I had a dry engine till recently which looked like was coming from the tappet covers. So off with the carbs and heat shield to get access to the covers. The cover with the vent on it was leaking and fortunately I had a spare rubber gasket. I glued the gasket to the cover and left it overnight to dry.
I cleaned up the oil on that side of the engine with cleaner and water. Re-assembled it all and hopefully got a dry engine again.
MGB tips mods and maintenance 
Remember to not over tighten it. It should be snug. You could apply a thin layer of RTV as an additional precaution.
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