At the risk of being pedantic, and because I neither have an A series c'case half on my bench - or the timing cover off mine - I have a query (mainly because I'm very interested in this mod and may well carry it out on mine if it as simple as it seems). Certainly BSA recognised the wetsumping problem and resolved it when they updated to the A50/65 models. I was hitherto unaware that they had done this.
My query is this: the attached photo seems to show the spring, hole and ball all the same size. I appreciate it is a sketch and not a drawing, but because of that it's unhelpful. Clearly the ball can't be the same size as the hole, or little oil would get past. Furthermore, my memory seems to tell me that the original hole in the casing is around 3/16" - or 5mm for the youngsters among us. Surely, then, the solution should be to open that hole to, say, 8mm and fit a 5/16" spring (that will be a clearance fit) together with a 1/4" ball? By opening up the hole a shoulder for the spring would automatically be created and a 1/4 ball in an 8mm hole should not obstruct oil flow. I believe the A65 spring had a reduced dia at one end, so could this be the solution?