The MGB crankcase ventilation system changed significantly during production, and the components required depend on which engine series is fitted to the car. The system’s purpose is to draw combustion gases and oil vapour from the crankcase and route them back into the induction system to be burned, rather than allowing them to vent to atmosphere. A correctly functioning breather system is important for engine longevity, oil cleanliness, and emissions compliance. The earliest 18G engines from 1962 used an open circuit breathing arrangement.
Crankcase fumes were vented through a pipe from the front cylinder block side cover (tappet cover) to atmosphere via an inverted J-shaped pipe, with a further outlet on the rocker cover venting into the front air filter. The metal oil filler cap on these engines was embossed with the names of recommended lubricants.
Closed Circuit Breathing (18GA Onwards)
From the introduction of the 18GA engine in February 1964, a closed circuit breathing system was adopted. The front tappet cover was redesigned to incorporate an oil separator, the breather pipe attachment on the rocker cover was deleted, and a new inlet manifold was fitted. Fumes from the oil separator were fed through a positive crankcase ventilation (PCV) valve mounted above the inlet manifold, which metered the flow of crankcase gases into the inlet tract under vacuum.
This arrangement continued through the 18GB and into the early 18GD engines. The PCV valve is a service item, its internal plunger, diaphragm, and spring wear over time, and a sticking or blocked valve can cause oil leaks, increased crankcase pressure, and rough idle.
Carburettor Crankcase Ventilation (18GG/18V)
From the 1969 model year, with the introduction of the 18GG engine and carburettor crankcase ventilation, the PCV valve was deleted. The oil separator was instead connected by a Y-piece directly to both carburettors, with the crankcase fumes drawn into the carburettor throats under normal engine vacuum.
This arrangement, using a Y-piece with hoses running to the front and rear carburettors, continued through the 18V-series engines to the end of production. The hoses, Y-piece, and associated clips differ between 18GG and 18V applications, and the securing clips at the carburettor end are specific to this system. The oil filler cap changed from a vented metal type on open circuit breathing engines to a sealed plastic type from the 18GA onwards.
Replacement caps are available in both the original black plastic and an alternative red plastic finish. For owners who have disconnected the closed circuit breathing system, whether as part of a performance modification or simply to eliminate oil mist from the carburettor intakes, a breather filter can be fitted to the front tappet cover in place of the original hose connection. These filters provide effective filtration and protection for the crankcase while allowing the engine to breathe to atmosphere. They are available with either a chrome or black top.
Complete breather hose kits are available for the carburettor crankcase ventilation system used on 18GG and 18V engines, containing the correct Y-piece, hoses, and clips as a single package. Individual hoses, clips, and the PCV valve with its service components are also available separately for owners carrying out targeted repairs. MGOC Spares carries all breather system components across the full MGB production run, from the early PCV valve and its service kit through to the later Y-piece arrangement, hoses, clips, oil filler caps, and aftermarket breather filters.