By Dave Porter, Technical and Customer Support at The Bonneville Shop
Converting Your 6-volt British Motorcycle to 12-volts can be a bit of work.“The conversion requires not just an upgrade of charging system components, but also auxiliary electrical components. This includes light bulbs, horn, fuses, and in some cases, switches, ignitions, and ignition coils.”
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If your British motorcycle was made before 1966, chances are it was originally specified with a 6-volt electrical system. A 6-volt system is adequate for daytime running, but with limited lighting, ignition, and battery selection, the choice to switch to a 12-volt system has become more common. In most cases, the 12-volt conversion can be made without irreversible modifications. The conversion requires not just an upgrade of charging system components, but also auxiliary electrical components. This includes light bulbs, horn, fuses, and in some cases, switches, ignitions, and ignition coils.
If you are uncertain about which components to replace, give us a call at 720-570-7884. You can also email order@thebonnevilleshop.com
Motorcycles Fitted with DC Generator Charging Systems
“It isn’t suggested to do this however, since you would need to double the speed of rotation of the generator to charge a 12-volt battery.”
For generator/dynamo models, the output of the generator is limited by the voltage regulator in place, and the speed which it can be spun. This is an oversimplification and there are other factors to consider. However, you could theoretically replace a 6-volt regulator with a 12-volt regulator and have a working 12-volt system. It isn’t suggested to do this however, since you would need to double the speed of rotation of the generator to charge a 12-volt battery. In other words, if charging capabilities of a 6-volt dynamo starts at around 2000 rpm, you would need to run up to around 4000 rpm to charge a 12-volt battery.
Some enthusiasts desiring 12-volt benefits while retaining a level of originality may consider altering the windings and field coil of the dynamo. This type of modification is best left in the hands of a specialist. Another option is to consider some of the modern aftermarket generators available. Suppliers of generators include Alton. Once you have addressed the charging capabilities of your generator, you can fit the appropriate 12-volt regulator. You also need the correct bulbs, horn, and battery.
Motorcycles Fitted with Alternators
For 6-volt British motorcycles with alternators, you will need to source a 12-volt regulator/ rectifier, or what I call a charging module. Today’s charging modules are solid-state, compact, and vibration-resistant. Some charging modules with MOFSET technology are even heat-resistant. They can be installed in an enclosure without any regard for the cooling effects of airflow. As part of the 12-volt conversion, the charging module replaces the Zener diode (voltage regulator). It also replaces the selenium rectifier, which is tasked with converting alternating current (AC) to direct current (DC).
If a contact breaker points ignition is going to be used, the original 3-wire 8-amp stator fitted to most later 6-volt machines can be used to supply the current. Simply pair the green/black and green/yellow wires together to form one lead. Then connect it to one of the yellow leads from the charging module.
The remaining white/green wire from the stator connects to the other yellow lead from the module. The brown or black lead from the module will be the power to the battery (through the ammeter, if fitted). The red lead is the grounding wire, at least on a positive ground system. Once the bulbs, horn, and battery are updated to 12-volt specification, you are now operating on 12 volts.
More Upgrades to Consider
Now that I have covered the most rudimentary conversion from a 6-volt system to a 12-volt system, let’s go forward to the next level of upgrades. The original Lucas 3-wire RM19 stator, as fitted to many 6-volt British motorcycles, had a current rating of 8 amps. That will equate to approximately 96 watts of power to run a simple electrical system. The majority of the late 1950s-early 1960s motorcycles were equipped with 96 watts.
If an electronic ignition system should be retrofitted in place of the troublesome 4CA points ignition system, a more powerful stator should be considered. The Lucas 47205 RM21 is a 10-amp single-phase stator with 120 watts of power to better satisfy the demands on the battery. This stator is fitted to 1969-1978 models.
Since the stator only comprises half of the alternator, attention must be paid to the magnetic rotor. Rotors must be well magnetized to optimize the performance of the charging system. This is especially true for running at lower engine speeds. A new rotor to pair with a 10-amp stator will go a long way to keep the battery charged. It will also satisfy the demands of electronic ignition on the battery. When selecting a charging module for your 12-volt conversion, be sure to select the appropriate unit to match the output of the alternator.
If deciding to further upgrade the charging system with a 16-amp RM23 single-phase stator, you’ll need a more robust charging module with 200-watt capability. One choice is Podtronics POD-1P-HP, which we offer under part number TBS-4035 (or in this kit with a stator, rotor and primary gasket).
Thanks for checking out this article. Please contact us if you need any further guidance with your 6-volt to 12-volt conversion.
Additional Resources
- View the entire stock of regulators.
- Check out our YouTube channel for more about British electric components.
- We also have the largest online selection of vintage Triumph parts to help keep your motorcycle running.
6-Volt to 12-Volt British Motorcycle Conversion FAQs
Can I convert my classic British motorcycle from 6 volts to 12 volts?
Yes. Many classic British motorcycles with original 6-volt electrical systems can be converted to 12 volts, often without irreversible modifications. The correct conversion depends on whether the motorcycle uses a dynamo or an alternator. You must also update the battery and other voltage-dependent electrical components, not just the charging system.
What are the benefits of converting a vintage British motorcycle to 12 volts?
Converting to 12 volts gives you a wider selection of compatible lighting, batteries, and ignition equipment. The conversion also provides an opportunity to upgrade charging capacity if you plan to install electronic ignition. However, changing voltage alone does not increase the system’s available power; the charging components must be appropriate for the motorcycle’s electrical demands.
What parts do I need to convert a 6-volt British motorcycle to 12 volts?
A typical conversion requires a 12-volt battery, compatible bulbs and horn, and suitable charging-control components. Fuses must also be addressed, and some motorcycles require changes to switches, ignition equipment, or ignition coils. Alternator-equipped motorcycles need a compatible 12-volt regulator/rectifier, while dynamo-equipped motorcycles require a generator and regulator arrangement capable of charging a 12-volt battery.
Can I convert a 6-volt motorcycle dynamo to 12 volts by changing the regulator?
The article does not recommend a regulator-only conversion. Although an original 6-volt dynamo may theoretically produce sufficient voltage at higher speeds, charging may not begin until substantially higher engine rpm. The article instead discusses specialist modifications to the dynamo’s windings and field coil or a suitable modern replacement generator, followed by the correct regulator.
Can I keep my original Lucas RM19 stator when converting to 12 volts?
Yes, the original 3-wire, 8-amp Lucas RM19 stator can be retained for the basic alternator conversion described in the article when using contact-breaker points ignition. It must be connected correctly to a compatible 12-volt regulator/rectifier. If you also plan to install electronic ignition, the article recommends considering a higher-output stator.
What does a regulator/rectifier do in a motorcycle 12-volt conversion?
A regulator/rectifier converts the alternator’s alternating current into direct current and controls charging voltage. In the conversion described, a modern combined charging module replaces the separate rectifier and voltage-regulation components. Choose a unit that matches your alternator’s output and motorcycle’s electrical configuration.
Do I need electronic ignition for a 6-volt to 12-volt conversion?
No. You can retain contact-breaker points ignition in the basic conversion described in the article. Electronic ignition is a separate, optional upgrade. If you install it, review the charging system’s capacity, stator output, and rotor condition so the battery can support the ignition system and other electrical loads.
Which Lucas stator upgrade should I consider for electronic ignition?
The article recommends considering the Lucas 47205 RM21, a 10-amp, single-phase stator rated at 120 watts. It provides more capacity than the original 8-amp RM19 arrangement, described as approximately 96 watts at 12 volts. Pair the stator with a compatible regulator/rectifier and a rotor in good condition.
Should I replace the alternator rotor during a 12-volt conversion?
Check the rotor’s magnetism and condition before deciding to reuse it. A rotor with insufficient magnetism limits charging performance, particularly at lower engine speeds. The article recommends considering a new rotor alongside a 10-amp stator to help maintain battery charge when using electronic ignition.
Which regulator/rectifier works with a Lucas RM23 high-output stator?
The article specifies a charging module with 200-watt capability for the 16-amp Lucas RM23 single-phase stator. It identifies the Podtronics POD-1P-HP, sold by The Bonneville Shop as part number TBS-4035, as one suitable option. Confirm compatibility with your motorcycle’s alternator and wiring configuration before ordering.