The center tube is not an anti drain back device, but serves an
entirely different purpose. If you remove the center tube you will
likely never find any differeence in function. There are nearly
identical filter mounts with no tube manufactureed and used in
Australia for many years (decades) without incident. (and don't
bother saying it's because the continent in upside down).
If the anti drain back valve fails, it may be possible for oil to
drain backward past a non-sealing pressure relief valve or through a
well worn oil pump. Even then it is very unlikely that the filter
will ever not be full of oil, because for oil to drain out some air
must get inside. Regardless of where that air might come from, the
only way it can get into the filter is by way of the main oil galley,
which is the only connection on the output side of the filter
mount. This is why you have to punch a hole in the filter to let air
in if you want the oil to drain out before removing the filter.
The standpipe will not retain oil in a filter that is draining down
anywhere. Regardless of which side of the filter may be draining, if
one side may have low oil level the oil can run freely through the
filter element to equalize oil lever on both sides. Any drain down
would then eventually lead to a completely empty filter. But even
then there would have to be some way for air to enter the filter.
The point that no one as touched on yet is, where does the oil drain
to when you punch an air hole in the filter? It drains down via the
main oil galley and drips out through all crankshaft bearings,
camshaft bearings, timing chain tensioner, and anything else that is
below the level of the oil filter in the oil flow path. Such oil
escape routes must be very small in order to maintain oil pressure
when running.
As such, the oil drain down is very slow even after you punch a hole
im the cannister, so give it at least 10 minutes to drain down before
removing the filter. That's why I mentinoed to punch the hole first,
then drain the oil, reinstall the drain plug, and maybe even put 4
quarts of oil in the sump before removing the filter.
Punching two holes is likely to speed the draining process
slightly. If you only punch one hole in the center, then air going
in will have to bleed radially outward through the top part of the
filter media as oil is bleeding inward farther down. With the second
hole outboard, air gets there easily allowing oil to flow inward
through the entire surface of the filter media.
If anyone is interested in the real purpose of the stand pipe, see here:
http://mgaguru.com/mgtech/engine/of100b1.htm
Barney
At 09:15 AM 7/29/2011 +0100, Paul Hunt wrote:
>Not so sure about that, the inverted filters should have an anti
>drain-back valve on the inlet, and the filter head should have an
>anti drain-back tube going up the middle of the filter. If the
>valve is still functional (as it should be) the filter will only
>drain as far as the top of the tube, leaving most of the oil still
>in the filter to come out when you unscrew it. Punching a hole in
>the bottom of a hanging filter *should* stop oil running down the
>outside as you are trying to unscrew it.
>....
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