BMC crankcase ventilation filter Crankcase ventilation is an important part of a healthy, properly working engine. Flow through the filter must be good, otherwise the filter soon clogs, which can have negative consequences. At the same time the filtration must be sufficient to prevent dirt and contaminants from reaching the engine. A good filter is a must – choose BMC! Choosing a filter: - The dimensions of each filter are as shown in the drawing and the specification table. - The rubber connection of the filter is fitted onto a pipe or flange with the same or a similar outer diameter as the inner diameter of the filter.