I have been contacted a few times by people looking for help with their magneto-equipped bikes.

Of course, the magneto means that the electrical and charging system is not needed to run the bike – it is purely there for lighting, horn etc…

This article was written with the Norton Dominator in mind, but everything covered is true for most brit bikes of the time, so this might be a useful page for non-Norton people too!

Alternator

It’s a personal choice, but I really like to specify a three phase alternator if I am rebuilding a bike.

The chances are that your single phase one, or your 6 volt one has seen better days by this point, so treat yourself to a new stator and rotor.

Three phase is a great idea for modern stop start traffic and lower speed around town and city riding, because you are charging at a lower RPM than the equivalent single phase unit.

Be careful that you don’t over spec the charging circuit though – bigger is not better in this case, and it is important that you are putting back in roughly the same as you are taking out!!!

The Lucas LU47252 (Wassell number WW10193L) is a 10.5 amp stator, so roughly the same spec of the original RM21 10 amp single phase stator that most of these bikes of the era would have had.

There is absolutely no need to go up to the 14.5 amp high power version unless you have a bike with electric start.

Regulator/Rectifier

There are a lot of reg/rec units on the market, but for me by far my favourite weapon of choice currently is the superb Shindengen SH775 regulator/rectifier.

I have written about open-type (also referred to by some as series-type) reg/recs extensively on this site ‘vs’ the much more common short-type alternative, so I don’t need to spend time covering the benefits here.

But for me, it is the best possible choice in terms of longevity, robustness and low maintenance.

Wiring these up is simple:

• 5, 3 and 1
  For connection to a three phase stator, the wiring order of the AC inputs don’t matter.
  If you use this with a single phase stator, connect the two wires to any three of these terminals.
• The black and red definitely do matter though!
  • 4
    The red (positive/earth) pin is toward the centre of the unit.
  • 2
    The black (negative) pin is on the outside of the unit.

The Shindengen is wired in the same way as most aftermarket reg/rec units:

As there is no electrical connectivity to the heat-sinking metal enclosure, this unit is suitable for use on positive earth or negative earth bikes.

Battery

I am a huge fan of the Motobatt AGM battery.

The Absorbed Glass Matt battery is sealed, so no worry about topping it up, venting it, or spilling battery acid on the paint.

These AGM batteries are superb over the winter months – no need to keep them on a trickle charger or anything like that.

Just give it a full charge using a decent quality charger (I like CTEK who can be found here) the night before your first ride of the new season, and you’re good to go for the year!

As an example, for the Dommie 650ss, we chose the MotoBatt MB5.5U AGM

This particular battery is pretty small in size (13 ½ x 6 x 13 centimetres) which meant that by playing with the oil tank and battery box mountings, we could slide the battery down between the two quite easily.
The battery box was now totally empty, and could be filled with a decent sized tool roll!

Key Switch

It is very difficult to secure a classic bike, but a key switch is a visual deterrent if nothing else.

In all honesty, these bikes don’t get left in public where a chancer would steal it anyway.

In the setup I have here, I have used a double pole switch which can commonly be found on scooters/mopeds/ATVs and costs between $10 and $20.

These are “four wire” switches, and handily one set of contacts is “on” while the other set is “off”

This means that when the switch is in the “off” position, the magneto can be grounded – neatly making it double up as an engine cut-off switch too.

Charge Warning Light

Instead of an Ammeter, I have used a Charge Warning Light.

My preference here are the brilliant little units from www.improvingclassicmotorcycles.com – this particular one is the Tri-Color LED which shows:

• less than 11.1V = slow red flash
• 11.1V – 12.6V = steady red
• 12.6V – 15.4V = steady green
• 15.4V – 16.1V = steady yellow
• above 16.1V = eye catching yellow flash sequence

Here is a link to a video on the ‘Improving Classic Motorcycles’ website.

To be honest, with a decent alternator, a good battery and the engine running totally independently off the Magneto, this charging system is totally over engineered.

My goal here though is literally fit and forget!

Headlight Switch

I went for the standard Lucas toggle switch here – familiar on most bikes of the time, and a very robust and long lasting item.

The LU31788 I use here is a three position Off-On-On switch

The lights turn on and off via a traditional Lucas 57SA toggle switch mounted on the headlight:

• (1) Off
• (2) Pilot Light
• (3) Dipped Beam

Be careful with part numbers if you are buying new – 57SA is the type (i.e., what it looks like, length of the switch toggle blade, size of mounting hole etc)

Here are the pinouts for the switch in the different positions:

There are several part numbers available under this type, some are two way (off-on) others have a different numbewr of connection terminals on.

The LU31788 is the part number you need!

Handlebar Switch

I have gone for the Wipac Ducon switch on the left handlebar to toggle between dipped beam and main beam.

It’s also the home of the Horn button.

In my opinion, these are the best of a bad bunch – all of the aftermarket ‘retro’ handlebar switches are really awful quality – especially the Lucas (Wassell) ones which is why I have gone up the Wipac route here.

Earth

Anyone that follows my blog will know that I am always rabbiting on about earthing.

On the Norton Commando (the topic of most of my posts) we are blessed with a bit of forward thinking.

Engineers at Lucas and Norton were worried about the Isolastic drivetrain mounting causing earthing problems, so in the main part ran earth wires (which should really be referred to as positive feed wires) between electrical components on the bike, rather that earthing out through the frame.

With one or two minor exceptions, this resulted in a reliable and robust electrical system.

I really like the concept of not relying on the frame for the ‘earth’ so whenever I can, I will always recommend running separate wires instead of earthing out to the frame.

It gives you a much more reliable electrical system!

Fuses

The reg/rec is on it’s own dedicated fuse which is wired directly to the battery.

The beauty of doing this is that if, for whatever reason there is an issue with the charging system, you can pull the fuse and still use the lights to get you safely home on battery power alone (the recommended battery above will give you a couple of hours).

Also, if there is a failure of the reg/rec, there is no risk of overcharging and boiling the battery dry – it’s a smart move.

I have found issues with the glass-style fuseholders in the past – the springs become weak over time, and eventually the circuit becomes intermittent.

As the fuse disconnects and reconnects to the contacts, a small amount of arcing occurs – over time a layer of ‘soot’ will build up over the contact patch, which in itself acts as an electrical insulator.

This can impact all sorts of things, not least an undesirable lighting disco effect when you ride over bumps!

I would recommend using automotive blade type fuses all round instead of the original glass type used on these bikes.

These are great, as blade fuses are available in every garage and petrol station, and are very resilient to vibration.

A 15-amp fuse in each will be fine.

Wiring Diagram

I have drawn two flavours of diagram here – the standard and more familiar positive earth diagram.

But also, a negative earth version too – this is useful if you have other, more modern bikes in the workshop alongside your 60s brit bike and are worried about flipping between positive earth and negative earth bikes.

I have kept the wiring standard, using period correct colour codes wherever I can.

Basic Motorcycle Wiring Diagram (Magneto) POSITIVE EARTH – PNG 3066×1841

This is also available for download as a PDF

Basic Motorcycle Wiring Diagram (Magneto) NEGATIVE EARTH – PNG 3066×1841

This is also available for download as a PDF

Whilst I appreciate the diagrams were made with a Norton 650ss in mind, the wiring is pretty universal so this article maybe useful for someone that is looking to build/rewire something similar from the era.