I second Gremlin on this one. A lot of care is needed when fitting the rocker gasket. I usually smear a very thin film of Gasket sealant around one side of the gasket and then stick it to the "spotlessly clean and dry rocker cover. Once set, then apply a thin film of grease to the open side of the gasket before fitting the rocker cover carefully back on the head. But before doing this, make sure the cover is not distorted, and always use a new gasket. The grease will allow the cover and gasket to be removed and refitted in an emergency, but in most cases a new gasket is required due to already being crushed once.