As Roger said it is common for cases to crack there due to over tightening of the transfer cover bolt. Regarding sealing the covers I always assemble them to the engine/block with no gaskets or silicone etc to ensure the faces are all true and mating correctly. If not then it is easy to either swap the covers over to see if they fit better upside down or LH to RH, then finally dress to ensure a perfectly square seat. This then does not involve machining crankcase and barrel together which for engines in bikes in impractical. Once square mating is acheived I use normal transfer port gaskets and don’t get problems with them leaking. I personally have an aversion to sticking engines together with RTV.