Since BSA forks are tapered to fit inside the fork yokes, their removal can be quite difficult. The best trick I was able to use for their removal was brute force. After removing the necessary securing parts (i.e., pinch bolts in the fork bottom yoke) and loosening the top chrome fork nuts, I supported the area between the steering head and frame on a saw horse, taped a thin block of wood to a ten pound sledge hammer (to prevent destruction of the chrome fork nuts when pounding), screwed the fork nuts down over a portion of the fork thread, and wacked the fork nuts until the legs broke free. This removal procedure required aggressive wacks from the sledge hammer but was effective. If available a "junk" cap screwed not quite down to the tube and
a large brass hammer a preferrable for this procedure.
To reinstall fork tubes into the triple clamps through the upper covers is difficult. Cut the head off a "junk" unit style fork cap that has the tapped hole for the dampener rod in a lathe. Drill and tap the hole thru the cap and screw in a 1 ft. length of 1/2" all thread rod. Lock it down with a nut. Screw this into the fork tube of the assembled fork leg. Insert it into the triple clamps through the cover. Put a large washer and nut on the protruding all thread and snug it up, pulling the tube into place.