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MG MGB Technical - crankcase ventilation/valve cover

Here's another question: what is the effect of the size of the hole in the valve cover hose restrictor in a 73/74 MGB? My car shows an oil seepage around the filler cap. The cap is new, nonvented and fits OK. Always had the seepage problem so I drilled the valve cover restrictor hole slightly larger did I mess up the situation?; seepage is still occurring. Thanks in advance.

You may have either reduced or eliminated the partial vacuum inside the crankcase, resulting in any gases escaping past the rings to pressurize the crankcase. The purpose of the restrictor is to maintain the interior of the crankcase in a partial vacuum which is created by the induction system. 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 carburetors 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 (technically termed "windage 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. In addition, without the partial vacuum provided 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 (BMC 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 (BMC 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. 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, depriving their upper sections of the additional lubrication supplied by the oil mist and the oil running down the pushrods from the rocker arm assembly. 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. 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 an induced vacuum, and as such the system contributes to long-term reliability and a prolonged engine lifespan.
Steve S.

comprehensive comment there!
My question is about the little pipe with a pin hole at the rear of the late model valve covers. It appears to me that the only function is to provide a partial vacuum to clean out the charcoal filter collecting fumes from the fuel tank.

On my 67, I have no such charcoal filter and I've soldered the pin hole shut. Have I made an irretrivable disatrous mistake??

Barry Parkinson

The purpose of the charcoal canister is to collect vapours from the expansion of fuel in the tank and float chamber instead of these being emitted to atmosphere. The purpose of the connection between the rocker cover and the canister is to pull a continuous amount of clean air through the canister which is what purges it of these fumes. If not purged the canister eventually cannot adsorb any more fumes and so they get into the atmosphere at best, or at worst it clogs and you get problems with fuel starvation or rich running. The rocker cover vent is also the only source of air to purge the crankcase of fumes and condensation, as the oil filler cap on these cars should be non-vented. By sealing up the rocker port you prevent any ventilation of the crankcase, which allows condensation to build up and cause internal corrosion - unless you fit a ventilated oil filler cap as well. On later cars the port is also part of the anti-runon system, disabling any part of it may prevent that system from doing its job.
Paul Hunt 2

The size of the hole in the valve cover restrictor tube has been restricted so as to not present a significant vacuum leak to the induction system. If you enlarge the hole size, then you will adversely affect the tuning at idle rpms. This may result in the loss of smooth idle, or inability to idle at correct low RPM.

The oil filler cap should form a tight seal and should not leak at all. You may be getting positive crankcase pressurization due to excessive compression blow-by from worn piston rings, or maybe the side cover oil separator is clogged with carbon deposits (this is common). To see if the oil separator is clogged, disconnect the breather pipe hose from the Y junction which goes to the carbs and see if you can readily blow air through the hose while it remains conected to the side cover.
Gerald O'

You will always be able to correct the mixture at idle with an enlarged port, but it will then result in an over-rich mixture off-idle, the vacuum 'leak' remaining relatively static and not increasing proportionally as the throttle is opened. The same thing happens with a genuine vacuum leak i.e. at a manifold gasket.
Paul Hunt 2

Dear, Steve S. in Virginia, Thanks very much. Now I understand the internal pressure dynamics and will be more attentive henceforth.

Are you an engineer or another turbo geek like me?

R.S.C Caskie

I'm a stone motorhead and a former commercial mechanic as well as a former tool & diemaker.
My father was an engineer and, due to my love of such things, I've inherited his textbooks. Since the laws of physics haven't changed, everything in the books still applies. I build engines for friends and a select few enthusiasts that I know personally. Although I retired from a career in Law Enforcement field about two and a half years ago, I started another career because my hobby can't be supported by a pension. Besides, I'm way too young (57) to sit on the front poarch in a rocking chair with a blanket on my lap and a cat dozing thereon. My cars, all built and painted by myself, are at present:
1972 MGB
1968 MGC
1972 MG midget
1959 MGA Twincam FHC (under construction)
1954 MG TF (under construction)
Steve S.

This thread was discussed between 07/08/2006 and 12/08/2006

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