How-To Replace Spark Plugs

Once upon a time everyone who rode a motorcycle was intimately familiar with the mechanics of removing and replacing spark plugs, especially if they rode a two-stroke. Everyone carried a spare set of spark plugs, every motorcycle came with a spark plug wrench, and everyone knew how to 'clean the spark plug', as one old-timer of my acquaintance used to call it. Fortunately that particular Saturday morning ritual is long gone and little missed part of riding a motorcycle, but that doesn't mean you shouldn't have at least a passing familiarity with the subject of replacing or cleaning your spark plugs.

First you start by locating the spark plugs. I know that sounds silly, but these days the spark plugs could be buried deep between the fins, stuck under the fuel tank, or hidden under multiple layers of plastic. The only sure thing is they're going to be located somewhere in the cylinder head.

Remove the coil's high-tension wire. Grasp the lead at the cap, impart a slight twist, or wiggle the cap to free it from the spark plug, and it should come straight up. In some instances the coils attach directly to plug; those might have to be unbolted before they can be removed.

To prevent dirt from entering the engine use a blast of compressed air to blow debris away from the spark plug. If you don't have a compressor, use a canned air duster or even a soda straw.
Apply the appropriate socket to the spark plug. IT takes a trained eye to recognize the size, and, unfortunately, your owner's manual probably won't list the plug's hex size. However it will tell what kind of spark plug your motorcycle uses. Armed with that information you can easily determine the size of the hex, either by looking up the spark plug manufacturer's online information or by asking that friendly guy at the motorcycle shop in your mooban (neighborhood)..

Remove the spark plug by turning the wrench counterclockwise. I've never heard of a left-handed spark-plug thread, so no worries here: lefty-loosey and rightty-tighty.

Compare the old spark plug to the new one; excepting wear, they should be identical. On occasion I've opened the box and found it contained the wrong plug, so confirm it.

Before installing the new spark plug, check the gap, preferably using a wire-type gauge. I know the box says pre-gapped; I know guys put plugs into engines without checking the gap every single day. I repeat, check the gap!

The next step is controversial, I always apply a dab of anti-seize compound to spark plugs before installing them. I've had plugs stuck in the heads and ruin the threads, so it's become a habit with me. Some guys resist the idea and claim it's not required. I use anti-seize and haven't stripped a plug in 10 years, but you're free to follow my advice...

Make sure the spark-plug seat is squeaky clean; any dirt will prevent the plug from seating properly.

Thread the spark-plug in by hand until they seat. The plug should go in smoothly; if it doesn't, stop and take a good look before proceeding. Wonky spark plug threads are easy to clean up, but once that plughole is stripped or cross threaded, it's much more difficult to fix it.

Once the spark plug is seated, it still needs to be properly tightened. All spark-plugs incorporate a crush washer, which acts like a gasket to seal the plug to the head. If the plug isn't properly tightened, that washer can't do its job, so there's a very real chance the plug won't transfer heat from the combustion chamber as efficiently as it should, which leads to elevated head temperature, detonation, and, if left to fester, an expensive repair bill. Obviously that gasket has to be tight, and to that end all manufacturers provide a torque spec for their spark plugs. Just as obviously that spec varies on the plug size.

If you have a torque wrench, just remember to reduce the torque by about 17 percent to accommodate the anti-seize. If you don't have a torque wrench, no problem. Every spark-plug box has instructions on wrench tightening the plug. Typically you'll run the plug down until the washer seats and then give the spark plug an additional half to three-quarters turn.

Before installing the spark-plug cap give it a dab of dielectric grease where it fits over the spark-plug. The grease will help seal out the elements and make it easier to remove next time.
  • Currently 3.50/5
Rating: 3.50/5 (2 votes cast)

Share It!