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Massive or fine wire spark plugs
What’s the difference?
By Lydia Patti

During the course of every routine aircraft piston engine maintenance inspection interval, the spark plugs are looked at and depending on their condition, they are cleaned or replaced as needed. When the plugs are replaced, massive wire electrodes are most commonly used, unless specific performance requirements dictate otherwise.

The overwhelming reason for not using fine wire electrodes is typically high cost -almost 2 1/2 times that of their massive wire counterparts. But should higher cost be a deterrent for not using fine wire plugs?

The difference between a massive and fine wire electrode Spark plug

The visible difference between a massive and fine wire plug is obvious - large nickel alloy electrodes vs. thinner platinum or iridium ground electrodes. Internally, the basic method of assembly is identical. The distinct differences are the center electrode material and the shape of the insulator or ceramic.

With a fine wire plug, the two platinum (or iridium) ground electrodes maximize conductivity and minimize wear.

The center electrode is made out of platinum, a precious metal which has a natural resistance to lead corrosion.

The long, thin insulator tip is resistant to oil fouling, fights lead build-up, and runs hot to burn off impurities, thereby maximizing service life. It also allows for more gas scavenging area to improve performance and reduce maintenance.

But while the cost of fine wires is about 2 1/2 times greater, the life is at about three times that of the massive electrodes.

'With all things equal, fine wire electrode spark plugs "burn" better and more evenly than massive wire plugs. For example, if you're experiencing excessive oil or lead fouling, using fine wire plugs will help to prevent premature failure of the plug due to firing tip build-up. This may also help to decrease the need for intensive cleaning of plugs early in engines where fouling is a problem or where operating conditions mandate.

Fuel consumption is another important consideration. Savings of almost 10 percent have been reported by fine wire plug users after switching from massive wire types.

Is plug maintenance different for either type?

Engine manufacturers determine the normal intervals of servicing spark plugs. Much of spark plug servicing is the same with both massive and fine wire types; however, there are special concerns for fine wire electrodes. Regardless, proper servicing can make a big difference to the life and performance of all plugs.

Its very important to examine spark plugs upon removal. Place the plugs into a numbered rack so that you can easily identify the plug's cylinder and position. This will aid in troubleshooting if you find there

is a need to look to other parts of the engine for possible problems and in-plug rotation.

Certain situations appear as visible signs in the spark plug firing end and can include a magneto that is not properly timed, a worn ignition harness, rings, valve guides, or cylinders; and certain operating conditions such as excessive ground idling or improper mixture. In a new or recently overhauled engine, you might see some oil fouling due to the piston rings not yet having been properly seated.

Plug rotation is important for both plug types since it helps to minimize uneven electrode wear. This is accomplished by rotating or moving the bottom Plugs to the top of a cylinder and vice versa.

Changing the polarity of the magneto is another way to enhance electrode wear. When the polarity is reversed, you are changing the direction of the spark. For example, the Path of the spark of a plug firing at a negative polarity will be from the ground electrode to the center electrode, resulting in the wear to be at the ground electrode. (Be certain to check manufacturer's service information.)

Careful handling of plugs is always important. The insulator material can crack if the plug is dropped, and the crack may not be visible to the naked eye. However during normal engine operation, the insulator material will expand due to heat. Any cracks in the insulator may expand and internal sparking (rather than tip firing) may occur.

If you have to clean and gap the plugs at the current service interval and there are no excessive deposits, soak the firing end in a degreaser such as unleaded gasoline or spray clean. Use caution not to contaminate the terminal connector end.

Lead deposits can be removed with a vibrator cleaner or with the use of a hand pick such as a dental pick. This must be done with extreme care to not put pressure on the insulator, which may result in cracking. Loose deposits can then be removed by using a minimum of abrasive blasting. Keep in mind that an abrasive blast lasting as little as five seconds can wear electrodes to the equivalent of 300 hours engine time. Use only glass beads for blasting fine wire plugs, and be sure there is no residual blasting material as it can be conductive.

Proper gapping is critical in performance and life of the plug. Key things to remember are:

Testing, installation, and storage of reconditioned plugs are essentially the same for both massive and fine wire electrode types. Be sure to read manufacturer recommendations for equipment you use and for the engine you are maintaining to insure proper specifications such as torque values for plug installation.

Conclusion

Despite their seemingly high up-front cost fine wire electrode spark plugs offer the user extended wear and performance that should more than make up for their higher price. And since fine wire plugs require the same or less attention than massive electrode plugs, maintenance will remain the same or be even less than what you're currently experiencing.

Lydia Patti is the manager of the Aviation Products business unit at Auburn Engineered Products Inc.