How to change Spark Plugs on a Classic Car

How to change Spark Plugs on a Classic Car
How to change Spark Plugs on a Classic Car

This guide captures ‘bygone’ advice on how to change spark plugs on a Classic Car. It is part of a series I am building that describes what to do on a Classic Car Basic Service.

Useful information for Classic Car and Retro Car enthusiasts, as many of these tasks are no longer required on modern day cars.

Remember folks these are ‘bygone guides’ … useful but the safety and personal protective equipment measures are reflective of bygone awareness. Stay safe.





Why service Spark Plugs on a Classic Car

Cleaning and ‘gapping’ spark plugs every 5000 km or 3000 miles is a relatively simple job. So simple, in fact, that you may be tempted not to bother­ especially if the engine is running well and starts easily.

But thls happy state of affairs cannot continue in­definitely. All the time you are driving, the gap between the electrodes of the spark-plugs is increasing by the small, but significant, amount pf 0.025 mm (one thousandth of an inch) every 1,600 km or 1,000 miles.

Eventually, as higher and higher voltages are required to jump the gap, you will find the car difficult or in cold weather impossible-to start. Engine running may become ragged, and fuel consumption will increase.

This is why overhauling the spark-plugs is an essential part of both the 5,000 km and 10,000 km service.


How do Spark Plugs work on a Classic Car

A spark-plug’s function is to provide a small air gap in the high-tension electrical circuit of a car. In trying to cross this gap, the HT current creates a spark and the spark ignites the fuel vapour in one of the engine’s combustion chambers.

Although highly volatile in air, the vapour needs a ‘fat’ spark to ignite it when compressed. To generate this spark a comparatively wide air gap is necessary. And to bridge the gap a high voltage-normally about 8,000 volts-is needed. This voltage is provided by the car’s coil and relayed to the spark-plug through the distributor.

At an engine speed of 4,000 revolutions per minute (about 110 km/h or 70mph in an average car) each plug in a four-stroke engine gives 2,000 sparks a minute and each single one of these sparks has to occur at precisely the right time-a split instant before the piston reaches top dead centre on the compression stroke.

In the combustion chamber of the engine, the tem­perature of the insulator nose must stay at or below 850 °C, or it will become so hot as to ignite the vapour before the pre-set time. (This condition, pre-ignition, causes acute misfiring, loss of performance, a loud knocking noise and a high degree of engine wear.)

On the other hand, the operating temperature of a plug should remain about 550 °C or carbon deposits will rapidly build up. And if the temperature should fall below 350 °C the plug will start to oil up at low engine speeds.


When to change Spark Plugs on a Classic Car

In spite of the stresses on them, modern spark-plugs have an average life of 12,000-16,000 km (8,000-10,000 miles) in four-stroke engines.

It is possible to keep plugs longer, but a plug that is nearly worn out can result in uneven running and a 20 per cent increase in petrol consumption, and can make the car difficult to start.

Further, an unreliable or neglected plug can be a safety hazard, for example, should it cause the engine to hesitate or misfire while the driver is overtaking. So to be on the safe side it is best to change the plugs for new ones every 16,000 km and to clean and re-set the gap every 4,000/5,000 km.

To get the maximum benefit from a set of plugs, start with a brand new set, a new set of contact-breaker points for the distributor and new plug leads if the old ones are worn. All three items are part of the spark­ producing ‘chain’, one weak link in which can undo the benefits of the other two.

Plug leads should be replaced if they have obvious physical defects like cracks in the insulation or sections where excessive heat has defaced it.


What to consider when buying Spark Plugs for a Classic Car

It is best to shop around when buying plugs. During a garage service you usually pay the suggested retail price for these items, but you can get a substantial discount by buying from a specialist supplier.

If in doubt which type of spark-plug will best suit your car the simplest solution is to stay with the make and grade of plug used as original equipment. (As car handbooks are occasionally out-of-date even when printed, check with a reputable dealer if you have any doubt.)

THE LITTLE GREEN SPARK PLUG COMPANY in SCOTTYS Supplier Library
This is a great place to start for further advice. They can also be found my Supplier Library Here

As the dimensions of a spark-plug can vary quite considerably, it is better not to guess which size to buy; instead check this point too with original workshop manuals.

A frequent cause of plug trouble is town driving, where the engine is idling a great deal and speeds up over short distances only.

In this situation, a layer forms over the plug electrodes and acts as a form of insulator. Once this layer reaches a certain depth it prevents full efficiency at wider throttle openings, resulting in mediocre acceleration and a lowering of maximum speeds.

In this case, sandblasting away the layer (see below) will erode an excessive amount of metal from the plug. One way to ‘clear’ the plugs is gradually to build up speeds to disperse the layer. A short progressive burst of acceleration often has the same effect.

But if these problems happen every time your car is driven in town you are probably using the wrong grade of plug. To decide which plug grade to change to seek the advice of a specialist.


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How to remove the HT leads on a Classic Car

The first stage in servicing spark-plugs is to remove their HT leads. But first you need to number them (see below) since, if you connect the wrong lead to any plug the engine could fire in the wrong order and perform very badly indeed.

To make sure that you return each of the HT leads to the right spark-plug, number each lead with a piece of adhesive tape
To make sure that you return each of the HT leads to the right spark-plug, number each lead with a piece of adhesive tape

For changing spark plugs, it does not matter which order you use in numbering the leads as long as you remember it.

But for some other tasks about the car you will need to know which cylinder is No. 1, which No. 2 and so on. So if the correct order is given in your owner’s manual, use it now to save confusion later.

Always remove the plug caps by pulling the caps themselves not the leads. The caps are not difficult to remove and using the leads to tug them off could damage the leads enough to interfere with the flow of current.

Be very careful about touching the plug leads or pulling off a plug cap when the engine is running. The high voltage running through the leads can give you a nasty jolt and when a plug cap is removed the voltage tries to jump the gap and rises in consequence.

The ‘bite’ from a plug lead is not dangerous in itself because, even though the voltage is high, the current is correspondingly low, so there is no danger of electrocution. But whipping your hand violently away from the source of the shock could send it into the fan or onto the hot exhaust manifold.


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How to remove the Spark Plugs on a Classic Car

Before removing the plugs clean off any dirt that has accumulated around them with a soft brush. To remove the plugs it may be necessary to buy a special plug spanner.

Before you remove the spark-plugs, use a soft brush to clean off any dirt or grease in the area around the spark-plug holes
Before you remove the spark-plugs, use a soft brush to clean off any dirt or grease in the area around the spark-plug holes

Some plugs are more difficult to remove than others, being deeply recessed into the cylinder head while others are set at odd angles. In most cases, though a box spanner long enough to clear the top of the engine-plus a lever to provide extra turning force will do.

Several types of spark-plug spanner are available. Make sure you choose a spanner that can reach all the spark-plugs
Several types of spark-plug spanner are available. Make sure you choose a spanner that can reach all the spark-plugs

Whatever tool you use, make sure it remains dead square to the plug. A spanner tilted to one side could crack the plug’s tough ceramic insulation, making the plug useless.

Rest the spanner squarely over the spark-plug. Do not tilt the spanner, as this could easily crack the plug's ceramic insulation
Rest the spanner squarely over the spark-plug. Do not tilt the spanner, as this could easily crack the plug’s ceramic insulation

Start by sliding the lever through the top of the box spanner so that there are equal lengths on either side for a two-handed grip. Leaving the whole length coming from one side only will give extra leverage, but will also tend to tilt the spanner.

You should not have to apply much force to undo a spark-plug. If a plug is hard to move it has probably been over-tightened
You should not have to apply much force to undo a spark-plug. If a plug is hard to move it has probably been over-tightened

If the plug proves hard to turn, try using your thumbs against the box spanner. Alternatively, try a longer lever, this time with most of its length protruding from one side of the spanner. Use one hand to keep the spanner square while the other applies force to the lever.


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How to clean Spark Plugs on a Classic Car

Once all the plugs are out, examine the plugs carefully – their colour and condition can tell you quite a bit about the condition of your engine.

Ideally the plug ‘points’ (the correct name is electrodes) and insulator nose should be a milk chocolate shade with a thin coating of carbon dust over them. A grey-white colour indicates an over-weak mixture, while a layer of black, fluffy soot is a sure sign of an over-rich mixture.

This can be corrected when adjusting the carburettor settings. Clean the spark-plugs with white spirit and a rag. If this does not get the electrodes clean, you will probably need new plugs.

With the spark-plugs removed, clean their electrodes in white spirit. Alternatively, have the plugs sand-blasted by a garage
With the spark-plugs removed, clean their electrodes in white spirit. Alternatively, have the plugs sand-blasted by a garage

Do not use a wire brush on the electrodes. Bits of the wire will adhere to the ceramic of the plug and eventually the power will ‘track’ down the surface of the plug instead of creating a spark. If you do then make sure you blast them out with compressed air.



How to reset the electrode gap on Spark Plugs

To re-set the gap between the electrodes you will need a feeler gauge. Feeler gauges come in both metric and inch sizes-for example, 0.6 mm or 0.025in. (stamped ’25’ to indicate 25 thousandths of an inch). Before buying, check in your owner’s manual what the correct spark-plug gap should be.

To check the gap, slide the feeler gauge between the electrodes and make sure it is a tight fit. If the gap is too wide, tap the side electrode gently with a screwdriver handle or similar light tool.

The electrode gap must be accurate to give a good spark. Use a feeler gauge to set it to the figure given in your handbook
The electrode gap must be accurate to give a good spark. Use a feeler gauge to set it to the figure given in your handbook

If it is too narrow, use a knife slipped under the side electrode and carefully lever it upwards a fraction.

As an alternative, this small tool from Champion combines a feeler gauge with a small device for bending back electrodes
As an alternative, this small tool from Champion combines a feeler gauge with a small device for bending back electrodes

After 10,000 km (6,000 miles) it is likely that the underneath of the side electrode will have become eroded to a noticeable extent. It is important to have a flat face opposite the central electrode and to achieve this, a points file (a cheap tool available from motor accessory stores) is needed.

If the electrodes need to be filed smooth. bend the side one with a broad.bladed knife or with the Champion tool
If the electrodes need to be filed smooth. bend the side one with a broad.bladed knife or with the Champion tool

The electrode should be bent carefully upwards, using a broad-bladed knife or purpose-made gapping tool, until the file can be used. File the face evenly until it is flat again and bend the electrode back until the clearance is correct.

File the face of the central electrode with a points file (from accessory shops). Do the same to the other electrode
File the face of the central electrode with a points file
(from accessory shops). Do the same to the other electrode

If the plugs are badly pitted and corroded after this mileage, replace them.


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How tight must Spark Plugs be on a Classic Car

One reason for plugs being hard to remove is that they have been put in too tightly in the first place, or that some obstruction may have found its way into the threads of the plug or plug hole.

How top change spark plugs on a Classic Car

Putting a few drops of engine oil on the thread of each plug is a fairly common practice, but do not use graphite instead of oil-it makes the problem worse.

The main thing to remember in replacing plugs is not to over-tighten. Hand screw the new or cleaned plug into place then, if it is a gasket-seat plug give it a quarter turn with the spanner; if it is a taper-seat plug give it a sixteenth turn.

If you have a torque wrench, the maximum setting for tighten­ing gasket-seat plugs is 25lb/ft, and for taper-seat plugs it is 8lb/ft.


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