Well, the air has to come from somewhere...
My feeling is that the oil leaks down through the non-return valve and
through the oil pump, and air gets sucked in through the
first bearings along the oil galleries.
Last year I changed my oil after not running the car for a week or so.
The filter came off COMPLETELY DRAINED. I did not spill a single
drop of oil while removing it. And it did have an anti-drain valve...
Doug Braun
'72 Spit
At 12:17 AM 5/24/01 , Carter Shore wrote:
>Here's a question:
>In order for the oil to drain out of the filter, it
>must be replaced by an equal volume of air (since oil
>is largely incompressible).
>So where does the air come from? The pressure side of
>the filter from the pump is filled with oil, the gears
>in the pump block the path, and the inlet is submerged
>below the surface of the oil in the sump. The outlet
>side of the filter is also filled with oil, which runs
>over around and through the oil galleys, and out to
>the bearings. The bearings are fitted with clearances
>of only a few thousandths of an inch.
>
>Seems to me that gap between the the bearings and
>journals provide the only path by which air could
>enter the system. Given the viscosity of oil, it's
>affinity for sticking to metal surfaces, and the
>tortuous path that it must traverse, I wonder how much
>of an issue this is.
>
>I've removed filters from motors that have sat for
>years, full of (ugly dirty) oil.
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