once you've removed the rocker and gasket,firstly make sure that where the gasket sits on the head is really clean,no sealant/paste anywhere.then put some sealant down only in the valleys on the cams/cam gears end on the head,and on the raised ridge at the other end around the dizzy.dont overtighten the bolts,and remember the tdc middle bolt is a different lenght to all the others...