chuck. Then it was centre-drilled, drilled and tapped to fit the piston rod.
The two parts were screwed together with thread-locker and placed back between centres where the piston was gently turned to its final diameter.
To cut the piston ring grooves, maximum rigidity was needed and so the piston was placed in the 3 jaw and the piston rod supported with the tailstock.
The piston was protected with slips of paper and the grooves were cut using a parting off tool. The resulting grooves were not perfectly concentric because of chuck run-out, but they were made slightly over deep to compensate.
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