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All engines provide some form of crankcase ventilation, normally drawing air in through a filter and exhausting it by means of a road draft tube or engine vacuum. On older American cars the inlet filter was in the oil filler cap and periodically replacing the cap -- and the filter it contained -- was a normal maintenance item. In the Volkswagen, air is drawn in around the fan pulley, exhausted to the air breather. There is no filter.
When building a Volkswagen engine, to insure dependable operation under all conditions the nose of the engine is machined to accept a shaft seal that fits around the hub of the fan pulley, the hub being modified to provide a smooth sealing surface. A filtered air inlet is provided elsewhere on the engine, typically on one of the valve covers. The filter is located on the firewall, plumbed to the engine with hose. The better breather filters provide for moisture trapping. The normal outlet is unchanged. The seal is commonly called a sand seal since it is a virtual necessity when running off pavement.
Without such a seal, whatever is in the air will be drawn into your engine. Even when running a full flow oil filter, it's wise to replace your oil periodically to get rid of chemical contaminants picked up from the air being drawn through the crankcase.
-Bob Hoover
-1995
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