Bob is entirely correct in saying that the duplex chain has different gearing for the dynamo to distributor. What the duplex chain did was to increase the rotational speed of the dynamo in relation to the engine so as to improve charging. But if the dynamo shaft is turning faster than the engine, you need to slow down the rotation of the distributor to suit the engine rotation speed. Hence the different dynamo and distributor gears for the duplex chain. It seems to me that there is evidence of these later (duplex type) dynamo and distributor gears wearing more quickly than earlier simplex type gears. This comes from the range of these gears currently in my workshop. However, I intend to use the duplex chain on my Healey, partly because that was what was fitted when I bought the bike and partly to see how it goes. My previous MkII Squares, (including my first Healey) had the simplex chain. Whilst I did suffer from poor charging at 12 volts with the original dynamo, the fitting of an alternator to the Healey transformed it. I used a Lucas alternator for this first conversion but I am about to make a Bennet Longman pattern one using his drawings and account in Cheval for the current Healey project.
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.