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Spark Plugs

Spark Plugs

Spark plugs: the spark that jumps across

To ensure that the high voltage generated by the ignition coil can be discharged with a clean spark in the combustion chamber, spark plugs are screwed into the cylinder (heads). In principle, the spark plugs consist of a central contact, the thread as ground contact and the insulator that separates the two. The insulator in the earliest spark plugs was still made of mica (a mineral), but since the 1910s ceramic has become the material of choice for the insulator. Between the center electrode as the continuator of the central contact and the ground electrode, which is welded to the thread, lies the spark gap. The length of this spark gap, the so-called electrode gap, depends on the installed ignition system, wear of the electrodes changes it. It is therefore necessary to check the electrode gap before installation and periodically in between. The electrode gap can be adjusted by bending the ground electrode without using excessive force.

What kind of spark plugs are there?

Apart from the brand names, they differ from each other technically.

  • Thread diameter and length: the spark plug must be neither too short nor too long. In the first case, the exposed threads in the cylinder head burn . In the second case, the threads of the plug that protrude into the combustion chamber become clogged, and you will have trouble removing them. And if the spark plug is too long, the piston may touch it.
  • Gasket: ring gasket or tapered seat? In any case, observe the torque when tightening.
  • Electrode shape: determines the position of the spark in the combustion chamber and the quality of combustion.
  • Resistance: electronic ignitions usually require spark plugs with a built-in resistor for spark suppression. These can be recognized by the letter R in the manufacturer's designation.
  • Heat value: there are "cold" and "hot" spark plugs. The colder ones for tuned engines that rev higher and generate more heat, the hotter ones for lower performance, lower revs and city cruising. It is important that the spark plug can burn free, otherwise it will become sooty. Soot (= carbon) is electrically conductive and can paralyze a spark plug.

How do I find the right heat value?

The problem should only occur with older engines. The newer ones from Evolution upwards use multi-range spark plugs. If it is clear which spark plug thread fits, you look for the right spark plug according to the W&W webshop. Before a test drive, of course, the carburetor setting is put in order. During the test drive, first bring the engine up to operating temperature. This means driving at least 30 km on a country road before the actual test begins: 10 - 20 km in all operating conditions. Then turn off the engine and unscrew the spark plugs. Now you can judge from the discoloration whether the spark plug fits. If you don't dare to unscrew hot spark plugs: no problem. Let the engine cool down and then screw. But under no circumstances let the engine run at idle for a long time after driving. This falsifies the spark plug reading.

  • With the usual unleaded fuels, the insulator should be light gray to very light brown. If the motorcycle has a regulated catalytic converter, gladly also fawn.
  • If the insulator is darker, the carburetor or injection can be adjusted to a leaner setting or the spark plug can be selected to be a little warmer.
  • If the insulator is anything between light gray and pure white, the spark plug is too hot, the fuel preparation is set too lean or the ignition is set too early.

What is critical in a modern spark plug? What do I use to select my spark plugs?

The easiest thing to do is to follow the engine manufacturer's recommendation. If they don't exist or you've lost the manual, check the wwag.com website and select spark plugs based on your Harley model. Before installation, please still check that the thread diameter and thread lengths fit. Then take a test ride and check the spark discoloration.

How do I change a spark plug?

It is best to do it when the engine is cold (except see previous section on test drive and spark plug reading). Experience has shown that spark plug threads are easiest to loosen when the engine is cold. On air-cooled Harley-Davidson engines, it is relatively easy to get at the spark plugs. To be on the safe side, blow out the area around the spark plug hex with compressed air so that no dirt can fall into the exposed plug hole. Take a good, suitable tool, place it on the spark plug without hitting the insulator, and turn it counterclockwise to break the threads loose. It is up to you to decide whether to continue screwing with your fingers or to unscrew the spark plug with the wrench.

It is best to screw in the new spark plug by hand, as you can see whether it is easy or whether the threads may be damaged. Hand-tighten and then put the good tool back on and tighten. Tip: most socket wrenches for spark plugs have a rubber insert inside that wraps around the insulator and prevents the spark plug from falling to the ground after unscrewing.

All spark plug manufacturers specify torque. Harley-Davidson does too, for Sportster cast iron cylinder heads from 1979 and all aluminum cylinder heads from Shovelhead late 1978:

  • 14 mm thread in cast iron cylinder heads 18-28 ft-lbs = 24-38 Nm.
  • 14 mm thread in aluminum cylinder heads 18-22 ft-lbs = 24-30 Nm
  • 12 mm thread 11-18 ft-lbs = 15-24 Nm
  • 10 mm thread 89-133 in-lbs = 10-15 Nm

I don't have a torque wrench handy. So what?

No problem. In addition to specifying the exact torque, spark plug manufacturers and Harley-Davidson specify the angle when tightening. No matter which thread, first tighten spark plugs by hand and then:

  • a new spark plug with an unused sealing ring is tightened 90° = 1/4 turn,
  • a used spark plug with a squeezed sealing ring is tightened only 45° = 1/8 turn.

This is not as accurate as using a torque wrench, but accurate enough for roadside wrenching.

Have any questions?

Our service team will be glad to help out: Mondays - Thursdays 08:00-17:00 CET, Fridays 08:00-16:00 CET, Phone: +49 / 931 250 61 16, eMail: service@wwag.com