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MG MGB Technical - Crankcase ventilation on '68 B?

Hello all,
I have a 1968 MGB that has new SU's from previous owner. I know my car's crankcase ventilation should route form the front tappet cover to a Y fitting and to each carb.

The issue is the new carbs do not have that hose fitting and the area appears to be solid where the fitting should be.

Do I drill and attempt to mount fittings (not so crazy on that) or get one of the older gulp valves? Or something else?

Right now there's a hose off the tappet cover just venting to the bottom of the car.

Thanks in advance, David

David Steverson

It shouldn't be that difficult to provide a carb port, it is only a drilling through to the space between the butterfly and the piston (I think!)

OTOH it will be easier to fit a PCV valve to the inlet manifold.

One of these forms of positive ventilation is *definitely* preferable to leaving the hoses disconnected, or even routing it into an air cleaner as was the case pre-PCV, as these give very little through-flow ventilation. And on switch-off and cool-down they will be sucking dust and moisture into the engine. Unless you only drive the car in warm weather you will get a build-up of condensation inside the engine and consequent corrosion. It's why PCV was fitted from Feb 64!
Paul Hunt 2


I have always understood that our cars up through 1968 had the PCV valve, and that the change crankcase ventilation via the carburetors did not happen until 1969? If you don't have the correct set of SUs, it would certainly be easy (and correct?) to add a PCV valve to the intake manifold plumbing as Paul suggested.

Cliff Maddox

It is important to retain the crankcase ventilation system. Properly maintained, crankcase gases are drawn into the combustion chambers of the engine by the vacuum created by the fuel induction system, either through the intake manifold as in the 18GB, 18GD, and 18GF engines, or through the carburettors as in the later engines. This permits the crankcase to function in a partial vacuum which not only reduces power loss due to the pistons, connecting rods, and crankshaft forcing the atmosphere inside the crankcase to move about (a condition that is technically termed “Windage Loss ” or “Pumping Loss”), it also causes oil mist inside the crankcase to condense more rapidly while being drawn upwards towards the camshaft and tappets. Because the oil mist becomes more highly condensed in the partial vacuum, more of it tends to fall into the sump rather than remaining in suspension as a fine mist and being drawn into the induction system. An oil separator is incorporated into the design of the front cover of the tappet chest in order to assist in preventing this. Due to the fact that the effectiveness of the system is dependent upon vacuum, all connections between the fuel tank, the vapor separator, the carbon canister, the rocker arm cover and its oil filler cap, as well as between the oil separator and the induction system, must be well sealed in order to maintain the optimum state of vacuum.

In addition, without the partial vacuum induced by this system, the pressurized gases inside the crankcase of the B Series engine would cause oil on the cylinder walls to be blown past the piston rings into the combustion chambers, leading to carbon buildup and consequent preignition problems. The carbon can also collect in the groove provided for the compression ring, causing the ring to seize (Bet’cha can’t guess how I know this!). In addition, an excess of these pressurized gases and oil mist would also be vented partially through its rocker arm cover, resulting in an oily film inside the engine compartment of engines equipped with a vented oil filler cap (Original Equipment Part # 12H 1836) of the 18GA, 18GB, 18GD, and 18GH engines, or, in the case of 18GJ, 18GK, and 18V engines with a nonvented oil filler cap (Original Equipment Part # 13H 2296), rather than traveling down through the pushrod passages in order to aid in the lubrication of the lower ball ends of the pushrods and the upper sections of the tappets as they should in both cases, pressurization of both the fuel tank and the adsorption canister would occur, interfering with its function. In order for the excess pressurized gases in the crankcase to arrive at the rocker arm cover, they would also have to travel up the past the pushrods and the oil drainback holes in the floor of the tappet chest. This means that the excess pressure of the gases would be forced upward around the tappets, decreasing the additional lubrication supplied by the oil mist and the oil running down the pushrods from the rocker arm assembly onto the upper sections of the tappets. The pistons would also have to work against the pressure trapped inside the crankcase, retarding their downward movement (i.e., “Pumping Loss”), thus causing more combustion heat to the transferred to both the cylinder walls and the roof of the combustion chamber, making the engine run hotter and reducing power output. Thus it must be understood that all of this is prevented by drawing all of the pressurized gases inside the engine out through the front cover of the tappet chest and into the induction system under the effect of an induced vacuum, and as such, the system contributes to long-term reliability and a prolonged engine lifespan.

If yours is an 18GA, 18GB, 18GD, or 18GF Series engine equipped with a PCV Valve (Original Equipment Part # 13H 5191), it should be retained in order to reduce atmospheric pressure inside the engine, thus reducing oil consumption and consequent accumulation of carbon inside the combustion chambers, as well as reducing power-robbing windage loss. However, the condition of the rubber diaphragm should be regularly checked. Should it rupture, considerable quantities of oil will be transferred into the combustion chambers through the induction system. In addition, should the compression rings start to fail, the resulting overpressurization of the crankcase will cause oil mist from the engine to saturate the oil separator tube of the early version of the front cover of the tappet chest and be transferred into the combustion chambers through the induction system, the consequent reduction of the octane level of the fuel-air mixture and carbon buildup eventually resulting in problems such as preignition, sometimes called “pinging.”

The front cover of the tappet chest for the later 18V engines (Original Equipment Part # 12H 4395), found on 18V-797-AE, 18V-798-AE, 18V-801-AE, 18V-802-AE, 18V-846-H, 18V-847-H, 18V-883-AE-L, 18V-884-AE-L, 18V-890-AE-L, 18V-891-AE-L engines) is preferable due to its better breathing characteristics and for having incorporated into its cover design an external oil reservoir/return chamber which minimizes the transfer of oil mist into the induction system. The later rear cover for the tappet chest (Original Equipment Part # 12A 1386) is less prone to distortion and leakage.

When replacing the gaskets onto the covers of the tappet chest, remember that the rubber O-rings on the bolts tend to take a set when left in place, so always replace them with new ones in order to obtain a good seal. Use either Hi-Tack or Permatex Aviation Form-A-Gasket sealant to glue the gaskets to the covers and allow it to harden overnight so that they will not move during installation. The nut for the shallow tappet chest rear cover should be torqued to 2 Ft-lbs, while the nut for the deeper tappet chest front cover should be torqued to 5 Ft-lbs. Exceeding these torque values may result in distortion of the covers and crushing of the gaskets, leakage being the result.
Steve S.

Many thanks for the replies. And Steve I do appreciate the info on this, things I did quite not understand are clear now.

Sadly, my engine number plate is missing, so who knows what's in there. But, it's easy enough to add the PCV setup, which is what I referred to as a gulp valve.

I'll start looking for one now, unless someone reading this thread has one to sell.
(the_steversons at

Regards, David
David Steverson

On the Right Hand Side of the block, in the area between distributor and oil filter, there are three numbers that form a circle and that are slightly raised, e.g. 30 12 71, which tells the day, month, and year during which it was cast. This will help to "narrow the field" a bit in figuring our when your engine was built. If the engine is original to the car, then being a North American Market 1968 model, The head casting number found on the top of the head should be 12H 2389. You should have an 18GF-(R)We-H engine.
Steve S.

Taking a simplistic view: The MGA had a simple U shaped pipe from the crankcase which vented downwards to atmosphere. The rocker cover had a simple pipe connected into the front air filter box. Some drivers disconnect this and extend the rubber pipe downward to vent the same.

I see nothing wrong with this set-up and converted my 66 B to the same. Runs okay with no problems and keeps it's tune. Been like that for years.

Take a look inside any car used for racing and you will see crankcase venting on similar lines.
roger lawson

Not as simple as you might imagine. The "A" and early "B's" used the vacuum caused by moving air past the end of the "U" tube to create air movement through the engine, starting at the valve cover vent hose. At road speed a fairly large amount of air moves through the engine. It is attached to an air filter for a reason: to keep extraneous particle out of the engine. So for years you have been drawing road debris into the engine through that rubber pipe. Read Barney Gaylord's articles on engine ventilation to understand how this works. Check out articles CV-101, CV-102 and CV-103.
Over the years, vastly different methods of ventilating the engine were used on our cars. Modifying the system ususally is not a good idea. The effect of any change needs to be clearly thought out, and switching parts as simple as the valve cover cap can be problematic. I recommend you re-connect the hose to the air filter cannister.
Thomas Lennon

"I recommend you re-connect the hose to the air filter"

Or in the case of a 66 to a PCV valve as the air cleaner probably won't have the port. With both tubes hanging down there *is* no through ventilation, only pressure relief. The requirements for racing engines are very different, they are unlikely to suffer from the internal condensation and corrosion that road cars get.

Even with one pipe connected to the air cleaner (as per pre-64) ventilation is iffy, and can draw air in either direction. When air isn't moving past the end of the 'road draught tube' (technical name for a bit of pipe hanging down by the bottom of the engine) the slight negative pressure inside the air filter will cause air with dust and moisture to be sucked into the engine through the open pipe at the bottom.
Paul Hunt 2

This thread was discussed between 15/12/2007 and 22/12/2007

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