Maintenance: Spark plug gapping and replacement

January 8, 2014

In the second installment of spark plug maintenance, I covered removing the plugs and how to use their condition to learn a little about the condition of the engine and how it’s been operated. Now it’s time to clean, gap, and reinstall the spark plugs. 

Jeff SimonThe easiest way to clean aircraft spark plugs is to use a special aircraft sparkplug cleaner. Your mechanic probably has one you can use, but if he or she doesn’t, you can purchase an inexpensive aircraft plug cleaner from any aircraft supply house.  It’s a special, air-powered cleaner that lightly sandblasts the plug electrode and insulator. If you use a plug cleaner that’s not specifically design for aircraft plugs, it may be too abrasive and you can ruin your plugs. Be especially sure not to use glass-bead media. It can get embedded into the insulator gap and eventually cause arcing in the plug. This is especially true for fine-wire plugs, which should never be used in any abrasive cleaner. For massive plugs with heavy lead deposits, you can use a vibrating cleaner or a dental pick to carefully remove the lead. For fine-wires, Hoppe’s No. 9 cleaner and a dental pick should be all you need to remove the lead without damaging the insulator or the iridium electrode. Finally, the outside of the plug needs to be inspected and cleaned on the plug threads where the plug goes into the engine as well as the lead threads at the top, making sure along the way that the barrel well is clean and free of any moisture or debris.

Once the plugs have been cleaned, they need to be carefully inspected.  If any of the ceramic is either chipped or cracked, the plug needs to be replaced. You should also replace the plug if the electrodes are badly worn or the shielding or barrel is corroded or damaged.

If you have massive spark plugs, it’s now time to set the sparkplug gap (do not attempt to adjust the gap on fine-wire plugs yourself). For gapping massive plugs, you need to use a feeler gauge and an electrode adjustment tool.  These are also very inexpensive and you can get them from the same place as the plug cleaner. Start by checking the plug gap using the feeler gauge. Chances are that the sparkplug electrode gap has grown in service due to wearing of the electrodes. If this is the case, place the plug in the gap-setting tool and gently move the electrodes in slightly. Check the gap with the feeler gauge for the correct setting. If the gap needs to be smaller, repeat the procedure. Never adjust the electrodes while the feeler gauge is in place. This can damage the plug. Most massive-electrode plugs use a gap setting somewhere between .016 and .021, but check your aircraft maintenance manual for the specific gap setting for your aircraft.

Before rotating and installing the plugs, they need to be tested. Check the resistance between the center electrode and the conductor in the spark plug well as described in the first-part of this series. Any spark plug showing a resistance of greater than 5K Ohms should be replaced. Also, if you have access to your mechanic’s “bomb tester,” check each plug for spark intensity.

Now that all of the plugs are clean, gapped, and tested, it’s time to install new gaskets and rotate them in the tray. Rotating the plugs in the tray is very easy. Simply swap plugs for cylinders one and four and from bottom to top at the same time.  Do the same thing for cylinders two and three. There is a similar pattern for six-cylinder engines; check the plug manual for the correct pattern. This rotation pattern does two things. First, it rotates bottom and top plugs. This helps even out problems caused by gravity such as lead or oil buildup. Second, it swaps plugs in polarity. This evens out the wear on the plug electrodes. Finally, put a light coating of aircraft sparkplug anti-seize on the threads. It is best to avoid using anti-seize on the first two threads to make sure you don’t get any on the electrode.

When reinstalling the plugs, put them in by hand until they are finger tight and then use a torque wrench for final tightening. Using the torque specified by the engine manufacturer is very important, as was recently reinforced by Vince Bechtel, Tempest’s director of sales and marketing: “We have seen that insufficient torque allows combustion gases to escape out of the cylinder. This leads to potential thread damage and can quickly erode the electrodes because the spark plug loses its ability to transfer heat.”

When reattaching the ignition wires, remember to use two wrenches when tightening the nut to make sure you don’t twist the wire. Also, be very careful not to over-tighten the nut. Check the maintenance manual for the proper torque.

Finally, perform a thorough ground check to make sure that the engine is performing properly before taking the airplane back into the air.

Social FlightJeff Simon is an A&P mechanic, pilot, and aircraft owner. He has spent the last 14 years promoting owner-assisted aircraft maintenance as a columnist for several major aviation publications and through his how-to DVD series: The Educated Owner. Simon is also the creator of SocialFlight, the free mobile app and website that maps more than 5,000 aviation events. Free apps available for iPhone, iPad, and Android, and on the Web at www.SocialFlight.com.