Classic British Motorcycle Alternator Information Single phase alternators become popular on British motorcycles in the mid 1950s as they, give better output than dynamos and need virtually no maintenance. The early RM12-15 series were constructed using separate winding bobbins pushed on to the alternator frame but these can become loose with engine vibration. This type don’t give good outputs as they did not have any other form of voltage regulation other than the physical size of the frame and having to switch in different sets of windings as the lights were switched on. Later RM-18 to 19 units saw improvements in output and a larger 74 mm rotor. The very late versions were fully encapsulated to prevent the bobbins vibrating loose. The RM19 was the most powerful 6 volt unit and is the most suitable for 12 volt conversion. All the round frame RM alternators use the same mounting pattern and are interchangeable. The RM12 & 14 had a hexagonal frame and can be replaced by round frame versions, provided you machine an adaptor ring and use the correct size rotor, the wiring will need to be modified as the RM12 had 6 wires. Whether you want to keep your bike original and 6 volts or you are up-grading your electrics to 12 volts we have the parts you need. You can convert later 6 volt RM19s to 12 volts by just using our universal solid state regulator and altering the connections. The best way to guarantee an improved output is to switch to a new Lucas alternator. These benefit from modern production methods and are made on brand new equipment. Single phase systems are the cheapest option whilst 3-phase systems give much better output at low RPMs. Improving 6 volt system is possible by fitting our 6 volt solid state regulator rectifier. This will stabilise the charging and stop batteries from boiling as well as allowing sealed & AGM batteries to be used. We are happy to advise you. Here are our recommendations if you want to up-grade your electrical system: 1. Replace open bobbin type stators with a fully encapsulated - the open type are very low output and not worth repairing unless you are building a concours machine accurate in every detail. 2. The 6 volt systems can be improved on by fitting our solid state 6 volt regulator rectifier unit. This will stabilise the charging and allow sealed & AGM batteries to be used. Doing so will not improve the output it will stop batteries boiling, bulbs blowing and make the lights as bright as possible. 3. If want to fit electronic ignition and keep 6 volt electrics you must use our 6 volt solid state regulator rectifier unit. From experience most electronic ignition manufacturers will declare the warranty void if the ignition fails and there is no voltage regulator fitted. These companies often recommend a modern regulator but stop short of saying that it a requirement. We feel it is a requirement to avoid damaging your new ignition and subsequent upset and disappointment for the customer. 4. If you want to run with no battery you will need a RM23 16 amp single phase or a 3phase stator to ensure there is sufficient power from the alternator to power the ignition while kick starting. In addition you will need a battery eliminator to stabilise the system.

©Marcus Rex 2016. All rights reserved. Rex’s British Alternator Info Rev 2 October 2016

5. On 12 volt systems replace zener diodes with a modern regulator. These give much better control over the charging and is recommended if you want to use an AGM battery. 6. Replace wet batteries with a sealed or AGM battery. This type of battery will hold a charge for a very long time and can be left unused with little ill effect (providing they were fully charged before being left standing). Cyclon batteries are designed to last for years without use. AGM batteries such as the yellow Motobatt are held in high regard with customers and size for size give more power than similar brands. 7. Avoid the lightweight lithium batteries, these have very exact charging requirements and will not stand the way old British motorcycles charge the battery. 8. Use an automotive type relay to by pass the headlamp switches and power the headlamp via the battery. These can be fitted inside the headlamp shell and wired so the normal switches work as before, all you will notice is that the lights will be much brighter and there will be a click from the relay when you switch them on. 9. Check all connections are in good condition! Rex’s stock a wide range of Lucas parts as well as our brand of PME high power, solid state voltage regulators, HT coils and wiring supplies. Alternators The RM 12-15 alternators use a 70.0 mm rotor which is no longer made, if you are replacing a 70mm stator the only option is to replace the rotor as well for the later 74 mm type. In most cases both 74.0mm rotor & stator can be fitted with no problem. A 70.0mm rotor cannot be used with a 74.0mm stator as the air gap between rotor & stator is too large to produce any power. Alternator types (using the LU54202299, 74 mm rotor) 47204 (RM19) 6 V, 3 wire 120W single phase replacement. Fully encapsulated, highest output 3 wire, 6 volt unit. 47205 (RM21) 2 wire 10 amp, 120W single phase this unit can be regulated to 6 or 12 volts and is a popular replacement for the RM19. A solid state regulator must be used with this unit if it is being used to supply 6 volts. 47239 (RM23) 12V, 2 lead single phase 16 amp. Improved output over the RM21. Recommend 12v single phase unit, can be used with a capacitor (battery-less) 47252 (RM24) 12V 3 phase 3 lead, 3 phase 10.5 amp stator. Fitted originally as standard to Triumph T140 pre electric start models (from 1979). 47244 High output RM24.12 V, 3 wire 180W 3-phase. A very popular and reliable upgrade of the electrical system. It can replace all above stators, not only producing more maximum power but importantly producing more power in the lower rev range. For best results the zener diode should be replaced with one of our combined 3-phase regulator/ rectifier units for maximum reliability. ©Marcus Rex 2016. All rights reserved. Rex’s British Alternator Info Rev 2 October 2016

6 Volt Alternators The RM 12-15 used a 70mm rotor and are very low output. If these have failed it is best to replace them as a whole alternator, IE the centre rotor and the stator. While these can be repaired it is more expensive than replacing them. Also these will not supply electronic ignition and lighting. We can supply a genuine Lucas 6 volt 120 watt alternator set complete with modern solid state regulator that will give good lighting and power electronic ignition. The RM12 had 6 wires and the wiring will need to be modified if swapping to a modern alternator. The later encapsulated RM18 & 19, 3 wire, 6V stators use an 74 mm rotor and will reliably supply modest lights and electronic ignition. The encapsulated RM19 is the only unit we recommend to convert to12 volts, however while this is possible the output is limited to around 100 watts. We can supply a modern 120 watt 6 volt Lucas alternator. The 12 volt RM23 is a much better option if considering a 12 volt conversion as it will easily supply super bright modern lights and electronic ignition. The 3 wire 6 volt alternator is often confused with 12 volt 3-phase generators as the wire colours are the same, however a quick look will confirm which type you have if you count the pole pieces showing in the centre; 6 = 6 volt. 12 volt generators have 9 poles. Our technicians are happy to discuss your requirements.Telephone 01580 880768. Battery eliminators (capacitors) If you decide you want to go down this route we strongly recommend that you use a separate battery eliminator (capacitor). Regulator/rectifiers with a capacitor built in are not a good idea as the capacitor does not like heat and the regulator produces a lot of heat! If an internal capacitor fails the whole unit is scrap so regulator/rectifiers and battery eliminators (capacitors) are best kept as separate items. All 12 volt, 3 phase generators and the 16 amp, single phase Lucas 42739 (RM23) unit will start a bike without a battery - if fitted with a battery eliminator, even with most electronic ignition systems. None of the other lower output, single phase stators should be used with a battery eliminator. Adjusting running clearance When mounted to the engine there must be a minimum 0.2 mm (0.008”) clearance between the rotor and stator all the way round. Less and the rotor may touch the stator as it heats up. If this happens the stator will overheat due to friction and fail very quickly, this type of failure is not covered by the warranty. Inspect the pole pieces and the rotor for signs of touching after initial fitting. The clearance is adjusted by slightly loosening the mounting bolts and - carefully - tapping the stator with a soft faced hammer then retightening. Our alternators fit all unit 500cc Triumphs, unit T120, T140, T150, T160, X75 all Commandos, 650SS, Atlas, BSA C15, B40, B25, B50, A50 & 65, and most other alternator equipped British bikes from 1950s to 1970s and earlier if the existing rotor is 74MM. NOTE: Some Triumph T20s have a taper on the crank, in order to fit the 74 mm rotor you will need to have an adaptor sleeve machined. T20SM models have a parallel fitting on the crank. When ordering for Triumph Cubs please check the crank for a 19 mm parallel fitting. Lucas is a registered trademark of Lucas Industries Limited ©Marcus Rex 2016. All rights reserved. Rex’s British Alternator Info Rev 2 October 2016