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The gap between the electrodes of the spark plugs: measurement, adjustment

All of everything, today I took apart one engine, and came across candles that were tightly killed, perhaps if the car owner had previously diagnosed the candles for their condition, repairs could have been avoided, and so I will try to tell you about how to understand the state of your candle motor, because it’s a candle that can tell a lot about the state of your car, and so let's begin.

And so what is a spark plug and what are its main parts? The spark plug is, first of all, a spark gap with two contacts, when current flows through these contacts, a high-voltage arc is formed which ignites the fuel mixture in our car.
On average, the life of a spark plug is 30-50 thousand kilometers (even though there are known cases of 100,000 for single candles), the main breakdowns of a candle are breakdowns of the dielectric insulator, as well as significant wear of the electrodes, which leads to a change in the gap and their shape, in subsequently, these malfunctions affect the stable operation of the engine, as well as its starting.

Electrodes of spark plugs.

The service life depends on the material used, usually in our time the following materials are used for this electrode:
- copper with a heat-resistant nickel coating,
- nickel alloy,
- iridium alloy
- with platinum deposition,
- silver coating
- gold plating
- palladium-gold alloys (used for racing cars),

Spark plug electrodes must meet the following requirements:
- high corrosion and erosion resistance,
- heat resistance
- sufficient heat conductivity,
- plasticity.
In addition, the material of the electrodes of the spark plugs must be technologically advanced and inexpensive, in order to be able to launch this design into mass production. As a consequence of this, the following electrode materials of spark plugs are still most common: iron-chromium-titanium, nickel-chromium-iron and nickel-chromium.

Now we will consider all the pros and cons of using this or that material for electrodes of spark plugs.
Copper electrode the spark plug improves heat dissipation, reduces the build-up of the spark plug at idle, and thereby extends the life of the spark plug.

Platinum electrode coating completely similar to copper, but more wear-resistant, which allows to reduce the diameter of the central electrode from 2.5 mm (ordinary candle) to 1.1 mm. In this regard, the beam of the discharge passing through the spark plugs is more collected (spot), which improves the cold start of the engine, increases the service life of the spark plug and, as a result of better ignition, reduces the toxicity of the exhaust gases, since their more complete combustion occurs.

Iridium electrode the spark plug has greater wear resistance than the platinum coating, which also reduces the diameter of the central electrode to 0.7 mm and even to 0.4 mm. At the same time, the electrical conductivity of this electrode is very high, which makes it possible to ignite the mixture at low on-board voltage (20% lower than normal), and also allows to ignite lean fuel-air mixtures. In addition, these spark plugs have a large operational resource.

Spark plug side electrode (electrode "mass").

In addition to the requirements that are put forward to the central electrode, this electrode should be well welded to the candle body, which, as a rule, is made of ordinary steel, and also must be ductile so that the gap between the electrodes can be adjusted. There are candles in which not only the central electrode is covered with platinum, but also the side. This improves the combustion properties and increases the service life. There are candles in which the central electrode is almost completely made of silver (99.9%) and designed for a service life of 50,000 thousand kilometers. The number of side electrodes changed over time: one, two, three, four. The advantage of multi-electrode spark plugs is a greater resource.

Types of candles by design:

candles with one electrode - have one central and side electrode,

candles with several electrodes - have a complex design of the central electrode with several side ones,

Iridium, platinum candles - have a thin metal central and side electrodes capable of self-cleaning.
Iridium and platinum candles will last a long time and will save fuel consumption, but at the same time they are quite expensive. More affordable and budget option are candles with several electrodes, which differ from single-electrode ability to develop greater engine power.

What is the clearance?

Any spark plug of an automobile internal combustion engine has two electrodes in its design: central and side. The first of these is a plus. It is through it that a high-voltage current generated by a coil is supplied through a contact tip, a rod and a conductive sealant (resistor). The side electrode is negative. It is welded to the body of the device and closes to ground through the thread and the skirt of the candle. A spark is nothing more than an arc discharge arising between the electrodes. It slips during the supply of an electrical impulse generated by the ignition coil. Its size and power directly depend on the relative position of the electrodes, i.e., on the distance between them, called the gap.

What is the clearance?

For each type of engine, depending on the type and octane number of fuel consumed, volume, developed power, pressure in the combustion chambers, different manufacturers of candles are provided by auto manufacturers with different characteristics. In other words, simply taking and rearranging them from the Mercedes to the Lada will fail. The gap between the electrodes of the spark plugs is their most important characteristic, which determines the stability of the engine, its power, the number of developed revolutions, fuel consumption, and the durability of the piston group parts.

Reduced clearance

The reduced gap between the electrodes of the spark plugs is characterized by a powerful but short-term spark discharge. Reducing the time leads to the fact that the fuel mixture does not have time to burn completely. As a result, the candle fills with fuel residues, the spark periodically disappears, the engine troit. Naturally, fuel consumption also increases. The amount of toxic substances in the emission increases sharply.

Such a phenomenon at high speeds often leads to the fact that the spark, being too short, does not have time to burst between the incoming electrical pulses, forming a constant arc. As a result, we can get burnt or completely molten electrodes, as well as inter-turn short circuit in the coil. This picture is supplemented by a difficult start-up of the engine and accelerated wear of the piston group parts.

Increased clearance

The increased gap between the electrodes of the spark plugs, on the contrary, leads to the fact that the spark lengthens, but becomes too weak to ignite the combustible mixture. In addition, the probability of breakdown of a coil, high voltage wire or insulator increases. If the distance between the electrodes is too large, electricity, which by its nature seeks the shortest way to equalize the potential difference, is easier to pass through the ceramics than to overcome the increased air gap. As a result, a spark in the cylinder either forms periodically or disappears altogether. The engine is choked with fuel, troit or stall. A common occurrence for the increased clearance is rare loud pops caused by misfire.

What should be the clearance on the spark plugs?

As we have already said, the distance between the electrodes of the candles for each engine is different. Owners of modern cars do not have to think about such a thing as a gap. Everything is simple here. There is an engine - there are certain candles for it with a certain distance between the electrodes. And the manufacturers of foreign cars categorically do not recommend their independent adjustment.

Everything is a bit more complicated with our machines. The gap between the electrodes of the spark plug for domestic cars can be from 0.5 to 1.5 mm. First of all, it depends on the type of engine. For carburetor motors with contact ignition, for example, the gap varies from 1 to 1.3 mm, and for non-contact - 0.7-0.8 mm. For engines with automatic injection, manufacturers recommend the distance between the electrodes in the range of 0.5-0.6 mm.

Why check the clearance? How often do you need to do this?

You ask: "Why check and adjust the gap, if you can buy the recommended candles, put and forget about them before the end of the set term of work?" The fact is that during the operation of the engine, the electrodes burn out. As a result of this, the distance between them increases. Because of this, it is recommended that single-electrode candles be checked at least after 10-15 thousand km, multi-electrode candles - after 20-30 thousand km.

How and how to measure the gap?

A special probe for measuring gaps will help determine the distance between the electrodes. You can buy it at any store specializing in the sale of auto parts. The only thing you should pay attention to when buying it is the manufacturer. You should not purchase a measuring instrument of dubious origin and quality. Deviation by hundredths of a millimeter can negate all your efforts to adjust the gap. There are three types of probes:

The first probe for measuring gaps in appearance resembles an ordinary coin with a rim surrounding it. It has different thicknesses at different positions of the circle. Moreover, a scale is indicated on the “coin” itself, indicating its value. The wire probe has a similar design. Only instead of the rim does the loop perform wire loops of different diameters. The most popular tool for measuring clearance is a Swiss knife meter. Here, instead of blades, steel plates of a certain thickness are used.

How is clearance defined?

First of all, the candle must be cleaned of dirt and carbon deposits, which may be present at its contacts. The measurement method for each type of probe is different. If you have a coin-shaped meter, place its rim between the electrodes of the candle. Slowly rotate it until it connects the contacts. Now look at the coin scale. The value printed on it at the location of the electrodes will be the size of the gap. To increase it, simply bend the side contact with the rim of the meter and check the distance again. To reduce the gap, the electrode must be slightly bent, resting it in some kind of stationary object.

If you have a wire probe, we measure by placing a wire loop between the contacts. Each of them has a specific diameter. The thickness of the loop, which will block the distance between the electrodes, will be the gap. The bending of the side contact is carried out using special curly plates located on the body of the wire probe. With a plate meter, it is easiest to check the clearance. It is enough to pick up a plate that will fit tightly between the electrodes and see its thickness indicated on its surface. Clearance adjustment is also carried out using the meter itself.

What should be the gap between the electrodes of the spark plugs on gas?

All car owners whose cars have been converted under HBO are wondering which candles are best suited for this type of fuel and what should be the distance between the electrodes. Indeed, the process of combustion of a gas-air mixture occurs a little differently than an air-fuel mixture. Firstly, propane has a large octane rating (105-115). Secondly, its combustion temperature is above 30-50 degrees higher than gasoline. Thirdly, a higher compression ratio in the cylinder is required for gas combustion.

In other words, if the engine of your car is designed for the 80th or 92nd gasoline, when working from HBO, ordinary candles will very quickly fail. In addition, the engine of the machine will constantly overheat, and the details of the piston group will wear out quickly. To avoid this, it is necessary to use candles with the minimum heating number provided by the manufacturer of the car. If the car’s engine is configured for gasoline with an octane rating of 95 and higher, conversion to gas will not affect it in any way. As for the clearance, it should be as recommended by the manufacturer.

Design features of some spark plugs

Modern technology allows manufacturers to produce spark plugs with improved power and size characteristics of an electric spark. This can be achieved through the use of precious and rare earth metals. Each of the manufacturers is trying to find the perfect design, in which the spark will be as powerful as possible, and the gap - large. Candles Denso, NGK, Bosch, Champion, despite the high cost, are in great demand because of their reliability and durability. So how do they differ from ordinary ones?

Take Denso candles, for example. Their electrodes are made of iridium, and the central contact has a diameter five times smaller than conventional candles. "What does it give?" - you ask. Firstly, this metal is very resistant to high temperatures. Secondly, if you use iridium spark plugs, the gap between the electrodes allows you to create the largest possible spark. So you will not only increase engine power, but also reduce the amount of harmful impurities in the exhaust. And thirdly, they will serve twice, or even three times longer than usual.

About the ignition

A candle is the last element that comes in contact with the fuel-air mixture. Its main function is to set fire to the fuel at the moment when the piston is at top dead center. Voltage is supplied from high voltage wires. In turn, the interval between the supply of a spark is set by the ignition coil. When the piston is at TDC, a spark runs between the spark electrodes. She is the catalyst for this process.

The mixture ignites, the compression energy is transmitted to the piston. After the removal of gases, the process is repeated again. However, in order for the spark to be able to pierce the electrodes normally and ignite the mixture, the gap should be optimal. During operation, it changes. So, distinguish between small and large gaps. How they affect the operation of the engine, we will describe further.

What is the effect of small clearance?

If the gap between the electrodes is small (less than 0.4 millimeters), this will primarily be manifested in the nature of the engine. In this case, the owner will observe misfire. Why is this happening? The fact is that a powerful spark is required to set fire to a combustible mixture. And since the distance between the electrodes is small, then its strength will be insignificant. As a result, the combustible mixture will ignite once. The motor starts to troit. On carburetor aggregates often "floods" candles. This is not very good, as the motor operates in overload mode.

Large clearance is not always good

Therefore, if the spark is weak with a small gap, it must be maximized. But it is not so. If the gap is 1.3 millimeters or more, the spark will simply not pass between the electrodes. The motor will start to troit again. In addition to this, a candle deposit will appear.

How often to check?

Note that the manufacturers themselves do not oblige to check and adjust the gap (including iridium candles). However, practice shows that over time this parameter automatically changes in one direction or another, since the candle is constantly under voltage and high pressure. What to do in this case? Experienced motorists recommend checking the clearance on spark plugs every 15 thousand kilometers and, if necessary, correcting the readings.

What is the norm?

So, we have certain data. Small clearance is everything below 0.4 millimeters. Large - 1.3 and above. What clearance should be on candles? It all depends on the type of food. On carburetor cars with a distributor ignition system, this parameter is 0.5-0.6 millimeters. But not all units should have such a clearance on the candles. The carburetor with electronic ignition works fine with a clearance of 0.7-0.8 millimeters.

Spark plug clearance for gas

This is a separate topic to consider. The gas mixture is slightly different from gasoline. A candle takes less effort to break a spark. Accordingly, the clearance should be about 0.7 millimeters. However, motorists say that even with 1.1 mm, a car with gas equipment works stably. Therefore, on gas units, the adjustment of the clearance of the candle does not differ from gasoline. There are also so-called gas candles. The thickness of their electrode is slightly less than usual. However, reviews of car owners say that it did not give much effect. Машина одинаково едет на газе как с обычными, так и с газовыми свечами. Это не более чем маркетинговая уловка.

Как выставить?

Если вы заметили, что автомобиль стал вести себя по-другому, появились пропуски зажигания, стоит проверить состояние свечей. They may have the wrong clearance. To adjust the parameter, disconnect the high-voltage wires and unscrew the candle with a special key (as in the photo below).

So, we pass to setup. Using probes, we determine the exact clearance on the spark plugs. Please note that on cars with a contact system (old Zhiguli and Muscovites) this parameter is 0.1 mm lower than on contactless ones. If the gap is less than normal, the electrode is bent with a flat screwdriver. If the value is higher, the element is pressed with the same tool, only on the other side. Some candles may have a lateral arrangement of the electrode (and there are several).

In this case, a gap is set for each element. Take-off should be minimal. Having set the gap of one candle, we twist it into place, put the armored wires and proceed to set up the next one. It is important not to confuse the location of the high voltage wires. Therefore, twist the candles one at a time, and not all at once. Or make special marks. Otherwise, the motor may simply not start. At this stage, the setting of the spark plugs can be considered complete.

Why is sticking so important?

A correctly adjusted candle is not only the key to the coordinated operation of the engine. The clearance parameter also affects fuel consumption. By setting the desired distance between the electrodes, you will reduce it by 5-6 percent. Also, the normal clearance positively affects the life of the candle itself, since it will no longer pierce the insulator.

If you regularly monitor this parameter, the life of the candle can reach 60 thousand kilometers. This applies to both injection and carburetor engines.


So, we found out what the clearance should be on the candles, and how to properly adjust it. As you can see, the procedure is very simple. Even a novice motorist can cope with it. In conclusion, we note that the gaps need to be checked on new candles (the central electrode may have been bent during transportation). That's all. We hope this article helped you solve this issue.