It's an anti drain-back tube. Together with the anti drain-back valves that
should be in spin-on filters it prevents oil draining out of the filter when
switched off, which would otherwise happen with an inverted filter, leading
to longer times to get pressure when restarting. However the valves in
filters can be very variable in construction and quality, and poor ones will
allow the oil to drain out more as the filter gets older, and some can be
pretty-well ineffective from day one, see
http://people.msoe.edu/~yoderw/oilfilterstudy/oilfilterstudy.html. Volvo
3517857-3/Mann W917 seem to retain the oil for their lifetime (3k miles in
my cars), better than the Unipart/Champion/Halfords typically available in
the UK which tend to tail off albeit it about 50% dearer. Some Fram filters
are reputed to be good but one I tried from a well known supplier in the UK
were noticeably worse. K&N Gold seemed as good as the Volvo/Mann but are
harder to get hold off and twice the price. One thing to be aware of is to
make sure your filter is at least an inch longer than the tube, some filters
that fit the thread are so short they tighten down onto the end of the tube
which prevents oil pressure developing.
PaulH.
----- Original Message -----
> ... I see a small
> metal tube about 4 inches long that is press fitted into the threaded
> hole that the oil filter element screws down onto with the metal tube
> fitting up into the oil filter. What is the purpose of this tube...
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