I have the original rocker cover for my 1098 cc traveller engine however the rocker cover gasket didn't make it when it came to working on the engine. I have bought a new gasket from ESM and have attempted to fit it, it went on (wasn't the best fit) but I had my suspicions that it would be a total seal. Having filled the engine up with oil my suspicions where confirmed as some oil leaked out, is this normal behaviour or is there a 'knack' to fitting them? I should mention that my head is not original and I believe it to be a mini head, would this also affect the rocket cover gasket fit?

Many thanks in advance
Tom