psps101 A Tour of The Harley-Davidson Factory

Proper Spark Plug Setup: 101

It doesn't matter if you drive a Dodge Dart or a Honda Prelude. It doesn't matter if you mow your lawn with a Lawn-Boy or a Snapper. Each of those machines has at least one spark plug. The more the better. Even better, if the plugs are properly set up, the performance goes up.

Harley-Davidson Sportster Spark Plug Setup

Though the steps followed here will benefit any engine, this article is geared towards Harley-Davidson Sportsters as they usually have only two spark plugs and their set up is critical, for the best possible performance.

Make sure your spark plugs are perfect. You have only two of them.
Get the most out of them.

Buy a whole bunch of spark plugs, of the same brand, whatever your brand. I kind of like plugs with the fatter center electrodes. With a multimeter, start comparing the resistance between each plug, across the center electrode. The difference in resistance between any two plugs may surprise you. Find the two closest plugs from the bunch, as the resistance across the center electrode of each plug will vary, sometimes greatly. These two plugs are the ones you will use.

The Nipple up top:
With two baby "vice-grip-type" pliers, hold the base of the nipple area (right on the tiny lip), just above the ceramic, with one of the small vice grips. With the little bit of room left on the nipple, use the other vice grips to tighten the crap out of the nipple onto the center electrode, trying not to distort anything.


You don't wish to tighten the nipple by grasping the 11/16" fastener (ground electrode) because you run the risk of actually spinning the center electrode (positive electrode) within the ceramic, thereby loosening it's tight fit within the ceramic, thereby minimizing the plug's performance. This will be evident if you find actual soot near the nipple of the plug ! (the top of the plug), not to mention the compression loss, however miniscule.

With one of those bladed feeler gauges, not the key fob type, select the proper gap for your bike. My Sporty calls for a gap of .038 to .041. I run .039, but I try different settings. Make sure that the ground electrode is dead center over the center electrode, using one of the vice grips, with the nose of the pliers wrapped in a thick towel. If you marr the ground electrode, you could be creating a hot spot, encouraging engine knock and ping during the hotter temps. Be careful not to disturb the center electrode "at this time".

DREMEL MOTOTOOL: Having checked the gap with your feeler gauge, take out the Dremel Mototool and set it up with one of those thin, red cut-off wheels. Get the tool going at a fast RPM. Carefully, slip the spinning cut-off wheel inside the gap of the new plug, perfectly parallel to the surface of the center elctrode. Perhaps you'll want to hold the spark plug in a vice, while you do this, to free up a hand. Now, with the Dremel Tool spinning really fast, touch the surface of the center electrode and then touch the inside of the ground electrode. Only a touch ! Don't mill the thing ! The more precisely this is done, the better. Why? Because when the two opposing surfaces of the gap are as parallel to each other as possible, less voltage is used for the best possible spark. Check the gap again after this and adjust accordingly. Be careful with the ground (-) electrode ( the "L" shaped thing). It's welded on, and there it should stay, and not end up bouncing around within your cylinder. Bend it gently.

This can be lengthy.
Indexing is where you attempt to install a spark plug and have the opening of the plug's gap pointing between the two valves, to get the best possible ignition going, in the cylinder. To accomplish the proper pointing of the gap, at the proper tightening torque of the spark plug is no easy task, to say the least. However, it must be done and is accomplished with the help of 12mm washers of varying thickness, which can work with, or even replace the standard crush washer usually supplied with a brand new spark plug. I try to use the standard crush washer, if at all possible.

Mark the plug, select a washer, and try it.
Selecting a spot on the opposite side of the spark plug, where the opening of the gap faces, make a visible, verticle mark with a pen, or some type of marker to make a permanent mark in the ceramic of the plug (to be easily seen). You could also try to scratch a verticle mark in the 11/16ths fastener on the ground electrode, if you wish. The object then, is to get the opening of the spark plug gap to face between the two valves, by having the mark you made in the spark plug's ground electrode to face out (away from the bike), when you've finally tightened the spark plug to proper torque. If you find that it didn't quite allow you to properly tighten the plug, with the mark pointing directly away from the left side of the bike, then use another washer. The different washer (shim) will "land" the plug in a different position, hopefully closer to the desired position, by blocking off a differnt amount of threads on the spark plug. Do not try to tighten the plug more than a 1/4 after the plug has contacted the cylinder head, with finger touch, using the collapsable crush washer. And do not attempt to turn the spark plug more than 1/8 of a turn when not using a collapsable crush washer.
OK, our plugs are almost in the best possible condition that this bike will ever see. The factory doesn't go through all this trouble, but you have to. You will notice a difference. How much? Tell ya later.

Pointy, prickly thingies glow first. Remember how I mentioned above that you don't want to marr or scratch the ground electrode, when you center it with the vice grips? It's because any scratch or sharp sliver or corner will become hotter than anything else in the cylinder during operation of the engine, and this could be enough, if the conditions are right, to get your engine knocking and pinging. You certainly don't want this, so prior to installing these wonderful plugs in to your bike you should ensure that there are only smooth, rounded surfaces.

Yes, more work.

Remove the "cut-off wheel" attachment from the Dremel Tool, and replace it with one of those sanding disks. With this setup, and at very high speed ( the higher the MotoTool speed, the finer the finish), try and round the lip of the spark plug, touching only the base of the threaded portion of the ground electrode. Look for other areas, from which you can remove any "corners", which will get glowing red hot and promote detonation while running your engine. Basically, any portion of the spark plug that will be inside the cylinder when installed must be rounded, or ground to at least a 45 degree angle from a right angle. (You may have seen how new spark plugs have a ground electrode that's strategically shaped to elimited right-angled corners). For areas that you can't get to with the small sanding disk, remove the sanding disk and replace it with the straight wire brush attachment which Dremel makes, to get into tight areas.

Cleaning Time
OK, blow the spark plugs clean of any residue (file shavings, etc). Get out the good old Marvel Mystery Oil. The best stuff on the planet. Always was, always will be. I would drink the struff, if I had the stomach for it. A little in the gas tank, a little in the oil tank quiets things, makes them last forever, and in some cases, brings them back from the dead ( but not a good additive for clutches ). Soak a spotless rag with some Marvel Mystery Oil. Tightly wipe the threads by "unscrewing" the plug in the rag. Avoid getting any of the wonderful oil in the gap area. Make sure the threads are moist with the oil.

Head Threads I was at the Chryslers at Carlisle show this past summer and I picked up a couple of round, brass wire brushes from the tool truck guy. The brushes' width is a little wider than the 12mm spark plug. They're the best thing for chasing out the head threads. Using one for each cylinder, dip it in the Marvel Mystery Oil and "screw" it in to the head. Keep in mind that the brushes I got were good quality, and didn't lose their bristles. I certainly would want them to land in the cylinder. You may want to fill a spray-type oil can, available at your hardware store, with some of the Marvel Mystery Oil, to spray around the brush and head thread area to keep things moist. You definitely don't want to screw up the threads. Now, just as you screwed the brush in to clean the threads, now un-screw the brush to remove it. Take your time.

Peformance Gain
You may or may not notice the type of performance gain as you would with changing a major component like cams or gearing, but then, how well do you know your bike.

A simple test could be to test your bike with the old plugs, before you begin this process, by taking note of the idle speed, when warmed. My tach once registered around 150 rpm faster at idle, after this process, compared to the old set up. It'll certainly vary from bike to bike.

Regardless of the rpm difference, I take great delight in preparing my spark plugs that same way, every time. I also run equal length plug wires, (FXR wires), as I made a special bracket to mount my coil between the jugs. This time-consuming process assures me that the bike's plugs are not suspect of robbing any of the bike's performance.

And now, back to Alex's Tour of the Harley-Davidson Factory