Crankcase Breathing

See also Engine Side Covers.

Non-positive system used until February 1964. This uses a hose from the rocker cover to the front air-cleaner (outside the filter) and a non-ventilated oil filler cap: (Clausager Original MGB)

There is also a hose on the front tappet chest cover clipped to a stud or blanking-plate screw where the mechanical fuel pump is on other models, just hanging down with an open end: (Auto-Part.com)

In February 1964 a positive ventilation system was implemented. Until October 1968 it consisted of a PCV valve (13H5191) mounted on the inlet manifold, plumbed to a port on the front tappet chest cover. The oil filler cap is now ventilated with a restriction and a filter: (Clausager Original MGB)

Ventilated oil filler cap, with filter gauze visible through the hole at the bottom. Non-ventilated caps are hollow inside and should not be used on engines with positive crankcase ventilation:

'A' is the rubber seal to the filler neck, 'B' is one of two air inlet holes:

One of the two air inlet holes, just above where the cap locks onto the pins in the filler neck. With a light shining straight down onto the hole you can see the filter mesh:

Cover 12H1399 (above) with internal filtering and oil separation, used with the PCV valve: (Chris Howells)

In October 1968 the PCV valve was replaced by carb ventilation, with ports on both carbs connected together and taken down to the front tappet chest cover. The oil filler cap is ventilated as before. This system was used on UK cars until the end of production:

Cover with external filter and separator 12H3684 used with carburettor ventilation until the 18V779/780 engines during the 74 model year. However the Parts Catalogue states that rebuilt engines used the cover below, as is the case on Bee's Gold Seal engine: (William Revit in Tasmania via the MG Enthusiasts forum.)

Cover 12H4395 (below) again with internal filtering and oil separation, used with original 18V797 and later engines, and for all rebuilt 18GG and later engines: (Chris Howells)

North American spec from October 1969 with the charcoal canister. Originally with twin SUs connected to the front tappet chest cover as before, but with a non-vented oil-filler cap. This version shows the later single Zenith/Stromberg carb and anti-runon valve: (Clausager Original MGB)


V8 with individual hoses and oil/flame traps from carbs to rocker covers:


Rear of the V8 air-cleaner box showing the breather intake filter with mounting clip and hoses, the oil filler cap in unventilated:


Clutter out of the way, the vent is a horizontal tube with a closed outer end pushed into the back of the block with a vertical tube pointing up to take the short hose to the filter: