Closer Look at a Motorcycle Engine Piston


Starting at the top; the crown of an aluminum piston in a modern petrol engine is limited to about 350c degree, but this is dependent on the particular alloy used to make the piston. In the past alloys such as Lo-Ex and Y alloy were used, and although they are still available today, the major piston manufacturers have developed their own alloy, with enhanced properties, to give the desired strength and corrosion resistance at operating temperatures.

The crown shape has developed as combustion chambers have evolved and as the move from two to four valves per cylinder has taken place. Popular in the old days was the hemispherical combustion chamber in the cylinder head. This works well enough with a flat top piston, but raising the compression ratio produces a piston with a large come, which exposes more surface to heat and alters the piston stability in the bore as the center of gravity is raised. This is as well as upsetting the hemispherical shape of the combustion chamber.

Most modern pistons generally have flat crowns except for cut-outs to avoid valve contact.
Just below the crown is the piston ring belt and in the sense that there are still three rings, not much has changed over the years since the early days of the combustion engine as far as the piston is concerned. However, there has been a revolution in piston ring design – but that is another story.

In the area of the lower ring there are usually some drilling connecting the ring gap to the inside of the piston. This allows oil scraped off the cylinder wall to escape and also lubricate the small end bearing surfaces in both the piston and the connecting rod. Be very careful if you are tempted to drill a few more holes as pistons have been known to split in two along this line.

Below the ring belt is the piston pin (gudgeon pin or wrist pin). The forces from the combustion pressure are transferred through the piston pin boss into the pin. The pin boss is attached very strongly to the crown by a column of metal or struts. There are two obvious differences between a classic piston and a modern one in this region. In a modern petrol piston the pin is as close to the bottom ring groove as possible and is much smaller both in length and diameter than in an older engine.

If you missed the first part of our 'Motorcycle Piston' special, you can find it here Pistons from your Motorcycle Engine
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