The MGC used two different crankcase breathing systems during production, with the changeover at the 1969 model year. Understanding which system is fitted is essential when ordering replacement components, as the two systems are completely different in design and share no parts. The purpose of both systems is the same: to remove the combustion gases and oil vapour that inevitably blow past the piston rings into the crankcase, preventing pressure build-up that would force oil past seals and gaskets and cause the characteristic oil leaks that plague poorly maintained classic engines. Both systems route these crankcase vapours back into the inlet tract to be burned in the combustion process rather than venting them to atmosphere, a requirement that was becoming standard practice by the late 1960s for emissions reasons.
Positive Crankcase Ventilation, Early Cars
Cars up to engine numbers 29G/3200 and 29GA/1400 were fitted with a positive crankcase ventilation (PCV) valve mounted on a bracket and connected by hoses with clips to the crankcase outlet and inlet manifold. The PCV valve is a metering device that regulates the flow of crankcase vapours into the inlet manifold based on engine vacuum, at idle, when inlet manifold vacuum is high, the valve restricts flow to prevent the crankcase vapours from leaning the mixture excessively, while at higher loads when vacuum drops, the valve opens further to handle the greater volume of blow-by gases. A complete service kit is available for rebuilding the PCV valve, or individual components: the plunger, diaphragm, cover, spring, and associated hardware can be ordered separately. The PCV valve cover changed at the same engine number breakpoint as the breathing system changeover, so the correct cover must be specified when ordering.
Hose is sold by the metre with four jubilee clips securing the connections. A sticking or failed PCV valve causes either excessive oil consumption (valve stuck open) or oil leaks and pressure build-up in the crankcase (valve stuck closed), and should be serviced at every major engine service.
Carburettor Crankcase Ventilation, Late Cars
From engine numbers 29G/3201, 29GA/1401, and 29GB/101 onwards, the PCV valve was replaced by a carburettor crankcase ventilation system. This uses a Y-piece connected by hoses to both carburettor air intakes, drawing crankcase vapours into the inlet airstream via the low-pressure zone at the carburettor mouths rather than via a metered valve at the inlet manifold. This system is simpler than the PCV arrangement with fewer moving parts and less to go wrong, though it relies on the hose connections being airtight for correct operation. Hoses are sold by the metre with four clips.
The Y-piece itself is a simple brass or steel fitting that should be inspected for corrosion and blockage during any engine service.
K&N Breather Filter
A K&N engine breather filter is available as an alternative to either closed-circuit system, venting filtered crankcase vapours directly to atmosphere through the oiled cotton gauze filter element. This open-breather arrangement is common on competition and spirited road use cars where the simplicity of an open vent is preferred over the closed-circuit recirculation system. The K&N filter prevents oil mist from depositing on the outside of the engine while allowing crankcase pressure to vent freely.
For cars used on the road, the closed-circuit system is the correct and original arrangement; the open breather is most appropriate for cars used primarily in competition or where the engine has been significantly modified and produces higher blow-by volumes than the original closed system was designed to handle.