Have fitted many forks / oil seals to BSA's over the decades, the last time during my GF restoration. I fitted stainless fork seal holders but, despite sealing the threads, they leak badly. I suspect the seal is not being clamped properly by the holder. Maybe the recess is too deep? Can't be bothered at present to look into it, but probably will when I get to strip the engine again to fit new conrods. BTW, I prefer rubber seals as I believe they seal better on the outside.