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MG MGB Technical - Rocker Cover Cap: vented or non ?

I am seeking advice on how to determine which is the correct cap. The obvious would be to replace with what is there now... but is the one there the "correct" type ? The cap (non-vented) is weeping oil and I suspected the gasket has hardened. The MG parts supplier claimed I had the incorrect cap. He supplied me with a "vented" cap. The engine on my 1978 B was replaced by the PO before I acquired the car 3 years ago. The engine serial number (18V511M278L) would indicate the engine was from a Marina and has Low compression. Unknown build date. The rocker cover has a vent stack located at the back that connects to the Charcoal Cannister. The collar the cap fits into stands about 1.5 inches about the cover body. The engine has a vent pipe near the tappet cover. I have not had any problems with the engine in the 1000 miles I have driven, so far. No oil leakage around the cover gasket or the cover bolts.
Does it make a difference which cap is used.. ? Is there risk of incurring damage to the engine over time with the "incorrect" cap ? Thanks.
R.M. Standish

Sounds like you should have the non-vented cap. There should also be a connection from the tappet side cover to a port at the base of the carb. You didnt mention if that was still there, or which carb(s) you may be using.
Oil leaking from the cap should not occur unless the engine is being pressurized by a lack of ventilation or serious piston blow by. With the cap off and the engine at idle try placing a 4"x4" piece of paper over the oil fill. If the PCV system is working as it should there should be enough suction to hold it place.
Andrew Blackley

Andrew:
Thanks for the suggestion to test the "suction". I'll try that. In answer to some of your questions: the tappet side port is connected to a rubber draft pipe leading to the base of the engine compartment. The carb is a Weber DGV with a K&N filter w/o an inlet port... hence the use of draft pipe.
Just for clarity, there is no oil "leaking" from around the cap. Just a trace of oily film. The gasket on the cap was very hard.. not rubbery... thinking this might be why a very slight "mist" of oil could be escaping.
The engine has very good compression and does not "burn" oil.
R.M. Standish

This thread was discussed on 09/06/2003

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