If installed right they should be leak free.
I use a light smear of black rtv and fasten the gasket to the cover and hold it in place with binder clips like in the link. Once it dries a bit I remove the clips and place the cover gasket down on a smooth surface and pile some heavy books on top to hold it down and cure overnight.
Bolt it on and you’re smiling.
https://www.123ink.ca/p-349041-n-010..._BwE#sku352331