Replacing spark plugs should be almost as easy as replacing light bulbs, but’ it’s not. Light bulb replacement does not involve the use of a socket wrench or gapping tools. A deep socket that fits a spark plug is not a proper spark plug socket. A spark plug socket has two things that distinguish it from a deep socket.

The first is a foam rubber insert that cushions the plug while it’s in the socket. The second is the hexagonal area around the top of the socket. The hexagonal part allows a wrench to be put on the socket if there is insufficient clearance for a ratchet. Motorcycles generally have plenty of clearance for ratchets and torque wrenches, but the ability to accommodate a wrench on the socket comes in handy for bikes with Fat Bob tanks and some automotive applications (like a Corvette with headers).

For several decades there were two common sizes of spark plug sockets.

These are proper spark plug sockets. They have a cushion part inside them to hold and protect the spark plug unlike a standard deep socket.

Shovelheads and 80″ Big Twin Evolution engines need a 3/4 ” spark plug socket, while Evo Sportsters and Twin Cams need a 5/8″ one. Then something happened: spark plugs that needed an 11/16″ socket appeared on the market, and 11/16″ spark plug sockets are often hard to find. I’ve seen people put a short length of foam pipe wrap (the product that’s often used to insulate hot water pipes) inside an 11/16″ deep socket to make their own. This technique works and is an inexpensive way to improvise a tool for the person who doesn’t change a lot of 11/16″ spark plugs.

Plug Gaps

Spark plug gap is important for proper firing. Some people think spark plugs are pre-gapped at the factory, which is not always true. But even if plugs are pre-gapped, movement during shipping could make the gap a few thousandths of an inch smaller. Gapping a spark plug is not brain surgery. All it takes are the right tools, some practice, and a bit of patience.

Carefully bend the ground electrode to set the gap. When the gap is set correctly, you'II feel a slight drag while passing the wire through the gap.

For starters, pliers and a feeler gauge are not the right tools. My preferred gapper is a good, old wire-loop tool where each wire loop represents a different gap size, and the metal tabs accommodate different ground electrodes. My least favorite is the disc with the ramp around the perimeter. The first disadvantage to the disc style is that it exerts pressure on the center electrode during gapping. The other disadvantage is that one side of the ground electrode may end up slightly farther away from the center electrode. On SplitFire and Screamin’ Eagle plugs, one prong of the ground electrode will be higher than the other.

Following three simple rules will allow you to accurately gap plugs. First, don’t try to bend the center electrode. Second, never bend the ground electrode sideways. Use the gapping tool to slowly move the end of the ground electrode closer to or farther from the end of the center electrode. Third, check the gap by moving the appropriate wire loop through the gap. The gap is correct when you feel a slight amount of friction as the wire moves through the gap.

Changing Plugs

Before removing a spark plug, wait until the engine has cooled off, and then use a shot of compressed air to blow dust and dirt away from the area around the

Use a shot of compressed air to blow dust and dirt away from the spark plug prior to removing it. This keeps destructive dirt from entering the cylinder.

plug. A spark plug socket and a ratchet are my preferred tools for this part of the process. After carefully removing the spark plug wire by pulling on the boot, not the wire, I remove the old plug with a spark plug socket and ratchet by turning them counterclockwise.

Once the old plugs are out, you’re ready to install the properly gapped plugs. Begin by putting a small amount of anti-seize lubricant on the threads of each plug. Modern Harleys have aluminum heads and spark plugs have a steel shell. Repeated heating and cooling of the cylinder heads may set off a chemical reaction between the steel spark plug shell and the aluminum cylinder head. The result is a spark plug that acts like it’s welded in place.

After putting the anti-seize lubricant on the spark plug threads, I begin screwing the spark plug clockwise into the head with my fingers instead of a socket and ratchet. This is a precaution that reduces the likelihood of a cross threaded plug and the expensive damage that could result.

After the new plug is finger tight, I reach for the torque wrench and spark plug socket. The torque spec for spark plugs in my TC 88 is 11-18 ft-lbs., so I split the difference and go for 15 ft-lbs.

This way, if my torque wrench is a little off one way or the other, I’ll still be within the required spec.

Is proper torque crucial for spark plug installation? Simply put, yes! If the plug is too loose, heat transfer to the cylinder head may be reduced, and the plug may overheat. If a spark plug is left really loose, it can actually work its way out of the head due to vibration and combustion pressure. And that is very bad. Insufficiently tightened spark plugs have also been blamed for combustion¬ chamber deposits finding their way into the threads of the spark plug hole.

Gapping tools come in a variety of styles. My favorites have wire loops or stalks for checking gaps and slots (arrows) for adjusting gaps. My least favorite, and the most inaccurate, tool has a ramp around its perimeter.

At the other extreme, making spark plugs too tight causes other problems. If a spark plug is over tightened, it’s likely that it will be more difficult to remove. Over tightening can also crush the gasket on a gasket seat plug. Extreme over tightening can damage the threads in the head and has been blamed for distorting the gap.

If you don’t have a torque wrench, make the new plug finger tight and use a ratchet to slowly tighten the plug to another quarter to half a turn. This method is clearly less precise than using a torque wrench, but it will get you somewhere in the 11-18 ft-lbs. range.

 Sandblasting Plugs

This is as good a time as any to describe the disadvantages of trying to clean spark plugs. Decades ago, every shop had an abrasive spark plug cleaner fastened to a wall or workbench and connected to a source of compressed air. Decades ago, bikers were a frugal group, and we didn’t have multiple parts sources.

Now, within 10 miles of my house there are seven auto parts stores that sell spark plugs that will work fine in a Harley. Within 25 miles of my house there are two well stocked Harley dealers and five aftermarket shops. Along with the availability of new spark plugs, the economics of cleaning used plugs enters into the equation. A new pair of spark plugs for my Twin-Cam costs about $8. Cleaning old plugs is usually not worth the effort.

There are also the physical and electrical aspects of cleaning used plugs. If any of the abrasive grit used to clean the plug remains in the business end of the plug after it’s been reinstalled, that grit may cause engine damage. Millions of sparks during thousands of miles have probably caused some electrode erosion. What started as a nice sharp edge around the center electrode is likely to now be slightly rounded. It takes more voltage to jump a gap to a round surface than a surface with a sharp edge.

Another reason to avoid cleaning and reusing old spark plugs is the gasket on gasket seat plugs. The part that looks like a two-layer washer where the threads end on a spark plug’s shell is actually a gasket. Like many gaskets, it’s meant to be used only once. The higher price or platinum plugs may tempt some people to clean them rather than replace them, but there’s a reason why platinum plugs should not be cleaned and reused. Although platinum withstands high temperatures very well, it’s easily worn away by the abrasive nature of the grit in the spark plug cleaner.

Plug Indexing

An article about spark plugs would be incomplete without mentioning spark plug indexing.  Indexing refers to positioning the ground electrode away from the center of the combustion chamber when the spark plug is installed. The theory behind indexing is that combustion (and power) will be enhanced if the ground electrode isn’t blocking the

Put an index mark on the spark plug's insulator to indicate which way the gap faces.

spark from the air/fuel mixture.

To index a spark plug, make a small mark on the insulator, as shown in the photo. Do  not  use a pencil to make this  mark; the  graphite in  pencil  lead con­ ducts electricity and  could  cause an arc  on  the outside of the insulator. After in­ stalling the spark plug and  torquing it to spec,  look  at  where the  reference mark ended up to determine where the  gap is positioned  in  the  combustion chamber. The goal is to get the gap oriented toward the center of the combustion chamber. You reach this goal by putting an indexing washer between the plug’s gasket and shell. Indexing washers come in

different thicknesses to permit various plug positions. Do not put more than one indexing washer on a plug.

Is the effort and expense involved in indexing worth it?  If you’re going racing and   need   every bit of power you can squeeze out of your engine, indexing is worth a try.  I’ve interviewed riders who experimented with indexing for street ridden motorcycles, and they couldn’t feel the difference. One well-known source of indexing washers put a disclaimer on its website stating that indexing spark plugs could not produce any verifiable improvement in performance.

 Reading Spark Plugs

Reading spark plugs has nothing to do with the letters and numbers on the insulator or shell. It refers to examining the appearance and condition of the ground electrode, center electrode, and nose of spark plugs after they have been removed from an   engine.  Reading a spark plug often provides clues regarding how well an engine is running or why it’s not running very well.

Black powdery deposits on the firing end of a plug generally point to an engine that’s running rich due to a fuel delivery malfunction or excess use of the choke or enrichener.

Black oily deposits indicate excess oil entering the combustion chamber. Powdery red or brown deposits are often caused by fuel system additives. I see this condition in older bikes when owners resort to lead substitutes and octane boosters to cope with modern unleaded fuel.

This index mark allows you to orient the gap (spark) toward the center of the combustion chamber instead of the wall of the cylinder/head.

A white center electrode with a blistered appearance is a sign of an overheated spark plug. Occasionally, I find a plug that is so severely overheated the end of the ground electrode has melted away too. These conditions may be caused by a too lean fuel mixture or too much ignition timing advance.  Sometimes it’s caused by using a spark plug with the wrong heat range, and the cure is as simple as installing spark plugs of the proper heat range.

What should spark plug look like if the engine is running properly? If the center insulator is pale tan, and there isn’t any noticeable electrode erosion after 7,000 to 8,000 miles of use, that’s an indication that the engine is in good condition.

This season, motorcycle rallies are set to kick off from all over the country. Numerous bikers will be partying for a week of festivity all expressing their love for motorcycles. You will encounter a lot of stories and building ideas to talk about with new buddies while you ride the times the time in
while in the rally. Make sure to drive protected and slip on the required protective equipment such as carbon fiber helmets. Good luck and have a fantastic run.