Converting from Positive Earth to Negative Earth
Converting early Spridgets from Positive to Negative Earth is a fairly easy process. However some early positve earth cars have had some "modern2 parts fitted that may need attention when the earth is changed. That said let us consider a standard car and sort that.
The first things to change are the battery cables. You will need new cables as the ends (pos and neg battery terminals) are a different diameter and will not swap easily. So you can buy new leads from motor shops or make them yourself. You can buy new battery clamp ends from motor shops that use screw clamps, these could be fitted to the original leads when the old clamps are cut off. So you should now have the positive terminal on the battery connected to the starter solenoid and the negative terminal on the battery connected to the car body/earth.
Now you need to change the wires connected to the coil. The coil terminals are either marked + and - or they could be marked CB and IGN. The white wire with a black tracer (connected from coil to distributor) should originally have been connected to CB on the coil. This needs changing onto the IGN terminal of the coil, whilst the white wire originally connect to the IGN terminal of the coil will now be fitted to the CB terminal. If the ar has a newer coil fitted to it with the + and the - symbols the connect the white wire to the + terminal and the white with black tracer wire connect to the - terminal. The engine will actually run perfectly with the connections eithe way round. But the way the wires are connected to the coil will detemine which direction the spark will jump across the plug gap. Getting the spark to jump the correct way will ensure the long life of he spark plugs.
Now it will be necessary to "flash" the dynamo. This process is required to "polarise" the dynamo to the new polarity, it is a simple process and is performed as follows. OK you need a piece of wire long enough to connect the positive of the battery to the small connection on the dynamo. This should just be connected for about 1 second maximum (it will spark a bit) and that is it, dynamo polarised!
That should be that but just a few points to check in case things have been modified. Check the wipers work OK. They may park differently and may need the arms removing and refixing in the correct park position. Check the heater motor, ensure that it is sucking fom the intake and blowing into the car. Some later heater motors are sensitive to this and it is simply a matter of reversing the 2 connections for the motor to make it work correctly. Check the fuel pump (electrical ones only). These should perform correctly but it is wise to check. Finally if someone has put in the car a radio that is a positive earth radio then it may have a switch on the bottom or the rear to change it to negative earth, if so then change it to negative earth. If it does not then take it out or it will burn out. You could always sell it to a concours buff.