Like Peter, I think that it is simply a matter of the position of the camshaft and the effects of the valve springs. The Longman alternator has no magnetic effects until switched on so it is most unlikely to be that.
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.