Tips on Wiring the BSA WDM20 Motorcycle

A few words about the wiring harness; you can easily make one yourself, using the wiring schemes in the handbooks. But use as much as possible the same wire coding; that will make life easier when you have electrical problems.

Another VERY good thing to do: run a thick, black cable from front to rear to act as a general earth/mass connection. Connect all lamp mass fittings and the frame to it, thus avoiding bad contacts due to ball/roller bearings, rust, paint etc.


Difficult to do yourself, but there are some people in the UK who can still apply this to your home-made harness. Instead I used that old black isolation tape (not the plastic kind) that seems to have been used on some early production bikes.

Bite the bullet

That marvellous invention of Lucas, the Prince of Darkness; you can find them on virtually all classic British bikes electrics in general, and on our BSA WDM20 in particular. They have - as we will see - an undeserved reputation of giving bad contacts, they will easily corrode and then are very difficult to take apart.

At left, you can see the real, original bullet, that must be soldered on the wire; a later type, that can also be crimped on the cable isolation. Both are 3/16" (4.76 mm)diameter, and made of tinned brass.

Next to this one is a modern bullet using a special crimping tool to grip both the wire and the isolation; often, this one is used to replace an original Lucas bullet. However, the diameter of this bullet is only 4 mm, so if you use the original Lucas female connectors, they will have a very loose fitting. A botch is to slightly squeeze bullet and/or connector, but you'd better use the connectors that go with this type of bullet.

And finally: the spade connectors that are now in general use, in motorcycles, cars and household appliances; if you are a purist, don't use them.


In my opinion, still the best way to connect bullets to the wire! In my BSA WDM20 emergency kit I have a cigarette lighter, a piece of resin-core solder, a few bullets and a tiny pillbox with pure lanolin grease.

I strip the wire for about 1/4", fold the core double to 1/8" , push it in the bullet, hold a small flame near the bullet tip, and at the RIGHT moment push the solder into the bullet tip hole. When the solder has solidified, pull to test, tip it in the lanolin while still warm and presto, you have a corrosion-free bullet connection. I admit, it takes practice to do a good job, but for me it works.


WDM20 58764

Version May 2007