Have had a persistent oil leak from around the sump plate and a couple of other jobs needed doing so i pulled the engine out and split the cases in the dim distant past this engine has throw a rod and the case has been welded and on examination its distorted near the sump plate I used Loctite Gasket Eliminator 515 but to cure needs no air I could see the case don't pull tougher tight can get 14th feeler in it I don't want to use silicone sealer as petrol attacks it.
I use Loctite 518, but looking at the data sheet there's not much difference between that and 515. They say it will fill a gap up to 0.025 mm. That's .010" and that's a pretty good gap, but you have to wait at least 72 hours to let it cure at that thickness.
A smattering: '53 Gold Flash '67 Royal Star '71 Rickman Metisse '40 Silver Star '37 Rudge Special sixtyseventy Lightboltrocket road racer...and many more.
There is silicon and silicon, 3bond supersealer is very good, its grey. Toyota use a black seal packing to seal sumps with no gasket and various other types of silicon based stuff for G/boxes etc. If these products had been available I'm sure BSA would have used them. I don't use many gaskets on my BSA and it seals better. If you have it apart why not build up the low area with JB weld then use a file to true it up.