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MG MGB Technical - Crankcase breathing and PCV valves

In the process of rebuilding my 18V engine I found that the catch tank connected to the front side chest cover with the breather tube was full with sludge, see image.

Clearly I would like to prevent this happening with the newly built engine, but how?

The rocker cover does not have a vent tube but does have a ventilated oil filler cap. The twin HS4 carburettors, with K&N filters, do not have provision for any connections to breathers. There is no PCV valve and the central port on the inlet manifold has a brass plug.

Does anyone have any advice/suggestions please?



Graham Gilmore

No through-flow ventilation hence nothing to take condensation away hence mixing with oil and the sludge.

If the carbs don't have PVC ports you really ought to fit a PCV valve to the inlet manifold somehow. The factory PCV valve used adapter 12H1405 with a sealing ring, you may be able to tap the manifold for that. Alternatively you could go back to the system used before PCV which is a 'road draught tube' hanging down from the front tappet chest cover.

Personally I would go for the PCV, getting a replacement inlet manifold from one of the used parts suppliers. You can get issues with the flanges on exhaust and inlet manifolds being different thicknesses needing cut down washers as spacers, but that may only affect North American spec.
paulh4

Thanks for your feedback Paul. Using a PCV is the conclusion that I was coming to. As pucker MGB PCVs are no longer available at a sensible price I did wonder about using a 'modern' one like this with a suitable tapping/fitting in the brass plug in the centre of the manifold. I understand that it not advisable to use one of the ready tapped ones either side of the centre.


Graham Gilmore

A modern one should be fine, I'd have thought, and less likely to fail. Yes, should be central so air flow has the same effect on both halves of the engine even though it is pretty small, which is why both carbs have one.

Incidentally does the catch tank have an outlet and where does it go? A comparatively cold surface like that would encourage condensation, like in the long oil filler tube on some BL models in the 80s. Ideally only air would be coming out of the tappet chest cover, oil should be trapped by wire wool in the outlet of the cover to form droplets out of oil mist which run back into the crankcase. If there is excessive oil coming out of the cover it could cause problems with the plugs.
paulh4

Those valves work well, but the size needs to be matched to the engine size.
A wrong sized valve will result in oil being drawn into the manifold.
Paul Walbran

Paul W, do you have any idea of size for the 1800 cc B series engine please? What does the 'size' refer to for a PCV? Thanks
Graham Gilmore

There is quite a bit on line on the subject. This from some smarty-pants AI thingy:

PCV valve size is crucial for matching engine displacement. Proper sizing ensures optimal airflow and effective crankcase ventilation. Here are some general guidelines:
• Engine Displacement: The size of the PCV valve should correspond to the engine's displacement. Larger engines typically require larger valves to handle increased blow-by gases.
• Common Sizes:
• For engines up to 3.0 liters, a PCV valve with a flow rate of about 0.5 to 1.0 CFM is often sufficient.
• For engines between 3.0 to 5.0 liters, a flow rate of 1.0 to 2.0 CFM is recommended.
• For engines over 5.0 liters, consider valves with flow rates exceeding 2.0 CFM.


pcv valve size table match engine displacement chart c - Yahoo Image Search Results


Jud
J K Chapin

Jud, thanks for that, should help Graham.

Graham, sorry, I didn't see your post till now, and I can't help directly as the only reference I have is the range of pcv valves available locally here and it's unlikely to be the same as you can access. But just in case: those here are branded fuel miser, they all look the same externally, part no pcv028 is the right size for small engines.
Otherwise the specs that Graham listed will help you identify.
Paul Walbran

I use a PCV that was intended for a Mazda or Toyota engine of similar size. Moss had a kit which came with a PCV for a large American engine and oil consumption was heavy. The engine consumes virtually no oil with the Japanese PCV.
Glenn Mallory

This thread was discussed between 25/11/2025 and 13/12/2025

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