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MG Midget and Sprite Technical - Alloy Rocker covers

I'm a sucker for a bit of bling and the alloy rocker covers look attractive. I notice they do not have a breather whereas my existing cover has a breather tube. It isn't attached to anything at the moment - should it be?

The alloy cover seems to have a blank circular area where a breather hole could be machined. Is this necessary?
Stuart K

Stuart

The breather isn't necessary if you use the standard breather outlet on the timing cover to the manifold or carb body, depending on the year of your car.

Any additional breather on the rocker cover may stop this system being effective in keeping a vacuum in the crancase, which allows oil to escape from the real of the crank past the scroll.

This would seem to be the case in your current setup?

So ally rocker cover with no 'breather' is fine.

A
Anthony Cutler

I don't agree. I think the vent on the timing cover is where the vacuum is drawn from, and the vent on the valve cover is where the air is drawn in to allow the vacuum system to work. It probably has a plug in it with a small hole drilled in that to limit the intake air.

The small hole in conjunction with the PCV valve prevents too much air from bypassing the carbs. At least that is how it works on some of the MGBs. If your current valve cover vent is not connected to anything, you should at least connect it to a small inline fuel filter to keep it from drawing dirt into your engine.

The proper set up for your car depends upon what you have. You might want to mention that so that someone who is familiar can verify what the set up should be.

You can modify the alloy valve cover or the cap.

Charley
C R Huff

Erm what vent on the timing cover? I don't see one. 1098 engine is different? My carbs/manifold aren't attached to anything. There are two connections on the intake manifold, one of which is blanked off and the other is connected to the vacuum advance tube that goes to the dizzy.

The cap on the alloy cover has a pinhole in it.
Stuart K

Stuart

It does depend on which model A series you have. While the 1275's had the sealed rocker cover and the breather shifted to the timing chain cover, earlier versions did not have the latter. Vents on the earlier engines were in the rocker cover and the pushrod cover. The pushrod cover was eliminated on the post-Cooper 1275's presumably in the interests of making it difficult to change the camshaft. (Or block rigidity if you are not a conspiracy theorist.)


There were also a number of ways of plumbing this in:

The pushrod cover breather was vented to atmosphere (these were the days prior to any emmisions consideration). At this time, the rocker cover breather went to the air filter.

In the mid 60's the PCV valve was introduced, feeding the breather directly to the manifold whenever the valve opened. (A long term problem with these valves is that one form of them dying results in oil injection to the inlet manifold, clouds of smoke from the exhaust and sometimes an unnecessary engine recondition in response.)

As far as I have ever been able to make out, The indentation on the rocker cover where the breather used to be is sealed. There is a small bleed hole in the plastic oil filler cap.

Then in the late 60's there was the vent to the carburettor body, between the piston and the throttle spindle.

One of the key things about the breather is that it lowers the crank case pressure to reduce the loading on the crankshaft seals (or lack of seal in the case of the midget rear main). If a partial vacuum is there, even better. The later systems were more effective in this respect. At least they are until the breather hoses block up. The gauze in the breather canisters themselves can also block up.

See http://www.mgparts.co.nz/burning_oil.html for more on the problems caused by malfuntioning breathers.


So the main thing is to make sure you have an effective breather system. If yours is a 1275, I'd do the simple thing and try the cover without the breather, relying just on the timing cover one, and see if there is any change for the worse. The cover is easy enough to remove and modify if needed.
If a pre-1275, you could modify the rocker cover to replicate the earlier system, or fit the 1275 timing cover. The latter is a lot more work if the engine is still in the car of course.
Paul Walbran

It is a 1098 engine. There is a breather from the tappet chest cover plate that drops down low near the sump and vents to atmosphere and there is a pipe out of the rocker cover that currently also is open to atmosphere. No timing cover vent. I suspect my rocker cover was originally connected to the airfilter but since I now have aftermarket airfilters it has been left disconnected.
Stuart K

Charley, the hole on the oil filler cap does not suck air in rather air breathes out and sometimes pushes oil with it. If this is not the case how do you explain the light residue of oil found around the cap, even on a B-series engine.
Daniel Thirteen-Twelve

Daniel,

I think the vented caps are used on the engines that don't have a PCV system, so they probably did mist out some oil. Mine has a PVC system, and originally had a non-vented cap. I had to modify it when I put a non-vented valve cover on it.

So, my cap sucks in just like the valve cover used to. I drilled a non-vented oil cap and put a brass plumbing fitting in it. I brazed the fitting closed and then drilled it out to the same size as the breather tube in the original valve cover. Then I put a rubber tube on the fitting and put an inline fuel filter on the tube, and left one end of the filter open.

You should be able to see the parts in the photo attached here. The fuel filter for the fresh air intake can just barely be seen below the right side heater hose to which it is tied with a nylock strap. I have never found any oil in the filter. The fuel filter directly behind the valve cover is actually the fuel filter, and it is not the one I added for ventilation.

The air is drawn out of the engine through the mushroom PCV valve seen between the carbs.

Charley


C R Huff

Charley

The later version where the breather is fed into the side of the carbs functions to exactly the same effect under any given conditions as the PCV valve - shut off under high manifold vacuum (otherwise too much oil gets sucked through), drawing from the crankcase under low manifold vacuum.

I've never seen it written anywhere in respect of these BMC engines, but to me your proposition that air is drawn in through the bleed hole makes sense. Functionally, this would be the ancestor of modern breathing systems.

I have seen oil get pushed out of the hole as Daniel describes when engines get a bit tired or the breather hose is blocked (the two usually go together). Such engines show other signs of excess crankcase pressure, like oil leaks from the crank seals.
Paul Walbran

The 1098s are similar to many engines of the period, when emissions started to be an issue - they consume their own fumes rather than venting them to atmos.

Mostly this means a tube from the rocker cover to the air-filter housing, so fumes are sucked in when the carbs are demanding air, creating a flow (it's really an early and less effective version of ported vacuum). There's not much vacuum on the 'air' side of the filter, so no PCV or other valve is needed to restict flow.

It made most sense when steel gauze filters were used, since the fumes helped to keep the gauze 'oiled', which was essential for their efficient operation. (Mind you - ISTRT only one side of the twin air-filters had the rocker vent pipe attached; but you can see this on the period Morris Minors and others with their big single filter.)

It makes less sense for paper cleaners, which tend to get clogged for engines that are venting excess.

Didn't realise you had a 1098!

A
Anthony Cutler

This thread was discussed between 07/07/2009 and 08/07/2009

MG Midget and Sprite Technical index

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