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MG MGB Technical - Emission Control Valve

I am considering fitting one of these to an MG XPAG engine in an effort to reduce oil leaks buy reducing crankcase pressure. I need to know if the MGBs with these valves had a special oil filler cap with possibly a restrited air inlet and filter. If so how are these identified and which years cars had them? Also, any information on how the system works would be appreciated.

M Blakey


The MGBs with the Positive Crankcase Ventilation (PCV) system used an unvented oil filler cap, but they had a vent tube in the valve cover that had an orifice button to restrict the airflow into the engine. I don't know the size of the hole, but someone will. It is probably something between 1/32 and 1/16 inch. This stuff might be different on this side of the water since the emission standards were different.

C R Huff

Mauriace. Charley is correct on how the later, 18V, engines were set up. They used a tight fitting oil filler cap, had an elbow set into the top of the rocker arm cover which had a small hole in it (with a rubber hose running from the rocker arm cover to the central hole of the charcoal canister) and a rubber hose running from the front tappet cover to either the Z-S carb or the intake manifold. (The factory illustrations do not clearly show where the hose goes after being hooked up to the front tappet cover and I do not have an example to hand to look at. Besides, you will not want to use a Z-S carb for your engine.)

That was the latest system, used after the requirement to contain fuel vapors by using a sealed fuel tank with a vent line, moving through a vapor separation tank, forwards to connect to a charcoal canister.

However, earlier versions of the positive crankcase ventilation system exist and have since at least 1968. All versions, until the requirement for a charcoal canister, used a vented oil filler cap with an integral screen filter. These systems used either a "Mushroom" valve, connected to the front tappet chest cover and the intake manifold, or they used a hose which came off the front tappet chest, hooked into a "Y" connector, then had a hose running to each carburetor (HS-4s I belive.)

Any of these three systems might be suitable to your needs with the earlier two systems being the easiest to set up. The 68 system would only require the addition of the mushroom valve and changing the front tappet cover to one having the pipe pointing upwards, then connecting the valve to the tappet cover and intake manifold.

The later system, using connections to the carbies, would require suitable HS-4 carbies, the proper style of front tappet chest cover, and the connecting hoses and "Y" piece.

The 18V style system would be modified so that the elbow on the rocker arm cover (which forms the intake portion of the system) is connected to a small air filter and you would still have to incorporate either the mushroom valve or the carbies designed to be hooked up to the tappet chest cover line.

Clausager's Original MGB has some photos of these systems and various retailers have line drawings of the systems. Between the two, you should be able to make a decision as to what you want to do and how to do it.

Les Bengtson

Many thanks Charley and Les.

M Blakey

This thread was discussed between 15/04/2011 and 21/04/2011

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