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24 July 2003 |
Here's something unusual that cropped up a little
while ago that I've never commented on before - main oil gallery plugs/bungs.
I'd been conversing via e-mail with a guy that was experiencing
weird oil flow/supply problems with his performance street engine. Initially
he'd been blaming it on the supposedly superior oil pump he'd bought. He sent
it to me; I inspected it and found it to be in perfect health with no signs of
causing the issues in hand. Firstly there was a claimed total oil drain down
from the pump causing lack of initial supply on start-up for a few seconds, and
then repeated crank bearing failures when used in anger (i.e. at high rpm for
protracted periods). Unless there is something drastically wrong with the oil
pump and fitment - cracked pump body, non-flat mounting surface, mounting bolts
too long, split gasket, etc. - it will not drain down simply because the inlet
and outlet ports are above the centreline, near the top of the pump as it is
mounted to the block. Therefore it will always hold oil. A delay in pick-up is
usually either down to an air leak in the pick-up system from the end of the
oil pick up pipe to the inlet side of the pump. This
could be a badly sealing gasket, missed-out 'O'-ring that goes between the
gearbox and the block, heavily silted up pick up pipe gauze, a pin hole in the
oil pick up pipe (either in the tube itself or where welding/brazing has been
applied) or maybe an issue with one of these pre-oil pump filters that can be
fitted into the gearbox where the pick up pipe is fixed to the side. It may
also be an issue with the relief valve - the seats can come adrift.
After several strip-downs, checks and re-builds - each
time with the same results detailed above - carefully checking everything I
outlined with no anomalies reported, I touched upon
the subject of the oil gallery plugs. Initially I had just asked whether he'd
actually removed the originals when cleaning the block after it had been
machined. Now, this is absolutely imperative since proper cleaning of these
main oil galleries that feed the crankshaft bearings and oil pressure gauge
connection cannot be done without doing so. The plugs are sited at each end of
the block, and are usually brass (solid plugs on pre
A+ blocks unless they've been changed, or core-type cup plugs on later blocks).
Weird oil pressure readings, including apparently slow oil pick up from
start-up, can be caused by a blockage or gunge at the oil pressure gauge port
right at the end, furthest away from the oil supply hole from the oil filter.
Serious debris build-up can also cause oil flow restriction to the main
bearings - particularly centre and rear ones.
The reply from this guy was 'yes, I have pulled the plugs
and replaced them with threaded plugs as per David Vizard's
advice in his book. That way I can easily remove the bungs at
each engine strip down to facilitate cleaning, which I have done'. Fair
enough. Only problem here is - and I've come across this many moons ago in a
block similarly prepared - the way these threaded plugs are fitted can cause
chaos. The existing holes may need drilling/tapping out to use readily
available grub screws as those David Vizard used
where old tapered plugs now not generally available. After some careful
questioning, turns out that matey-boy here had
drilled/tapped the holes out to suit a straight UNF grub screw to such a depth
without realizing that when he tightened the screws into place, the tapered
ends where almost completely blocking the galleries off that supply the end
main bearings! I instructed him to grind the tapered ends off the screws, then re-fit them using bush/bearing fit liquid only as deep
as they need to go to be level with the end on the block. Doing this eradicated
the main bearing failure and apparently 'slow' oil pick up issues in one hit.
My advice is - just use the latest cup-type gallery
plugs in all blocks - easy to fit, easy to remove. If you must use screw-in
bungs, then make sure they are as short as you can get them, and when screwed
in they do not cover the main bearing feed galleries.
Useful part numbers:
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12G3053 |
Brass oil
gallery plugs, latest 'core plug' type, 4 for large bore block, 2 for small
bore block and Cooper S. |