Clean both surfaces thourally as super1c said, On my 600 twin those are kind of like a compression gasket mine were 3 layers of metal then an outer sealer of somekind, My dealer told me to install them with the sealant side towards the manifold he's a hillbilly though so if U hear different tell me, But mine does not leak now so something good must have happened,( In my opinion though if I have to take my manifold off for whatever the reason I will install brand new gaskets again, Once U disturb them they're ruined I think. Plus a too rich situation either from too much fuel or a oil pump thats doing wrong will exacerbate the issue hope this helps.