Rocker oil supply

Some owners have less oil supply to the valve gear than they would like and I have given the problem some thought.

The flow to the rockers is taken from the scavenge return at the oil tank. If you ever glance inside the tank after starting you will see a steady stream of oil flowing back into the tank. After a while the flow comes to halt and just occasionally splutters. This is correct since at first, the sump is very full from drainage and the pump has lots of oil to shift. In these conditions there is probably sufficient back pressure to push oil up to the rockers. However when the normal state of oil return is reached, the oil will contain lots of oil bubbles and only spurts of oil. This again is as it should be.

If you try to force more oil to the rockers by restricting the outlet to the oil tank, you would think this would be a simple method to provide more oil. Well if all the oil coming back to the tank was in a continuous stream without gaps consisting of large air bubbles then it would be. The reality is that the air bubbles cause a problem.

With a warm engine that is running steadily you will have a shot of oil moving up the pipe from the scavenge pump. It will be followed by a bubble or a âlengthâ of air also being pushed along by the next shot of oil behind it. When the oil reaches the restricted outlet to the tank, a pressure is created back down the scavenge pipe. As the oil shot eventually passes along on its way out, the following air bubble is now in line with the rocker feed pipe union and so the air will immediately get pushed up the rocker pipe. The greater the restriction the more air will go up it and become compressed behind the oil which is already up the long pipe. Eventually the next shot of oil gets to the rocker feed union, and some of that oil will be forced up behind the air bubble. If the next air bubble is quite large it will not only travel up the rocker pipe as before but it will still exist when the oil in front has fully exhausted into the oil tank. As there is very little restriction to air being released into the tank outlet compared to that of the oil, the air bubbles exhausts into the tank very quickly and collapses the pressure behind it. When this occurs, the air bubble in the rocker pipe previously moving up to the rockers driven by the oil pressure can now expand and push back into the scavenge pipe and pushes the oil below it back out into the main feed or the oil tank.

You can see then that with a spasmodic air bubbled filled scavenge flow there is a very likely possibility that oil has a very difficult time ever getting to the head, and will reciprocate back and forth along the pipe to a great extent.

Increasing the restriction at the tank MIGHT have some benefit but the air bubbles will be also be more pressurised and will doing there best to resist this, with a chance that it might hardly have any effect at all. Remember this is not a continuous oil flow of oil, if it was, then yes, any restriction would create a corresponding increase to the rockers, but the bubbles create a problem by the fact that a gas is compressable.

However it seems quite easy to lessen the problem.

As air is much lighter than oil and is always trying to rise to the surface. The rocker take off should be in a position that is at the bottom of any pipe. Ideally the return pipe should be horizontal just before it gets back to the tank and the rocker take off should come off at the 6 oâclock position and travel down an inch or more before heading for the rockers. Much greater pressure is required to force a bubble downwards in a fluid that it is to let it rise naturally or forcibly upwards.

In everyday terms, without major surgery, a possible practicable solution might be obtained if you turned the take-off union as far round as you can, so that the takeoff union is below the 3 oâclock position and preferably at the 6 oâclock position (probably not possible unless lengthening the pipe is done but I donât know) This might result in less air attempting to rise up the pipe and a more continuous flow achieved. It wonât be as effective as the previous suggestion though.

For an experiment, it would be easy to monitor the flow, by using an old rocker pipe and cutting the pipe quite near to the take off point at the bottom and introducing a length of clear plastic pipe. The pressure here is very low, so leakage problems should be zero. Observations can be taken and an idea of how much oil or air is passing along can be ascertained, depending on the angle. You will almost always have very tiny bubbles trapped in the oil and these will provide a marker to see the flow. The big bubbles or gaps in the oil flow are the ones to exclude. Rotation of the oil union can be experimented with to see if any benefit can be obtained.

The ideal flow will be continuous but not too great a quantity. Valve gear does not require a great deal of oil as all the actions are reciprocating ones which is less punishing wear wise. It does require at least some oil though and some owners do find the rockers are running dry. However with too much oil to the head you will almost certainly burn more oil as it sucks the excess down the inlet valve guides..

I hope this has been interesting and may lead, with some care and careful adjustment, to a more consistent lubrication of the cylinder head components, without needing to fit a tighter restrictor union. It may prove that just by turning the union by some degrees downwards, it will solve the problem, but then again it may not. Thanks for reading this.

Les H