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MAGNETO MYSTERIES EXPOSED There is much discussion among pilots and mechanics about the virtues of one magneto installation or manufacturer over the next. Each product has its advantages and detractions. Generally speaking: The biggest advantage to magneto ignition is that it is simple and reliable. The biggest problem with magneto ignition is that it is simple and reliable. Most often, support of the product and its weight (for aircraft) are the critical determining factors for most owners. Unison/Slick has worked for years to capture leading spots in aviation and does very well on both of these areas (weight and performance). However, other manufacturers are still well supported, if not slightly more obscure. The Bendix units, despite AD’s and detractors ($$) remain popular and very viable units. Many/most of the light aircraft magneto installations have as their genesis, ready availability as magnetos used on early tractors. Aviation adopted the products as the early engine manufacturers sought reliable vendors capable of mass production. (the FAA has of course effectively outlawed this practice) As noted in the opening paragraph, one of the best, and worst, features of magneto ignition is that it is reliable and that it demands very little maintenance. Because these units do not demand maintenance to work reliably, they are ignored, and few owners do any maintenance to them until some failure such as hard starting appears as a problem which interrupts their flying interest. Virtually all of the vintage magneto brands do require some basic maintenance, and without it, they will eventually ruin themselves and cease to function. Failures in this mode are more expensive in a lump sum, but about equal in price to routine maintenance that should be completed on the units. Many of the repetitive AD notes and service requirements by manufacturers have resulted directly from this lack of maintenance, and the manufacturer’s desire to require owners and mechanics to properly maintain those ignition products which are in service. (it helps keep the lawyers away.) Lubrication Maintenance frequently NOT done is a lubrication service of the magneto’s rotating armature bearings. These are usually NOT sealed, and are usually packed in grease within the housing. Under warm operating conditions this grease bakes, hardens, and eventually fails to lubricate the bearing into which it was packed. After several hundred (thousand?) hours, the bearing fails, allowing the armature to contact other internal parts and damage the magneto case, primary shoe and coil which provides the spark. At this point repair may be impossible- and no prior notice of impending failure may have been noted by the operator. Early Scintilla/Bendix and other mags had lubrication ports for oil can oiling of these bearings at each inspection. For reasons of competition and to improve mass production, better greases were used and routine lubrication was relegated to a service note in the manuals. That note is often ignored by mechanics in the field. Construction The advent of plastics and better bakelite products in the 40’s gave way to sealed coil units that after many thousands of hours seem to fail. Moisture and internal corrosion from condensation appears to be the culprit. Disuse can accelerate this problem in moist environments. E gap or efficiency gap (points setting) Magnetos work best and deliver the hottest spark when the energy inductance to the coil caused by the rotating magnetic shaft is interrupted at the strongest point. This interruption is done by adjusting the points against a cam to cause them to open at just the right moment. Points that open sooner (smaller gap setting) or later (larger gap setting), are less efficient, and produce less of a spark than those tuned to the most efficient specified setting. This point gap varies by manufacturer but is generally not less than .012" and rarely greater than .025" (see maintenance manual. Points fail when the condenser fails, by causing the transfer of metal on the face of the points that changes the E gap. Also the fiber rubbing block on the points can wear from corrosion on the cam, and lack of cam lubrication. Eventually the points do not open, and spark ceases from the magneto. Over a long service life and from constant heat and cooling of the magneto, the rotor shaft may loose its magnetism. Or the coil may begin to break down. These failures are often noted in magnetos where they start well "cold" but the engine becomes hard starting once it warms to operating temperature, or after it becomes heat soaked. The lesser magnetism or failed coil results in a lesser spark, and harder starting. Pretty simple stuff. Hard starting usually is the result of one or more of the above problems that have not been addressed in the prior 500 to 1000 hours of operation (20 years). A partial list of popular magnetos used on light vintage aircraft follows. Case Magneto These are becoming less common. Case manufactures(ed) tractors, and got in the magneto business for support of their own vehicles. Most of these units are un-shielded units which will create radio noise. I believe that Case has become part of the International Harvester companies. Very light weight and good for non-radio airplanes and restorations seeking extreme originality. Eisemann magneto Eisemann magnetos came in both an unshielded version (bakelite cap) and a shielded version which has a metal case. Parts support continues to be available from New Old stock and from American Bosch Bendix magneto (Scintilla) Bendix was the big hitter in aviation magnetos during the golden years and later. Its reputation for reliability (and its performance), have resulted in many recurring AD’s to force owners and mechanics to maintain the units which seem to run for years with trouble free service. These are slightly heavier than many other mags for similar application. The construction makes for expensive repairs and reliability, which is one of the primary reasons for the Slick obsolescent program- to remove them from service. Slick magneto (Unison industries) Slick once produced a "disposable magneto" that was not repairable and was to be used for a thousand hours and then thrown away. Most of these are now removed from service and standard, serviceable, and more reliable units are replacing them. Unison is in the leading edge of aircraft ignition now and delivers great products, reasonable prices, and good service. Part of this is their interst in a dominant presence in aviation ignition. Slick/Unison has developed a very successful buy-back obsolescence program undertaken with rebates paid to remove other manufacturers from service. Slick mags are lighter than most other brands, and seem to now have a good service history with few maintenance headaches. Often they can be bought with a new harness, for the price of overhauling a Bendix unit. (be warned- you may need to buy new gears or adapters also) Savage Magneto Service I have had many dealings with Savage who does top notch aviation electrical restorations which work like original, new equipment. The price is not cheap, but the work is right! Standard Magneto Odds N Ends (not necessarily aviation suppliers) http://www.taylorcraft.org/Eismann AD 69-09-01.HTM http://www.skytronicsinc.com/avco_lycoming_engines.htm http://www.magnetoparts.com/index.html And for those who are familiar with Lucas, prince of darkness- a sporting option: http://www.magneto.co.uk/Frame.asp Doug Combs |
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