The top cap of the Bastard is made of cast iron. This not only looks nice and robust, but is also a solid precision instrument. Provided you pay a little attention to it. Cast iron should be seasoned to keep it in top condition. This is also called “Seasoning”. Seasoning your top cap properly will keep it indestructible and operating smoothly. Seasoning causes oil to bond with the cast iron and form a protective hard polymer layer. This layer protects the cast iron from weather, wind and temperature. Moreover, it allows the two parts to fit together better so that the top cap can optimally do what it is meant to do; regulate or seal off air circulation.
Is your top cap not closing quite right? In most cases, this problem disappears over time as the kamado is used more. The top cap needs to "silt up" a bit, so to speak. So give it some time.
Optionally, you can try to help this process along by seasoning the top cap with a mixture of charcoal and linseed oil.
For this, take some of the finest grit at the bottom of a bag of charcoal and mix it with the linseed oil to form a porridge of yoghurt thickness, rub the moving parts of the top cap with this. Further seasoning instructions can be found here.