The MGA With An Attitude
OIL DRAIN DOWN and Change of Oil Level -- OF-115

On 10/11/2013, Trevor Charlton in England, UK wrote:
"It takes days for the oil level on the dipstick of my car to return to its maximum level. On a recent trip to France I was doing about 300 miles a day and topping up to max every day for 2000 miles. When I got home and left the car for 4 days the oil level was well over max mark".

Most of the oil circulating while the engine runs will drain back to the sump really fast. It has to. At 4000 rpm crankshaft speed the oil pump will move slightly almost a pint of oil per second. An engine in good condition will take about 12 quarts per minute through the engine when running at 50 to 70 psi relief pressure. Any excess flow from the pump goes over the pressure relief valve returning directly to the sump. Oil lowing through the engine is returned mostly through the crankshaft bearings and camshaft bearings, and other minor points in the lower part of the engine.

A smaller portion of the oil flow goes to the cylinder head to lubricate the rocker shaft and valve gear. This takes a little longer to return to the sump, but it certainly must flow downward as fast as it is delivered upward. You may know that when you pour new oil into the valve cover it will flow down to the sump about as fast as you can pour it in. If you check the dip stick half a minute after pouring in new oil you will find it is nearly all in the sump.

When you make a pit stop and want to check oil level, give it just a minute or two to drain down before checking the dip stick. Then you can top it up is necessary, and check the dip stick again a minute later.

I presume that the oil level rise you see on the stick a day later is due to a very small amount of oil that stands in the cylinder head "top floor" and takes somewhat longer to drain back to the sump. There is also a flat floor in the tappet gallery where a small amount of oil may be retained for longer time. And of course some small amount if oil will cling to vertical surfaces for a while due to oil viscosity, and most of that will drain back a little later.

In some cases ail might drain back from the oil galleries in the engine block. Most of this oil may return to the sump by way of the crankshaft bearings. Most notable may be oil from the rocker shaft assembly draining down. For this to happen air has to find its way into the rocker bearing journals and into the rocker shaft. Similar for oil that may drain out of the camshaft bearings with air having to find its way through the bearing journals to get into the oil galleries. This will be a slow process, but a day or two might be enough time for it to happen.

An oil filter with a leaky anti-drain-back valve (or none) might allow oil to drain backward through the oil filter. For this to happen oil will have to also drain backward through the oil pump. This also will be an exceedingly slow process, unless the oil pump is well worn with unusually large internal clearances.

Oil level should be between the marks on the dip stick a minute or so after engine shut-down. The engine is not much concerned if the oil level is a bit higher after being stationary for a day. Within seconds after it is started up again it will achieve a dynamic condition where a little bit of the oil is stuck upstairs, and oil level in the sump will be normal. You won't see the elevated oil level again until after it is shut down for an extended period of time, and the excess oil will "vanish" (run and hide upstairs) immediately after next start up.

If you want to think about changes of oil level, you might think about what the oil level might be while the engine is running at full chat. With 12 quarts per minute circulating through the engine, it will take a short time for that oil to return to the sump. For the short time that oil is stuck upstairs, oil level in the sump will be lower. How much lower might give you some pause. Suffice it to say, the oil level must never be low enough to suck air through the pick up pipe at full chat. Oil might be about two quarts low before you might notice low pressure flucuations on the oil pressure gauge caused by sucking up some air with the oil. By the time oil is that low you might also notice slightly lower oil pressure at hot idle due to elevated oil temperature caused by reduced oil cooling via the sump walls.

Distance between the two marks on the dip stick represents about 3/4 quart of oil. Being one quart low (slightly below the low mark) is not detrimental to the engine. Being a smidge above the full mark after settling when not running is also no concern.

Being half a quart or more too high is a concern and may be detrimental to the engine. If the crankshaft and/or con-rod big ends dip into the oil while running it can froth up the oil full of air bubbles like whipped cream. Crankshaft bearings in particular do not like air bubbles which displace oil that should be lubricating and cooling the bearings.

Bottom lime is, check oil level a short time after shutdown, like when you stop for fuel fill up. Do not be concerned about slight overfill after sitting for a day, as it will not look like that when running.

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