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Magneto spark tester.


A spark tester is used to check the spark intensity after servicing a magneto with an impulse coupler. The magneto is a Fairbanks Morse FMX4B16A for a VW conversion.

Science revolves around laws, theories, and conjectures. Aviation science in particular stresses the importance of various laws and theories in the study of flight. None are more important than Newton’s laws, for which Sir Isaac surely sits at the top of the heap with his “big three,” which are: (1) inertia, (2) force/momentum/time, and (3) for every action there’s an equal and opposite reaction.

Though not likely to be as celebrated as Newton’s Laws, I have developed a few of my own. “Bob’s First Law” is the number of tools you own is proportional to the amount of time you’ve been working on airplanes. The proof? It’s in all the shops and hangars of everyone who’s ever laid a hand on an airplane.

The base was milled from a block of Delrin plastic. But any nonconductive material that can be machined will work, including hardwoods such as maple, birch, or cherry.

Most tools are acquired during the build phase, but new tools are always needed for ongoing maintenance and upgrades. Some tools are used all the time. Others may be used only once a year or even less. But when a tool is needed, there’s no substitute, be it a strap duplicator, a gear puller, a slide hammer, a pin spanner, a spring compressor, or—this month’s project—a magneto spark tester.

In order to function properly, magnetos must be timed both internally and externally. Internal timing is done to maximize the intensity of the spark. External timing sets the spark relative to piston top dead center. Most every pilot is familiar with the buzz box timing tool for setting external timing. The buzz box is also used to set internal timing (see “Internal Timing for Slick Magnetos,” November 2013), but to test internal timing, a magneto spark tester is needed.

As with many tools, there are fancy and basic. Fancy spark testers have a compartment that can be pressurized to test the magneto output under conditions similar to the combustion chamber. This project consists of two insulated electrodes with an adjustable gap. The idea is, if the spark can jump a nominal gap of 1/8 inch or more (more is better), then the magneto will have no problem firing 0.015-inch-gapped plugs in the combustion environment.

A small trim router with a 1/8-inch round-over bit was used to edge-radius the corners before milling. Note the plywood spacer to provide outboard support for the router.

Magneto spark testers like this project are inexpensive to buy, but they’re so simple to make, why buy one? The material costs are minimal. Most of us have an old spark plug lying around. For the rest, all that is needed is a long 1/4-20 threaded bolt, material for the insulating block (any type of machinable plastic or even wood will work), some wire, and an alligator clip.

Milling the gap on the mini mill with a -inch four-flute end mill at 1200 rpm.

Checking the dimensions with a micrometer.

To use the tester, adjust the gap between the electrodes on the tester to about 1/8 inch. Hook a high-tension lead from the magneto to the spark plug. It doesn’t really matter which terminal on the mag you use, but I like to use the number one cylinder for reference. Connect the ground wire from the adjustable electrode to the mag housing (leave the P-lead terminal unconnected/open). Steady the mag in one hand, and use the other to vigorously “flip” the impulse coupler until the plug fires. A four-cylinder magneto should fire within four “clicks” of the impulse coupler. Be aware you might get a mild shock if your hands are near the distributor terminals.

Drilling the respective holes for tapping.

The No. 7-size hole for the -20 tap was set up and drilled in the mini mill. (Right) The 27/64-inch drill bit for the spark plug tap (M12 x 1.25) was too long for the diminutive mini mill, so the hole had to be made using the drill press.

If you get a spark, your mag is good to go, but it’s a good idea to repeat the test to determine the maximum gap the spark will jump, which you can then log for future reference. Mostly, however, the spark is either healthy, or it isn’t. If the mag is new or freshly rebuilt and it can’t produce a spark to jump the initial 1/8-inch gap, the problem is most likely the internal timing needs to be rechecked and reset. If that doesn’t fix it, check (in order) the points, the coil, the magnets, and the distributor.

Fine threads in soft materials can often be power tapped with a hand drill. When in doubt, use a vise! If you don’t have a tap to match the spark plug, the threads can be turned off the plug to make a press fit into an unthreaded hole in the base block.

All in all, magnetos are simple mechanisms that are inherently reliable as long as they are maintained according to the factory specs (usually every 500 hours). Note that most manufacturers warn against running a mag without all the plug wires connected (to the engine or a grounded test fixture). For a few hand-driven rotations to test the spark strength, this is not a problem.

The fixed electrode was a modified NGK B9ES spark plug (about $3). The ground tang was broken off before using the lathe to remove about inch of the threads. This was done to improve the visual exposure of the center electrode.

A 2-inch lag bolt was used to make the adjustable electrode. The only modification was to lathe-turn the tip to a 0.100-inch diameter post to match the spark plug electrode shape. This step can be skipped. It does not improve the spark’s ability to jump the gap. It was done only to make the tool look a little more professional.

When the internal timing is correctly set, a quick flip of the impulse coupler fires the spark across the gap.

Additional reference: “Magnetos for Half VW Engines,” KITPLANES, July 2016.

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Bob Hadley
Bob Hadley is the R&D manager for a California-based consumer products company. He holds a Sport Pilot certificate and a Light-Sport Repairman certificate with inspection authorization for his Jabiru J250-SP.


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