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MG MGB Technical - Valve Cover Venting

I have a 1980 MGB. I am currently using the original metal valve cover. I recently purchased an alloy valve cover from Vicky Brit. I expected this to be a 5-10 minute installation. However, a couple of things worry me. The original valve cover has the breather tube with a small hole (5/64ths" diameter) and a hose running from the breather tube to the charcoal cannister. The original has a non-vented oil filler cap. The new alloy valve cover has a breather tube, but the hole is much larger than the original at 3/8ths" diameter. It also has a wonderful looking chrome oil filler cap, but it is also vented with a 1/16th hole on the top side and the same on the underside.

Should I flow some solder into the holes on the chrome cap to make it a non-vented cap? Should I somehow make the breather tube opening smaller? Or should I plug the breather tube and charcoal cannister and just leave the oil filler cap vented?

FYI - most of the original emission controls have been removed and I have dual SU carbs.

Thanks for your advise,
Dave Kalp

Hi Dave
Your engine was originally designed to run fully sealed for emission purpouses therefore it has a non vented cap and your fuel tank vents through the carbon canister. Your question's answer depends on what other emission parts have been removed------ If you still have a vac. supply to a pcv valve in the breather system you will need to seal up the hole in the filler cap and refit the canister hose. ( a suggestion here would be to tap a thread in the 3/8 hole and find a fitting to screw into it with the correct restrictor size id to take your canister hose )-----If the vac. supply to your breather system has gone and it breaths to atmosphere and you're not having any breather problems with your current setup I would block the filler cap hole up anyway just to keep the top of your motor clean, and either reconnect the cannister hose as above(it would probably look better with a hose on the fitting) or block the fitting in the tappet cover and leave the cannister open to atmosphere
Whatever you do DO NOT block off the cannister as this is your fueltank breather. Hope this has been of some help

The rocker arm cover (BMC Part # 12H 3252) of the North American Market 18GJ, 18GK, and 18V engines is equipped with a 5/64” restrictor tube in order to prevent the fresh air that is being drawn in from overly diluting the fuel-air mixture and causing lean running. The size of the hole in the valve cover restrictor tube has been restricted to 5/64” 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 speed. This may result in the loss of smooth idle, or an inability to idle at the correct low engine speed. It will also result in an overly-rich mixture off-idle, the vacuum leakage remaining relatively static and not increasing proportionally as the throttle is opened. Much the same happens with a genuine vacuum leak, i.e., at the intake manifold gasket.

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 on the roof of the combustion chambers as well as on the piston crowns, 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 the breather tube of the rocker arm cover, and 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. In the case of 18GJ, 18GK, and 18V engines equipped with a nonvented oil filler cap (BMC Part # 13H 2296), rather than allowing oil to travel down through the pushrod passages in order to aid in the lubrication of both the balled lower ends of the pushrods and the upper sections of the tappets as they should in both cases, pressurization of the fuel tank as well as the adsorption canister would occur, interfering with its function. A seemingly incurable seeping leak at the base of the rocker arm cover is the usually the first external symptom in such cases. 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 both the oil mist from the crankshaft 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 as well as to 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.
Steve S.

Engines without the carbon canister use a non-vented rocker cover with a vented oil filler cap - and the cap contains a restriction and a filter, it is not just a hole, a simple oil-filler cap with a hole drilled in it will draw in dust and moisture. Engines with a charcoal canister use a vented rocker cover and a non-vented oil filler cap. The cover vent has the restriction, and the PCV valve or carb ventilation continuously draws fresh air from the atmosphere through the charcoal granules (which act as the filter) to purge them of any fuel vapours adsorbed from tank and float chamber filling and expansion, and then through the crankcase to take away oil vapours and condensation to be burnt in the combustion chambers. Neither is a 'sealed' system, or a closed-circuit system as Clausager states, but a through-flow system.

If you fit a non-vented rocker cover with a non-vented oil filler cap you will block the ventilation system and could get a build-up of condensation and hence internal corrosion. The very least you should do with the new rocker cover is to use a proper ventilated oil filler cap. If you aren't required to have a functioning charcoal canister for annual inspection, then you can just leave it how it is, the tank and float chamber(s) will vent as before through the charcoal canister, but it won't be purged, so it could eventually block up. The most immediate thing you may notice is that the anti-runon valve may cease to function, as this depends partly on the suction from the rocker cover to operate.

If you want to remove the canister, anti-runon valve and their respective plumbing you must pipe the float chamber vent port(s) down past the engine and exhaust for safety. If you want to remove the vent pipes and separation chamber from the tank you must seal the port at the tank i.e. outside the car and use a ventilated fuel filler cap. Don't remove the chamber and pipe to the front of the car and leave the port open in the boot/trunk, or it will fill with fumes, and there are electrics in there, like the fuel pump sparking away all the time.
Paul Hunt 2


I wanted to thank everyone for the comprehensive explainations and advice regarding the installation of my new alloy valve cover. I think I have solved the problem. I took an old valve cover I had on the shelf and removed the breather tube with a hacksaw. Then using a punch I removed the 5/64ths "restrictor" from the tube. After a little fiddling I got the restrictor into the 5/8ths ID breather tube on the new alloy valve cover. I soldered the retrictor to the tube. Then I placed a drop of solder into the top and bottom vent holes in the oil filler cap to make it non-vented. I then hooked up the hose from the charcoal cannister to the restricted breather tube. After 25 miles of stop and go traffic and back road twisties everything seems to be working and the polished alloy valve cover looks a good bit better than the old original painted one.

Again thanks for all your help.

Dave Kalp


Good work. I have the same cover and had to do the same thing except I used a thread pipe fitting and had the tools to thread the breather tube. Drilled a 5/64" hole in the pipe fitting and screwed it into the breather tube.

I think somehow you ended up with the wrong oil cap. That one should have been on the W/out breather tube valve cover. I have a small hole on the bottom, but none on the top.

Ray 1977mgb

Another option take valve cover to local parts store select a freez out plug that fits over tube with a gentel tap, and drill hole
Fritz Warner

This thread was discussed between 26/04/2008 and 28/04/2008

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