Workshop Basics

Fashion a bespoke set of HT leads for your classic for £20

How to make your own HT leads.

To get a spark to jump a 0.025in (0.65mm) plug gap in fresh air, you only need a few hundreds volts. When the plug’s under pressure in the cylinder and surrounded by swirling fuel/air mixuture, however, you need thousands.

With such high voltages, high tension (HT) leads play an important role. At the heart of the lead is a conductor. Surrounding this is insulation to stop the spark taking an easier route to earth before it reaches the plug. It’s essential that the conductor, the insulation and the connectors at either end are in perfect order. All deteriorate – so HT leads should be viewed as consumable.

Pre-assembled HT lead sets can be bought for many classics. If they’re not available or if your classic’s been modified, you can make your own. It’s a simple and satisfying task. It allows you to pick-and-choose high quality components and route the leads in a superior manner. In fact, even if pre-assembled leads are available, there’s a lot to be said for doing it yourself.

All the energy carried by the HT leads gives off lots of radio frequency interference (RFI) and electromagnetic interference (EMI). EMI will wreak havoc on the electronic systems of later classics – including the ignition itself. Both RFI and EMI are controlled by putting resistors in the HT circuit. These can take the form of in-line resistors, spark plug caps, resistor spark plugs or – most commonly – the leads themselves.

Leads, plug caps and connectors are available from a range of classic suppliers. Good quality parts for a four-cylinder car will cost no more than £20. The only specialist tool you’ll need is a large crimper for the connectors. We’re using the cheapest type – widely sold for less than £5 – which is perfectly adequate for DIY use.