Volkswagen 1.9-Liter TDI Owner's Manual page 9

In comparison with the conventional
parallelogram-shaped link between the
piston and connecting rod, the trapezoidal
connecting rod eye and piston hub have a
larger contact surface area at the piston pin
owing to their shape.
This distributes the combustion forces over
a larger area and relieves the load on the
piston pin and connecting rod.
Engine – Mechanics
Force Distribution in a Parallelogram-Shaped
Piston and Connecting Rod
Contact Surface
Force Distribution in a Trapezoidal Piston
and Connecting Rod
Contact Surface
Combustion Force
SSP209/008
Combustion Force
SSP209/009
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