How To: Magneto
How to Rebuild a Magneto
The magneto is an almost magical instrument. It will generate enough electrical power to light a spark plug, but it doesn’t require a battery or a coil. Simply spinning the magneto armature inside the magnetic casing, generates an electrical current. Since this is to be a short article, I’ll skip the Physics lecture. Seven or eight images are below with details. Suffice it to say that the armature (a cylindrical coil of very very thin wires) when spinning inside a magnetic field, induces an electric field. A set of contact points, opening at just the right time, taps that electric field, and sends the current to the spark plug.
Let’s say you have a bike with a mag. Possibly the bike has never run, and doesn’t generate a spark. Or it has been getting increasingly difficult to start the bike, especially when it’s hot. Now is the time for a rebuild. You don’t have to send it off to a specialist, and spend $800 or more. You’ll find that a magneto is a pretty simple piece of equipment, and that you can restore its sparking power with a short amount of time and money.
- If the motor starts easily when cold, and get hard to start when hot: check the condenser (capacitor).
- To see if the condenser is failing: run the motor in the dark and look for a continuous ring of sparks at the points as they spin around.
- A magneto can generate a spark in a plug much easier when the plug is not inside the combustion chamber. At 7:1 compression, the mag has to work much harder to overcome higher resistance. If you mag sparks during testing held against a cylinder fin, that doesn’t mean it will work in a running motor.
Shown the photo below is a typical magneto, disassembled. This model is an EIC, single cylinder, from the 1920’s. A Bosch will look about the same, or a Lucas, Miller, etc.
The basic elements of the mag can be seen. From left to right, there is the end casing, a bearing, the armature, another bearing, then the magnet (a horse shoe shape) and the other end casing. The points are inside the right end case, and attach directly to the armature with a taper fit.
Disassembling the mag is straight forward. Just two notes to remember. Many magnetos have a screw in the magnet or central case. This screw fits into the V groove of the armature. Remember to remove this screw before pulling out the armature. The V groove is made from plastic, and it will crack if it is forced against the screw. The second thing to remember is that you should keep a steel (must be ferrous –no aluminum or stainless) bar or plate across the open end of the horseshoe magnet. Without this bar, the magnet can lose its strength, while sitting on your bench.
You can attempt to re-wind an armature, but it is a whole lot of work. And you need to build or buy a winding machine, source about 3 miles of the correct wire, and spend a lot of time and trials. There are a few folks out there who still rewind them, typical prices are about $150US for just a rewind. Most of the people who advertise “magneto rebuilds” in the magazines and other places DO NOT rewind. They will gladly take your money and mail the armature to the specialist who does the work.
Be sure to get a new condenser (capacitor). Don’t be tempted to use an NOS (new old stock) item. They get old, even sitting on the shelf. The old paper cover items weren’t that great in the old days, and are worse now. Spend the money on a decent one from a reputable supplier.
Bearings and shims are pretty straightforward. Inspect for wear, replace if needed. Shoot for just a few thousands of an inch of axial freeplay. Do not over oil or grease them. Use a paper washer behind the bearing race to keep it electrically isolated from the mag housing.
Points are available from specialists’ shops. The earliest magnetos used platinum points. These will cost you a pretty penny! One thing to remember is that when you replace the points on any type of motor, the performance will drop for a while, while the points bed in. Don’t fret. But don’t replace points if you don’t have to. A simple clean up with a points file to keep them flat is all they typically need.
This should be re-magnetized during any mag rebuild. A good magneto shop will have a big electromagnet to do the job. Don’t be tempted to build your own electromagnet, or use a small one. If the EM isn’t strong enough, it will not give the magnet enough strength! The operation takes about 5 seconds on the proper EM. Pay the shop guy a few bucks for the use of his equipment, then get back to re-assemble your mag.
Like the fix-it book say: re-assembly follows the opposite steps used in dis-assembly. J It shouldn’t be difficult. Be careful with the safety gap screw. Check the bearing preload. Don’t tighten screws too tight or they will break or strip threads in the housing.
Connect your magneto to a cordless drill with some rubber fuel hose and two hose clamps. Connect a spark plug to the lead, and a bit of wire to ground the mag housing to the threads of the spark plug. Spin the drill, checking for spark at the plug. A strong mag will spark across a large gap at atmospheric pressure (ie not inside the motor). Try a gap of .030” or more to simulate the larger resistance that the plug will see in the combustion chamber. Run the drill motor for while to be sure that the mag is working well. You might even want to bench test it at higher temperature.
- Make sure that the mag housing has a good ground connection to the bike frame and/or motor.
- Set timing with mag fully advanced.
- Check timing on both timing cams for a V twin.