Craig, the center thread is 1/4 BSW same as cover screws.........Try Norman's idea for the plunger spring to see if this helps. if so, then compress the spring in the jaws of a vice until it coil binds, this should shorten it by about 2 mm or so and then it will work fine. On nearly all older engines I've found the cam plate spindle is loose.........The center bolt/lock wire trick is good.......I use a liquid thread sealant on them, just on the upper threads........this seals from an oil leak, plus helps stop it coming undone........I use a product called Cyberbond........SH 27........This is an amber color anaerobic thread sealant that is excellent........especially on fuel taps and many other oil unions on these bikes. You can smear a little around the exposed threads shown in your picture, then undo the spindle several turns to get the sealant on the upper threads of the spindle, then wind it in tight with a good screwdriver, one that you can put a spanner on if possible . Make sure you are happy with the shift of the gearbox before doing this last step.........The sealant I mentioned is great for all the oil return lines across the heads using the stock lower seals and small 1/4" dowty type seals under the bolts atop........even a tinny amount on the taper faces of the pipes them selves.......I never get any leaks from this area if this is all done carefully.