Classic Motorcycle Magneto Tests & Repairs Of The Electrical Components (Continued) Brush Spring Repairs

This series of articles is shared with permission of the author who wishes to remain anonymous.
In addition to hardness, spring pressure is the other important parameter for a brush. I determined from measurements of the depth of the recess in the HT pickup that compressing the spring until the bottom end of the brush is 5 mm above the surface is a good value to use to simulate the average pressure on a brush when it is partially worn. Since Pressure = Force/Area, if a spring has the incorrect spring constant it will exert either too much or too little force on the rubbing surface of the brush (Force = -spring constant x displacement). The next photograph shows how I made this measurement using an electronic scale with a resolution of 0.01 g:What I found was the brushes that were in this magneto had springs that are ~25% stiffer than those on Lucas HT brushes. This means that not only were the “KEV” brushes the restorer used significantly softer than proper ones (so they would have worn away faster), they were subjected to 25% greater pressure on the slip ring (wearing the brush away even faster still). Had I not replaced these brushes with proper ones the slip ring soon would have been covered with a conductive coating of carbon that likely would have resulted in an internal short, bringing the motorcycle to a halt.Contact Breaker Points

Although the points in this magneto were fine, had they not been, and if there were no replacement readily available (e.g. cannibalized from my ZE1 magneto), I would have had to rebuild them. This can be done by silver soldering short lengths of pure tungsten TIG electrode rod over the ends of the worn points. Tungsten wets easily with certain silver solders, and a single 7″ piece of 4 mm-dia. pure tungsten rod is enough to restore a lifetime’s worth of magnetos. The end of one of these TIG electrodes is shown in the next photograph using my unrestored Bosch ZE1 for scale.

Note that if you rebuild your own points you need to use pure tungsten TIG electrodes, which are painted green on one end, as in the photograph. Do not use rods with either red or yellow ends because they contain 1-2% radioactive thorium. Because of the nature of the radioactivity, in solid form this isn’t an issue, but you don’t want to breathe the dust and have it permanently settle on the surface of your lungs. Obviously, it is difficult to form the W pieces into the necessary shape without creating dust. Also, the reason for thorium is that it makes it easier for the TIG electrode to emit electrons, and that is the opposite of what you want in contact breaker points. When the points separate, you would like it to be as difficult as possible for an arc to form.

Because platinum oxidizes only very slowly, Pt points (actually, Pt alloyed with Ir, Ru, and/or Os) were used in the earliest magnetos before it was realized laminated armatures significantly improved the low speed operation for which it had been felt Pt provided an advantage. Perhaps because of this, and citing a “lower surface resistance,” shop manuals often say Pt should be used for the contacts in magnetos intended for higher performance motorcycles. However, despite what is commonly written, my measurements show that the difference in surface resistance between the two metals is negligible unless the magneto sits unused for some months in a humid garage, in which case tungsten can benefit from a light burnishing to remove the resistive oxide.

The next photograph shows that the resistance across the tungsten points in a Lucas KNC1 magneto is only 9.4 milliOhm (actual values varied between 2 and 11 mOhm when measured after successive make/breaks when rotating the armature by hand). I found the same range for the W points in the Lucas KVF beside it. An important fact is that both of these magnetos were given new tungsten points 16 years ago and have been sitting unused inside the house ever since. Even after this extended time, the oxidation of the W was negligible.

Since 10 mOhm is less than 2% of the resistance of the primary coil itself, the contact resistance of these points will only reduce the primary current, and hence the output of the magneto, by ~2%. What this means is, even if the resistance of Pt points were zero, the output of two otherwise identical magnetos with Pt and W points in good condition would differ by a negligible amount.

Although Pt will resist oxidation in a humid environment better than W, it reacts with organic vapors to form PtC, which is both resistive and rapidly erodes the contacts. The choice of materials for magneto points is addressed in Chapter V ‘The Magneto-Type Ignition Systems’ of Electrical Contacts Data Book: Materials, Designs, and Applications (P.R. Mallory & Co., 1945): “… because of the location of the magneto and its design, the contacts may be subjected to oil mist. In such cases it has been the practice to use tungsten for the contacts, because platinum and its alloys have a tendency to become brittle… [causing] high contact resistance and a corresponding high rate of electrical erosion.” Since organic vapors typically waft about on old motorcycles, my conclusion from such reference books as well as my own measurements is that tungsten is the better choice for motorcycle magnetos in general, and for this Bosch ZEV in particular. Send questions or comments to

This entry was posted in Harley Tech and tagged , . Bookmark the permalink.