The vacuum pump in diesel engines creates a vacuum for the power brakes. This O-ring ensures that there can be no leakage between the pump and the crankcase.
This is not a standard rubber ring, but a ring specially designed for this application, hence the different colour. Do not be tempted by cheap reproductions (recognizable by their black color) that do not withstand high temperatures.
Always install a new sealing ring when the vacuum pump is removed from the housing, as reuse will inevitably lead to leakage.
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