Paul, -- You have the flow path right, but the gauge pressure tap is
wrong. The gauge port is not directly connected to the large
outletport at the RR corner of the block. The outlet fitting at this
point has an extended tube inside the block that isolates the outlet
port from the gauge port. The gauge port is at the back end of the
main oil gallery in the block, downstreanm from the filter. The
gauge will register oil pressure as presented to the crankshaft main
bearings. If the oil filter is clogged or leaking badly, it is
possible to have god oil pressure before the filter, and poor
pressure shoeing on the gauge (insufficient oil to the crankshaft).
See notes and picture 3/4 down the page here:
http://mgaguru.com/mgtech/engine/of101.htm
A few pages to follow have more information on oil flow path, oil
gallery drillings, port plugs, and either excess or deficient oil
pressure issues.
It sounds like you have run across some faulty oil filters blocking
the flow. Since the pressure relief valve comes before the filter, I
have no idea why they leak. Could be a bad filter base seal ,or
perhaps a faulty female thread teh prevents the filter from screwing
on all the way. I recently ran across an incomplete thread like that
in a NAPA Gold (WIX) oil filter (just one in a lifetime).
Barney Gaylord
1958 MGA with an attitude
http://MGAguru.com
At 03:39 PM 4/13/2013 +0100, PaulHunt73 wrote:
>.... three or four times in the last few years there has been a
>problem in that the filter doesn't seem to have sealed properly, and
>chucked out about a cup-full of oil at first start. .... This time
>it didn't seal at all. .... These are Mann filters, with a bypass
>valve as well as an anti-drainback valve.
>
>What's the direction? I thought it came from the pump out of the
>back of the block past the oil gauge tapping, through the cooler,
>into the filter head .... up the outside of the filter, then down
>through the middle and into the block for the bearings.
>....
>
>.... immediately before oil started gushing out there was a sort of
>squeal from the direction of the filter, as if there was high
>pressure somewhere, but almost nothing was shown on the gauge. If
>there was something wrong with the filter such that it wasn't
>passing oil i.e. both the filter was blocked and the bypass failed,
>surely that should have shown on the gauge if that's between the
>pump and the filter? ....
>....
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