Spark Plug Tricks

Spark Plug Removal:

The combination of aluminum cylinder heads and steel threaded spark plugs is inherently dangerous. The problem is differential rates of expansion and contraction versus temperature for these two metals. The aluminum head cools and therefore shrinks faster than the steel plugs. When these forces are at odds, you get serious clamping forces on the spark plug threads. So even though you didn't over-torque them upon installation, you can easily strip the threads in the soft aluminum head if you remove your plugs carelessly.

Skipping the theory, there are two permissible conditions when it 's OK to remove spark plugs on a GL1000 (or any modern engine for that matter):

Any other situation puts you at risk!

Spark Plug Installation:

Equally dire things can happen when installing plugs. The problem you're trying to avoid here is cross threading. To avoid this disaster, always screw the plugs into the head by hand until you're absolutely sure they're threaded correctly. Once correctly started, I use a small palm ratchet on a short extension to screw them all the way in …just to the point where the gasket just begins to offer resistance. Then I switch to a standard ratchet for final torquing. Here's how set the torque accurately without a torque wrench:

It's an OK idea (but not necessary) to use a very small quantity of anti-seize compound on the plug threads before installation. I do not use any anti-seize myself. I prefer to install them "dry" and clean. If you do, just be sure not to place any anti-seize on the last 2 or 3 threads nearest the tip of the spark plug. Neither Honda nor NGK recommends anti-seize because it affects torque values. If you decide to use anti-seize and plan to use a torque wrench to install your plugs, reduce the target torque value by 15%. Otherwise, use the method described above.

Spark Plug "Indexing":

Here's an old racer's trick guaranteed for a slight fractional increase in horsepower. Grab a handful of new plugs that have been gapped and are ready for installation. With a Sharpie, place a mark on each plug (on the porcelain part) opposite the point where the side electrode attaches to the spark plug tip. Now (through trial and error) experiment with different plugs in each cylinder until you find one which will orient the mark you made on the plug as close as possible in the direction of the intake valve / intake runner when correctly torqued. Note: don't set the final torque now…just estimate the final position based upon where the mark would be if you added another 1/4 turn (for new plugs).

Unless you're very lucky, you may have to go through a dozen plugs to get a good match for each cylinder. Perfection is not necessary. What you're trying to do is orient the plug so that the intake charge filling the cylinder can "see" the spark gap directly and is not shielded in any way by the shadow of the side electrode attachment point. This promotes fast and complete flame propagation that yields maximum power.

This is tedious and time consuming, but I always take the time to do this for my own bikes. Once you've indexed the plugs, you're set for the life of the plugs. Just make sure you always replace the plugs back into the same cylinder whenever you remove them for maintenance. I've seen this trick make a noticeable difference in bikes with slight plug fouling problems and slight idle roughness. Recommended!

Spark Plug "End-Gapping":

Here's another racer's trick worth knowing. This is basically a poor man's "Split-fire." By the way, I don't recommend Split-fire spark plugs. Their design concept has slight merit; however, they seem to have serious manufacturing and quality control issues. Splitfires are the only brand I've ever seen disintegrate into multiple pieces during normal use.

"End Gapping" is a very simple idea and best of all it's free. Just takes a few minutes of your time to execute. What you do is lop off the last little bit of the side electrode so that the end now bisects the midpoint of the center electrode. I use a hacksaw to accomplish this surgery and dress up the electrode after making the cut with an ignition file. Make sure to re-gap after making the cut.

Again the theory here is to present the spark directly to the incoming fuel/air charge without any impediment from the side electrode.

The only down side is your plug life will be reduced because you won't have as much available range to adjust the gap to compensate for wear.

Heat Range Selection:

For 99% of rider's, the recommended spark plug heat range is best. For the GL1000, this is the ubiquitous NGK D8EA plug. Honda and spark plug engineers collaborate to pick the optimal recommended spark plug heat value. Modern spark plugs accommodate a very wide range of operating conditions.

Deviation from the recommended heat value is not advised unless:

Deviation from the recommended heat value can result in catastrophic spark plug failure which can ruin an engine. You're never at risk for this with the factory recommended spark plug.

Spark Plug Selection:

The only real spark plug "improvement" worth considering for most riders is to upgrade to premium plugs like platinum or iridium (in the recommended heat range). These have a greater ability to resist fouling, have longer service life with less gap maintenance and have the performance advantage of "end-gaps."

NGK DR8EIX Iridium Plugs

Important Resistance Issues:

There should always be some resistance in the high-voltage, secondary side of the coil circuits (spark plug wire side): either a resistor plug, a resistor spark plug cap, or resistor spark plug wires.

You always need at least one of these elements. Otherwise, the coil discharge will be too rapid, resulting in shorter, less useful spark. This will also risk overheating the coils and reduce their life expectancy.

Using any two of these forms of resistance is probably OK if you are overly concerned about radio static and interference, but you should avoid using all three forms of resistance at once. That will hurt performance...especially at higher rpms.

For example: the NGK DR8EIX iridium plug recommended for the GL1000 is indeed a resistor plug (hence the "R" in the code). If you run this plug, you should pair it with aftermarket ignition wires / caps of the nonresistance type. Or, if you stick with the OEM wires and NGK caps, you should probably eliminate the removable resistor in the cap.

To eliminate the resistor from NGK / OEM style spark plug caps here's what I do. Get a handful of suitably-sized brass machine screws that have the same OD as the resistor - about 5mm (SAE-sized #10-24 or #10-32 brass machine screws also have the right diameter and are available at any hardware store). Then, cut sections of these brass machine screws to the same length as the resistor you are removing (about 18mm). This makes an excellent resistor substitute and you don't have to worry about the interaction of dissimilar metals.

Spark Plug Gap Deviation:

Finally, if you've upgraded to an electronic ignition (like Dyna-S) AND have aftermarket 3 ohm coils like Accel or Dyna - the spark plug gap should be increased to .032" - .035" to take full advantage of the ignition improvements you've invested in.

Special Note to Restorers:

The Honda-specified spark plug for the GL1000 is the NGK D8EA plug. However, when the first GL1000s were introduced, the specified plug was NGK D8ES-L. The "S" designation indicates that it was a "special" design and the “-L” designation means the heat range is a half step "cooler" than the norm for D8EA plugs.

Honda recommended the D8ES-L for all of the 750/4’s from 1969 through 1977, the CB350F, CB400F and the GL1000 through 1977 were also included. Other Japanese manufacturers used them as well.

If you are doing a concours-level restoration, you may be interested in this. Pete Boody has managed to secure a stash of these long-obsolete plugs. They are the absolute "correct" spark plug for early GL1000s (and many other vintage Hondas beyond the ones listed above).

Pete has priced these unique plugs at a very reasonable price.

NGK D8ES-L Plugs - for highest level restorations

Contact Pete directly to buy a set of these plugs:

Pete Boody

Pistol Pete's Services

Oliver Springs, TN

email: petesgl@comcast.net

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