Work­shop Ba­sics

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

Practical Classics (UK) - - CONTENTS - With Theodore J Gil­lam

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.

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