Improving Crankcase Ventilation

What is PCV?

(Positive Crankcase Ventilation)

Ventilation of the crankcase is necessary to remove blow-by gasses and to prevent condensation of moisture in the crankcase and rocker cover.  In modern engines these gasses and any entrained oil mist are conducted back to the induction manifold in a sealed system and are burned in the combustion process thereby reducing emissions to the environment.

How does the Magnette ventilation system work?

The ZA and ZB Magnettes are equipped with a Road Draft Tube which is one of the earliest methods used to provide active crankcase ventilation.  This system discharges to the atmosphere instead of burning the gasses in the engine, and in a worn engine, this can lead to emission of smoke and loss of oil from the draft tube.   After a fast run this will be evidenced by drips on the garage floor.

The direction of gas flow in the Magnette ventilation system is not as it might first appear. There is a hose connecting the rocker cover to the air cleaner and one might assume that the induction vacuum is drawing gasses from the rocker cover, but this is not the case. The hose connects to the outer casing of the air cleaner where there is little or no suction.  In fact, the lower end of the draft tube, which is in free air below the engine, is cut at an angle such that when the car is in forward motion a small negative pressure is created.  The upper end of the draft tube communicates via the front tappet chest cover with the crankcase which correspondingly experiences a very slight vacuum. This vacuum draws air from the outer part of the air cleaner into the rocker cover and in this way an active ventilation flow is established - so long as the car continues to move forward at a reasonable speed.

On the ZB Magnette there is a restricting orifice in the stub of tubing where the hose attaches to the air cleaner (I am not familiar with the ZA but I imagine it will be the same).  This restrictor is designed to ensure that there is a small negative pressure in the crankcase when the car is moving.  This negative pressure has a further advantage in limiting oil leakage from engine seals especially in engines such as the early BMC B-series which employ a crankshaft scroll rather than a rear lip seal. However, when the car is stationary with the engine running, the crankcase becomes slightly pressurised by the blow-by gasses, and this will exacerbate any tendency for the seals and scroll to leak oil.

How to improve ventilation

It can be seen that the Magnettes, in common with the MGA and early (3-main) MGBs, have a very poor ventilation system because it only functions when the car is in significant forward motion, and even then the draft tube provides a rather weak suction effect and discharges to atmosphere. This scarcely merits the description “Positive Crankcase Ventilation”

It is possible to fit a PCV system from an MGB to the Magnette. This will utilise the much more powerful vacuum available at the induction manifold to draw the blow-by gasses into the engine and burn them.

MGBs from engine No. 18GBxxxx onwards employed PCV systems which were improved in stages during the development of the later models.  The first improvement was to fit a pressure control valve (Photo 1) which connected directly into the balance tube of the induction manifold.

PCV Valve


This drew gasses from the front tappet chest cover in a controlled manner.  Fresh air was allowed into the rocker cover, controlled by an orifice in the oil filler cap which was vented and contained a filter. There was no tube on the rocker cover to connect to the air cleaner.  In later versions the vacuum take-off was relocated to ports in the constant depression region of the SU carburettor throats. This system had the advantage that a pressure control valve was not required, and the tappet chest cover was connected directly to the carburettor ports via a “y” connector.

For the Magnette, the earlier system using the PCV valve is probably the easier to adapt unless you have already fitted the later MGB carburettors equipped with vacuum ports.

Items required

  1. Late MGB Tappet chest cover with separator  Part No 12H1399
  2. Bolt for above (longer than Magnette item) Part No. BH605201
  3. Gasket for side cover Part No. 12A1139
  4. Smiths PCV valve  Part No. 13H5191
  5. Hose tail  connector
  6. DIY Mounting hardware and hoses for the Smiths valve.

The tappet chest cover is a bolt-on Item, but is thicker than the Magnette side plate so needs the longer bolt. The end of the take-off pipe faces upwards almost directly beneath the Magnette cross brace.  It would be possible to modify this pipe to bring it further back so as to clear the cross-brace but I found I could manage without doing this.

The Smiths valve needs to be mounted with the diaphragm horizontal.  On the MGB, the PCV valve is mounted on the balance pipe utilising a hose connection which faces straight upwards. Although there is a tapping in this location on the Magnette balance pipe it is inaccessible beneath the air plenum (unless neither the air plenum nor air cleaner is being used). The valve contains a diaphragm that needs replacing from time to time as a maintenance item because if it splits it allows excess air into the induction manifold.

I decided to mount the valve body using a length of 1/8” thick steel strip (blue) bolted onto the second manifold stud (See Photo 2).


This positioned the valve well clear of the air cleaner so that connections could be made using rubber vacuum hose.

It was necessary to create a vacuum tapping somewhere in the induction manifold to connect the hose. I had already fitted a hose tail into the rear blanking plug of the balance pipe to provide vacuum for my brake servo, so I simply tee-d into the servo hose (See Photo3)


Connection to Servo Hose


It would have been neater to have fitted a second hose tail into the front blanking plug, but I took the easier (?) way out.

Since the hose would run close above the exhaust manifold I used a length of steel tube connected at each end with silicone tubing so as to avoid problems with the heat from the exhaust.  In fact the silicone tubing alone would probably have been sufficient.

The standard Magnette sealed oil filler cap can continue to be used along with the hose connection between the rocker cover and the air cleaner.  Apart from the PCV valve and its connections, the engine compartment looks much as it did originally.


 Andy Dear 2012

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