Crankcase Ventilation

G'day all

We all know that if you block off the engine breathers in your Sprite or Midget, you are likely to get serious oil leaks - mainly from the rear main bearing. Good crankcase ventilation is very important, either by simple open vents (such as on the Bugeye engine side plates) or the PCV (Positive Crankcase Ventilation) of the later engines.

Most of us assume that the cause of crankcase pressure is combustion gasses getting past the piston rings and into the sump. However, here is another explanation for the cause of the problem. When you read it, like me you might think that the logic is obviously flawed. However, I sat down with pen and paper, and couldn't find a mistake in the calculations. Perhaps I missed something! If you find the mistake PLEASE let me know.


There are varying degrees of symmetry and our engines are not symmetrical vertically.

With the engine at 1/2 stroke, and the crank journals at 90 degrees to the vertical, the con rod is at an angle, and the piston has therefore moved more than 1/2 way down the bore. Similarly, the one on its way up will have moved less than 1/2 way, so the two together will have pressurised the sump.

If you were to draw a vertical line from the piston centre down through the axis of the crank shaft and a second horizontal line at 90 degrees to this through the journal at 1/2 stroke, then the con rod itself will form the third side of the triangle, and will lie along the hypotenuse.

The (full) stroke is 3.2" and the rods are 5.75" between centres.

The simplest way I can describe it is to say that:- when the crank is at 90 degrees from TDC, the pistons are not half way down the bore! At this point in a 1275 motor the piston is at 57% of its stroke. Hence when the crank is in this position all 4 pistons are at 57% stroke. Compare this when the crank is at TDC or BDC, you will have 0% stroke on two and 100% on the other two, making an average of 50% overall. Hence my 7% volume variation.

Conversely when No 1 piston is at 50% stroke, the crank is 82 degrees after TDC. This phenomenon is accentuated by the length of the con rod, ie: the longer the con rod the nearer to 90 degrees half stroke is.

On a 4 pot motor, the motion of the pistons moving will cause a pressure ripple at twice engine crank speed - 2 positive (and 2 negative) pulses per rev. This is why on an otherwise air-tight engine you can feel pressure pulses if you remove the oil filler cap or some engines will throw oil out of the dip stick hole if the latter is removed with the engine running.