Here you go, Mike!
Barney's post...
On Sun, 19 Oct 97 14:45:32 -0400 Larry Macy
<macy@bblmail.psycha.upenn.edu> writes:
>On 10/18/97 11:29 PM so and so Scott Gardner said. (And I quote)
>>.....Can anybody tell me the exact path the oil takes through a 18V
engine? Please include the filter, oil pressure relief valve, and cooler
in the path. I'd especially like to know where the oil pressure gauge
sender is in the path as well.
>> Also, where does the oil go when the oil pressure relief valve
opens? Is it possible for the main bearings to be completely starved of
oil and still have pressure on the gauge? Somebody mentioned oil galley
plugs on the bottom of the engine. What are they, where are they
located, how many are there, and what would the effects be if they were
missing?
>>.....
>
>Scott
>In most engines the oil goes to the cam first and then through the
galley to the mains and then from there to the con rod bearings. Makes it
easier to to drill. the relief valve is on the way to the cam. If this is
diff in the MG - 18V engine, somebody will say - Barney?? Lawrie??
Uh, wrong. And yes, it's different in the MG B-series engines.
Oil is picked up from the sump via the oil strainer and riser tube, and
passes through the pump and into the left rear corner of the engine
block. There it encounters the pressure relief valve where excess oil is
forced past the relief valve and returns directly to the sump. Oil not
dumped by the relief valve passes into a cross drilling at the back of
the block to emerge at the external pipe fitting at the right rear corner
of the block.
If the engine does not have an oil cooler, the oil passes through a steel
pipe to the oil filter housing. If the engine does have an oil cooler,
the oil passes through a hose (and maybe also a pipe) to the cooler,
through the cooler, and back through another hose (and maybe a pipe) to
the filter housing.
When the oil enters the filter housing it passes into the outer portion
of the filter, then inwards through the filter to the central space, then
back through the filter mount into the engine block. There is also a
pressure relief valve in the filter mount (or in the filter itself in
some cases) which will relieve itself if the filter gets dirty and
clogged, and the oil which cannot pass through the filter element will
bypass the filter element and enter the engine block directly without
being filtered. If you change your oil and filter regularly this
condition should never occur, and all the oil entering the engine block
should be filtered.
Once back in the block the oil enters a drill hole (oil gallery) running
the length of the engine from front to back (right side of the block).
At the front, a small hole allows a bit of oil to pass through the timing
chain tensioner where it flows onto the timing chain before returning to
the sump. Just a couple of inches from the back, a port is tapped from
the gallery to the outside of the block where the flex line to the oil
pressure gauge connects. In three places (five for later engines) there
are drillings from the main bearing journals to the gallery, where oil
flows from the gallery to the main bearings. So, the pressure gauge
registers system pressure after the filter and just before the main
bearings.
There are diagonal holes drilled in the crankshaft from the main bearing
journals to the rod bearing journals, and oil flows here to get to the
rod bearings.
There are more holes drilled in the block from the main bearing journals
to the camshaft bearing journals (three or five places), so oil flows
from the main bearing area to the cam bearings.
There is also one vertical hole from the rear camshaft bearing journal
where oil can flow upwards to the top of the block, through the head
gasket, through the cylinder head, through the rear rocker shaft
pedestal, and into the hollow rocker shaft. From there the oil flows out
through radial holes in the rocker shaft to lubricate each of the rocker
arm bearings. In each rocker arm there are two small drill holes.
One hole runs latterly through the rocker arm from the rocker bushing to
the threaded adjuster screw hole, and is plugged at the outer side of the
threaded hole. Oil flows from the bushing, through this drill hole, out
around the wasp-waisted girth of the adjuster screw, through a radial
hole into the center of the screw, and out a hole in the bottom end of
the screw to lubricate the ball and socket joint at the top end of the
push rod.
The second hole in the rocker arm comes out at an angle from the top
shoulder of the rocker bushing area so that some oil is squirted in the
general direction of the tip of the rocker arm. With a little luck (and
a bit of splashing around) this lubricates the rubbing end of the rocker
arm and the tip of the valve stem, as well as having a bit of oil splash
through the valve spring and enter the top of the valve guide to
lubricate the valve stem. The oil entering the valve guide will
eventually exit from the bottom of the guide into the port above the
valve head. With the intake valves, the oil will enter the combustion
chamber where it will be mostly burned along with the fuel/air mixture.
In the case of very loose valve guides, excessive oil passage here can
create smoke in the exhaust, wet the spark plugs, and leave carbon
deposits in the combustion chamber when the oil burns of gets baked on to
the surfaces. With the exhaust valves, oil from the guides is generally
blown out the exhaust port creating soot or wet oil in the tail pipe.
Sometimes this oil can be burned in the exhaust stream if it is hot
enough, if there is a little oxygen left over in the exhaust (lean
mixture at the carbs), and especially if there is a catalytic converter
being fed fresh air from an air pump.
Oil escaping from the rocker bushings and the drill holes in the rocker
arms runs down through the holes in the cylinder head around the push
rods where it flows onto the tops of the cam followers (tappets or valve
lifters). There it lubricates the ball and socket joint at the bottom of
the push rod as well as the tappet and the bore in the block that the
tappet rides in. A small amount of oil escapes from the bottom of the
tappet bore, while most of the rest returns to the sump via drain holes
beside the tappets.
Oil escaping from the connecting rod bearings is thrown around the inside
of the engine. Some gets thrown up onto the cylinder walls to lubricate
the cylinders, pistons and rings, and some gets thrown onto the camshaft
where it lubricates the cam lobes, the bottom end of the lifters, the
distributor drive gears, and the oil pump drive gears. Oil escaping from
the rear cam bearing lubricates the tachometer drive gear(s). This oil
and oil escaping from the main bearings and other cam bearings returns to
the sump by gravity.
One key point in the oil flow path is at the right rear corner of the
engine block. Here there is a special fitting where the external oil
line connects. The longitudinal oil gallery passes through the cross
drilling at the back of the block where this fitting connects. The
fitting has a tubular end which enters the block to receive the oil from
the cross drilling. This tubular end also blocks the path from the cross
drilling to the oil gallery in the block. If a fitting is installed
which does not have this extended tubular end, oil can pass directly from
the cross drilling into the gallery in the block, completely bypassing
the oil filter (and oil cooler if fitted). The cooler return line
fitting at the filter mount is such a fitting without the tubular
extension.
It is possible (but not likely) for the main bearings to be completely
starved of oil and still have pressure on the gauge. There would have to
be an obstruction in the flow path between the oil gallery and the main
bearings. One way this could happen would be for the bearing shells to
be lacking the oil supply hole that receives oil from the engine block,
but every main bearing set I have seen has the hole in the center of
every bearing shell, the upper and lower shells being identical. It's
hard to imagine enough gunk left in a freshly rebuilt engine to actually
clog up the supply holes to the bearings. Another unlikely possibility
would be for the crankshaft bearing journals to be machined one or two
thousanths of an inch oversize, or for the bearing shell to be one or two
thousanths of an inch undersize, such that there is no clearance for the
oil to pass through the bearing.
Oil galley plugs (usually press fit brass and sometimes threaded steel)
are used to close ends of drilled oil holes where oil should not flow
out. There are at least four brass oil gallery plugs inside the B-series
engine, a few threaded plugs on the outside, and two pressed steel plugs
in the ends of the valve rocke shaft. Some of these plugs are external,
and if left out would result in pressurized oil loss to the outside of
the engine. Some of these plugs are internal and if left out would
result in oil by passing freely back into the engine oil sump. In either
case, there would be a dramatic reduction of oil pressure.
End of answers to the original questions. Take Tech 102 for further
education..
Barney Gaylord
1958 MGA with an attitude
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