The twin lobe cam A7/642 used from 1939 has the same 1" base circle as the single lobe cam so uses the same length pushrods. The earlier F cam and the A6/664 cam have a 1.5" base circle so need pushrods 1/4" shorter than those for the later cams.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.