I think this is the same problem as appeared in this forum in October 2018 as 'Another 403 problem'.
In that thread, Dave Dale said (I hope it's OK to quote you, Dave):
"A few years ago I had the same problem with my 403. As Geoff K correctly observes, the threads in the manifolds were corroded and allowed the bronze nuts ( correctly called 'peg nuts' with reference to the tool that is used to tighten them ) to slip over the threads when an attempt was made to tighten them. Spencer Lane-Jones had supplies of replacement nuts in stainless steel and in three slightly dissimilar sizes of thread, to accommodate varying degrees of manifold thread wear. If one of the three sizes is a close fit, the nut can hopefully be tightened up without too much force or used as a die to cut its own thread into the manifold by reshaping the faulty existing thread, thus obviating the need to subject the manifold to a proper re-thread. At least, that is how I understood it at the time and it worked for me. You could try […]
office@spencer-lj.com Telephone: 01985 847133. Hope this helps.
Dave Dale."