Fuel injection has become the norm on modern motorcycles, and "plug cover" is no longer a problem. However, the electrodes are worn out by the continuous electric spark that triggers combustion, so it is essential to replace them periodically. It is often thought that the work is easy, but it is also a delicate part that can lead to problems if it is not handled properly, so it is necessary to be careful.
The importance of spark plugs is increasing along with the high performance of engines.
In the case of air-cooled engines, the base of the plug is exposed so grit and dust can easily accumulate, so before loosening the plug, use an air blower or a long bristle brush to remove any dirt. In the case of water-cooled 4-stroke engines with deep plug holes, they should be blocked by the plug cap, but the cap may have floated or shrunk and water may have entered, so look into the back and check before loosening.
Plug wrenches are sometimes included in the on-board tools, but when purchasing a general-purpose plug wrench, select the correct size for your motorcycle's plug. For NGK plugs, the hexagonal diagonal of the C plug and DCP plug is 16 mm, the D plug is 18 mm, and the B plug is 20.8 mm. If you have plug wrenches of these three sizes, you can install and remove any plug, but be careful because the thicker the wall thickness of the socket part and the larger the outer diameter, the more it may interfere with the plughole with a small hole diameter.
While the trend toward hybrids and EVs is accelerating in automobiles, gasoline engines are still the overwhelming majority in the motorcycle world. Therefore, spark plugs, which trigger explosive combustion in the air mixture, are also essential.
Along with the evolution of fuel supply from carburetor to injection, ignition system has also developed from contact point to transistor to digitalization. Nowadays, ignition and fuel are one and the same, and are operated by precise control to meet even stricter emission regulations.
Spark plugs, which are exposed to harsh conditions in order to ensure reliable ignition of dilute fuel in the combustion chamber where intake and exhaust gases are mixed together like a storm, basically need to be replaced according to the mileage, although there are products that use precious metals such as platinum and iridium to improve durability. A special nickel alloy is used for the electrodes of ordinary spark plugs, and electric sparks from high voltages of 20,000 to 30,000 V are generated from the ends of the cylindrical central electrode. As the driving distance increases, the ends of the electrodes wear out and become rounded, and in some cases the electrodes become shorter.
Then, the gap between the outer electrodes sticking out from the side widens, and the state of loss of ignition which makes it difficult for sparks to fly easily occurs, and the engine performance may decrease. If the performance of the ignition system has been degraded by the aging of the four-cylinder engine of the out-of-print motorcycle, there may be a vibration that is bothering when idling, or there may be a symptom that the engine does not blow up well when accelerating. It is quite common to find that the cause of the problem is a worn spark plug after all, after switching parts to see if it is the carburetor, ignition coil, or plug cord.
The plug manufacturer's website states that the standard replacement distance for normal plugs is 3,000 to 5,000 km. It is surprising that this is the same span as oil changes, but at least in the opinion of the plug manufacturer, it is not a part that should be used for 10,000 to 20,000 km. On the other hand, when it comes to motorcycle-specific plugs, the range is 8,000 to 10,000 km (in the case of NGK's MotoDX plugs), so if your motorcycle has the right size and you want to install motorcycle-specific plugs that include life, you may want to make a choice.
In any case, there is no doubt that spark plugs are a part that must be replaced based on mileage.
- Point 1: Spark plugs are one of the most important parts of gasoline engines, even though fuel injection and digital control of ignition systems have become popular.
- Point 2: Spark plugs should be replaced according to the mileage, but the plug manufacturer's recommended replacement distance is surprisingly short.
Slim plug wrenches are required for water-cooled engines with narrow and deep plug holes
Rusting of the hexagonal part at the bottom of the insulator is a common problem with air-cooled engine motorcycles stored outdoors. Even if it is rusted, sparks will still fire, but the mileage must be quite high for it to be this rusty, so it is best to replace it with a new one when you notice it.
The gap between the center electrode and the outer electrode (plug gap) is an important factor in the formation of ignition sparks, and in the case of NGK plugs, there is a part in the part number that indicates the gap, and if there is no indication, the standard gap is 0.7 to 0.8 mm. The narrower the gap, the easier it is for sparks to fly even if the voltage generated by the ignition coil is low, but the harder it is to grow into a large flame, and the wider the gap, the bigger the sparks fire, but the more power the ignition coil needs. Even if you buy a new plug in a box, it is a good idea to check it just in case there are any variations before installing it.
In the days when only carburetors and kick-starting were used, it was not uncommon to use too much choke when starting the engine, causing the plug to be covered and making it impossible to start. Every time this happened, the plug was removed and wiped with a rag, cleaned with parts cleaner, or even more roughly, roasted with a lighter. In other words, in the Showa era, there were much more opportunities to take the plugs on and off than now, and if the engine was not in good condition even a little bit, it would be difficult to start. Unplug it and take a look.This was the norm for the riders.
However, nowadays, injection and cell motors ensure that the engine starts reliably at all times, and there is no chance of the plug being covered. Also, due to the increase in the number of water-cooled engines and cowl-equipped vehicles, there are more and more models that do not allow for easy removal of the plug. Because of these factors, it seems that there are many riders these days who have never done any spark plug maintenance. However, as mentioned earlier, the plug is a consumable item, so it must be replaced, either by asking a motorcycle shop or accessory shop, or by doing the work yourself.
A plug wrench is essential for installing and removing spark plugs. It may be included in the on-board tools, but can also be purchased as a universal tool. The plugs installed on different engines vary, and the thread diameter and the opposite side of the hexagonal part where the wrench fits are different depending on the plug, so you need to get a wrench that fits your motorcycle. If you are using an air-cooled engine that has a lot of space around the plug, any wrench can be used. However, if you are using a water-cooled 4-stroke engine that has a compactly designed plug hole that is deep, a wrench with a thicker socket may not be able to reach the plug due to interference.
Also, when removing the plug from a water-cooled 4-stroke engine, make sure there is no water or oil in the plughole, and if there is, blow it out with an air blower or wipe it off with a rag before loosening the plug. The plug hole should be sealed by the plug cap, but the cap may shrink over time and allow rainwater to enter, or the headcover gasket may deteriorate and oil around the head may leak out and accumulate.
If there is a build-up of grit or dust around the plug, it must be removed by air blowing or scrubbing with a brush, whether water or air-cooled! If you neglect to do this and loosen the plug, water will flow into the combustion chamber and gravel will clog the plug hole, which will require extra work to remove. It is important to understand that doing a little work before the work is done will ultimately save time.
- Point 1: The plug wrench that fits the plughole of the cylinder head is indispensable for installing and removing the spark plug.
- Point 2: When removing the plug from a water-cooled 4-stroke engine, remove water, oil, and dust from the plug hole.
If you use a plug wrench from the beginning of the installation, it may cause an accident.
If the plugs are changed every 3,000 to 5,000 km as recommended by the plug manufacturer, there is no chance of the threads sticking, but there must be many riders who are concerned about galling and want to apply heat-resistant grease. However, the plug manufacturer (NGK) does not recommend this because they are concerned about over-torquing, so when applying the grease, it should be tightened carefully.
The structure of the rubber tube dedicated for connecting and disconnecting the plug that I once got somewhere is simple, so you can substitute the rubber tube that you can buy at Home Depot.
The rubber tube is not as rigid as a plug wrench, but if the plug is at an angle, it will stop turning as soon as it does, so you can notice it before the threads are gnawed. When using a plug wrench, it is important to limit it to using the extension bar and not to turn it with the handle to avoid problems.
The area that requires the utmost care when installing a new plug is the few threads where the plug threads engage with the plug hole threads in the cylinder head. If gravel or debris is caught here, or if the plug goes in at an angle, the female threads on the head side will be damaged due to the difference in hardness of the materials used for the plug and cylinder head.
In order to avoid such problems, it is important to blow and clean the air before removing the plug, and also to install the plug by pinching and turning it with your fingers, without using any tools for the first few strokes. This way, even if the plug is slanted at the entrance of the plughole, it is easy to notice the problem because it will not turn with your fingers when you lightly bite it. On the other hand, it is the worst to start turning the plug wrench with a long handle.
The torque applied to the plug is increased by the use of the spindle, and even if it is a little sluggish near the entrance of the plughole, it may somehow be able to enter. By the time you realize that something is wrong, it's already too late. This kind of mistake tends to be made not only by maintenance beginners but also by veterans with a lot of experience, so it is important to keep your eyes open and stick to the basics.
However, there are many engines where the plug hole is too deep or the gasoline tank is close to the cylinder head, and it is not possible to turn the engine by pinching it with your fingers. In such a case, insert a suitable rubber hose into the plug and carry it to the plug hole. After the first few threads are firmly engaged, remove the hose and replace it with a plug wrench for retightening.Flexible spark plug fitters sold by Daytona.The body of the plug can be bent freely, so the plug can be attached securely even in a narrow space.
In some cases, instead of using fingers, rubber hose, or plug fitter, the plug is carried to the plughole with the plug set in the plug wrench, and the shaft of the extension bar is turned to engage the threads. However, if you start turning the plug with the wrench tilted against the plug, the head of the insulator may break, so be careful. Since the insulator of the spark plug is made of ceramic, it can be broken by a slight shock if it hits the plug in the wrong place, just like a rice bowl.
Once you have successfully crossed the few mountains at the entrance, you can then use the plug socket to tighten the plug to the proper torque. The tightening torque depends on the thread diameter of the plug, and a thicker plug should be tightened to a greater torque than a thinner plug. It should be noted here that grease should be applied to the threads. In order to prevent sticking due to the heat of the engine, it is nothing special to apply heat-resistant copper grease to the threads before installation. However, according to the plug manufacturer, the use of lubricants or anti-seizing agents is not recommended because the friction of the threaded part is reduced by applying grease, and there is a risk of over-torque even when tightened to the standard tightening torque.
If you still want to avoid sticking plugs, you can use the rotation angle to control the tightening. The spark plug gasket is in contact with the cylinder head and becomes airtight while being crushed by the application of tightening torque. Therefore, when the plug is reused, it must be tightened in a collapsed state. The angle of tightening depends on the thread diameter of the plug, but in the case of DPR8EA (thread diameter φ12mm) installed on Kawasaki GPZ400, when the gasket is new, it should be tightened by 90° after touching the cylinder head, and when it is reused, it should be tightened by 45° to get the proper torque. When the gasket is new, it should be tightened about 90° after it touches the cylinder head.
Replacing spark plugs may seem like a simple task, but it is one that requires a surprising amount of care and attention. However, replacing a worn out plug with a new one will definitely improve (or rather, restore) the starting and acceleration performance, so if you have never tried it before, why not give it a try?
- Point 1: When installing a new plug, turn it with your finger until it is firmly attached to the threads.
- Point 2: Even when you can't turn with your fingers, use a rubber tube or special tool, and don't suddenly turn with the handle when using a plug wrench.
