Triumph Bantam Cub Electrics

Electrical information for the Bantam Cub variant of the Triumph Tiger Cub

All T20B models, both Bantam Cubs and Super Cubs, were fitted with electrics based on the BSA Bantam D10. The system is 6 volt, positive earth with a three-phase alternator and a rectifier supplying a battery mounted under the seat.

Lucas & Wipac Components
Wiring Loom & Horn
Electronic Ignition
12 volt Conversion

Lucas & Wipac Components

Traditionally Triumph appeared to favour Lucas components, while BSA seemed to prefer Wipac. However, this was not consistent in practice and these seem to be the most common parts used on the T20B.





Alternator (3-wire)

Lucas (or Wipac )

RM18 (or 1G1768)

See wiring details below



2DS 506




45150 - MA6

1.9" diameter x 4-5/8" long

Contact Breaker Plate



One set of points fitted.

Contact Breaker Points






425377 (DCB117)


Headlamp Shell


MCH 56

See 'Rebuilding' page for details

Headlamp Lens

Lucas (or Wipac)

516828 (or S0507)

5-3/4" diameter

Headlamp Main Bulbholder



For British 'Pre-Focus' bulb - see below

Headlamp Pilot Bulbholder



For BA9s bulb - see below

Speedometer Bulbholder



For BA7s bulb - see below

Ignition Switch


88SA 34427


Ignition Switch Wiring Socket




Lighting Switch


88SA 34289


Lighting Switch Wiring Socket




Main Beam Warning Lamp

not fitted


See note below *

Horn/Dip Switch


S3857 "Ducon"




8H (6V)


Brake Light Switch


S4010 "Series 185"


Rear Light Assmbly


LU53454 - L564

Takes BAY15d bulb - see below

* Note: Some BSA Bantam D10 models had a blue, main beam, warning lamp fitted in the central hole on the top of the headlamp, between the ignition and lighting switches. However, the Triumph T20B Bantam Cubs and Super Cubs did not have that option fitted and instead had a black, rubber, blanking grommet in that hole.

Back to top of page


Most Bantam Cubs and Super Cubs had Lucas alternators fitted, but some did leave the factory with Wipac alternators. All the alternator stators have six coils wired in three pairs. Two coils (one pair) are used for charging and normal running. The other four coils (two pairs) provide additional power when the lights are switched on.

Alternator Schematic Diagrams

Alternator schematic diagram as shown on Triumph & Lucas wiring diagrams.

Alternator schematic diagram as shown on BSA & Wipac wiring diagrams.

As shown on Triumph & Lucas
wiring diagrams

As shown on BSA & Wipac
wiring diagrams

Both the above diagrams are electrically identical, the connections are just pictured differently.

The Lucas and Wipac alternators are physically and electrically interchangeable. The only significant difference between them is the colour of the wires as shown below.





Green & White (or Light Green)


2 Coils - charging

Green & Black (or Dark Green)


4 Coils - lighting

Green & Yellow (or Mid Green)


Back to top of page

Wiring Loom & Horn

The wiring loom for both T20B models is very similar to both the Bantam D10 loom and late, standard, T20 Tiger Cub looms with separate ignition and lighting switches.

There are very minor differences in some of the wire lengths between the Bantam and Cub looms, but the only really significant difference seems to be the location of the horn. The BSA Bantams and the majority of T20 Tiger Cubs had the horn mounted under the seat near the battery, but the horn on the T20B Bantam Cubs and Super Cubs hangs down under the front of the petrol tank. It is attached to the frame by a small flat bracket and protrudes below the tank, close to the headstock.

If you can't source the correct T20B loom then either a D10 loom (or even a late T20 loom) can be used with just a minor modification to accomodate the horn. Simply run two wires from inside the headlight shell to the horn under the petrol tank and connect one to the switch and the other to a live (negative) connection on the main loom.

Note that although the Bantam Cub is largely based on the Bantam D7 machine, the Bantam D7 wiring loom is not suitable because the D7 had a single, combined, ignition/lighting switch.

Back to top of page


The Triumph Parts Book lists the original, lead-acid battery as a Lucas MLZ9E. This was a 6 volt, 11Ah battery that was 4.75" wide x 2.5" deep x 5" high. (This is considerably wider than the PUZ5E battery that was fitted to standard, T20 Tiger Cubs.)

If you're looking for a lead-acid replacement then the Yuasa 6N11A-1B or an equivalent should fit. Alternatively, many modern, gel batteries will fit.

Yuasa 6N11A-1B or equivalent battery will fit.

Back to top of page


T20B Bantam Cubs and Super Cubs use standard 6 volt bulbs that are still readily available. See the 'Suppliers' page for sellers.


P36d (312)
6 volt, 30/24 watt
Double Contact
British Pre-Focus
Double Filament
Lucas LLB 312

P36d (312) bulb, 6 volt, 30/24 watt.

Pilot Light

BA9s (641)
6 volt, 3 watt

Miniature Bayonet
Single Filament
Lucas LLB 641
Replaces obsolete LLB 988.

BA9s (641) bulb, 6 volt, 3 watt.

Speedometer Light

BA7s (282)
6 volt, 0.6 watt
Miniature Bayonet
Single Filament
Lucas LLB 282
If appropriate, 1.2W options are available.

BA7s (282) bulb, 6 volt, 1.2 watt.

Stop & Tail Light

BAY15d (352)
6 volt, 18/6 watt
Double Contact, Offset Pins
Medium Globe
Double Filament
Lucas LLB 352
Or LLB 384 21/5W may be eaiser to source.

BAY15d (384) bulb, 6 volt, 21/5 watt.

Back to top of page

Electronic Ignition

The traditional ignition system fitted to the T20B is adequate, but requires regular attention to keep the points clean and the timing correct. Retaining the points does keep the bike original, but increases maintenance and can be unreliable.

There are several options for upgrading to modern electronic ignition. Some systems retain the points and simply use electronics to significantly reduce the electrical current through the original points. This can considerably extend the life of the points. Other systems replace the points completely with an electronic sensor, and some even replace the coil as well.

Suppliers of electronic ignition systems suitable for Tiger Cub engines include Boyer Bransden and others. Always follow manufacturer's instructions and seek advice first.

Advice and information from other Cub owners can be found on the forums and websites listed on our links page.

Back to top of page

12 volt Conversion

Many owners continue to run Bantam Cubs and Super Cubs with the original 6 volt, positive earth electrics. However, some have chosen to upgrade to 12 volt, or even convert to negative earth. Such changes can be straightforward, especially if done as part of a major rebuild. However, they can be expensive and might not be worth it unless you are doing a high mileage or riding competitively.

Unless you have detailed electrical knowledge and experience of auto-electrics, before considering modifciations I strongly recommend seeking advice from a professional, and via forums and websites such as the ones on our links page.

The 'Electrical' section on the 'Suppliers' page lists sellers of both 6 volt and 12 volt components.

Back to top of page

Questions about Tiger Cubs? Why not try some of the independent forums such as:
Tiger Cub & Terrier | The Triumph Forum | Triumph Rat Forum | or others.

Boring legal bit... All information on this website is provided in good faith. The accuracy of the information cannot be guaranteed.
No responsibility can be accepted for any consequence arising from the use of this website. You must independently verify all information..
The owner of this website has no connection with or interest in any organisation mentioned except as a customer or casual subscriber.

Copyright © 2011-2023