3 phase alternator

The question often comes up, what is the cheapest, easiest way to upgrade a Norton Commando charging system?

In my opinion, a great option is to go for a three phase stator:

  • Lucas RM24
  • 3 Phase
  • 10.5 amp
  • Part Number LU47252
  • Also found under Part Number WW10193L
Grant Tiller

This stator has roughly the same output as the standard RM21 that came on the bike – however, the rated output is produced at a much lower RPM making this an ideal solution for around town and in stop start traffic conditions.

As always, I would recommend that you ride with your headlight on, as this will always help with the longevity of the components that make up your charging system.

The factory standard zener diode can handle this with no problem at all, so no need to change that.

However, it will be necessary to change your factory rectifier from a single phase one to a three phase one.

Fortunately, these are available at a very low cost indeed (around 10 dollars) – plus you get the advantage of something that is encapsulated in resin so is shake-proof (something that the original Lucas single phase rectifier suffered with.

A good three phase rectifier would be something like the 36MT60 – it is a very common component, and there are plenty of places online that sell them (you are not limited to classic Norton dealers)

The component looks like this:

Grant Tiller

Don’t be worried about wiring it in – it really couldn’t be simpler!

Grant Tiller
Grant Tiller

Here is a set of factory wiring diagrams with the three phase rectifier unit added into the circuit in lieu of the single phase one.

Pre-1971 (ammeter in the headlight shell and Wipac Tricon type handlebar switch) PNG 3066×1841

Grant Tiller

1971 (three wires to the master switch) PNG 3066×1841

Grant Tiller

1972 onwards (four wires to the master switch) PNG 3066×1841

Grant Tiller

This would also be a better system than the high power single phase stator that was fitted to the MK3.
Dropping one zener, and moving the remaining one to the DC side as shown below will up the system voltage by 0.7 volts – sounds like a silly little number, but that is actually quite useful, and was an achilles heel back in the day that upset the early electronic ignitions.

1974 MK3 Early Bikes – there were around 2,000 bikes that were built around the December 1974 timeframe that have three additional fuses that can be found in the headlamp bucket.

These bikes are also wired with the old Lucas 3AW 3 wire ‘silver can’ assimilator.

1974 MK3 (Early) PNG 3066×1841

Grant Tiller

1975 MK3 this is the most common configuration, and takes us through to the final Commando that rolls off the production line.

1975 MK3 (Original) PNG 3066×1841

Grant Tiller

1975 MK3 Canadian Market – there were legal requirement in Canada around the headlamp being on while the engine was running, so the wiring diagram includes changes needed (swapping out the Warning Light Assimilator 06-6393 for the Headlamp Warning Unit 06-6392). Note that a different Master Switch is also required.

This is covered in the Factory Wiring Diagram, by notes.

1975 MK3 (Canadian Market) PNG 3066×1841

Grant Tiller


A couple of points about the way these diagrams have been drawn:

  1. Where the same colour wire goes in to and out of a single connector, that connector has usually been omitted from the drawing.
    It’s obvious on the bike, is easy to spot and easy to troubleshoot.
    Leaving them off the diagrams makes them a LOT easier to read, and considerably less cluttered.
  2. Wherever the earth or ground side of a component goes back to the battery, the drawing shows a red earth symbol:
    Grant Tiller
    In reality, this could be connected either to a red wire in the bike’s wiring harness (loom) OR it could be attached to the frame or engine of the bike.

    I have shown the red earth symbol each time in order to massively simplify the diagram, and make it a lot easier to understand for everyone.

    I have also coloured them red as a gentle reminder that these bikes are wired positive earth!

4 replies

  1. Grant,
    I have a 1974 Commando that I want to keep with positive ground. I have the 3-phase alternator with a Mosfit regulator and now installing a tri-spark ignition. I am also going with the icm unit along with the oil pressure sending unit.My question is how to connect the wire going to the electronic tach.12V single coil.
    Any help would be appreciated
    Mike Yingling

  2. Hi there. What a brilliant and helpful page, Thank you so much!
    Am i right, if I only need to change the alternator(stat or), and rectifier.?
    And that I can leave the roter, and that there is No need for the podtronics box?
    Would be so satisfying to change as little as possible on my 71 fastback.

    • Hi Lars,

      Absolutely right!

      The rotor can stay exactly as it is – they do lose their magnetism eventually, but provided it can hold it’s own weight if you hang it off a screwdriver, it will be fine!

      Make sure you buy the 10.5 amp LU47252 and NOT the high powered LU47244 and you should be good to go!

      Hope that helps,


Leave a Reply

Your email address will not be published. Required fields are marked *