This is the Black Plastic Version For Hexagonal Brass Top use the same part number but add 'HB' to the end (eg AUC 8102HB). For Round Brass top add 'RB' to the end of the part number. These must be ordered seperately. Dampers are designed to Slow the rise of the Suction Piston in the Carburettor in order to give a richer mixture on snap acceleration. They vary considerably from type to type by varying the clearances in the assembled Components. The oil in the Dashpot is forced through the Assembly and thus Slows (dampens) the Piston rise. A further difference is that some are externally vented (hole in the top) and some are internally vented (angled hole in the Suction Chamber) these are termed dustproof and non-dustproof respectively. It is essential that the correct part is fitted to ensure smooth Running of the engine.