Appendix III: Anatomy of a Post-WWII Lucas K2F Magneto

Although most students who took high school biology dissected a frog or fetal pig, I suspect few people who own a motorcycle have dissected a magneto. Much is to be learned in both cases (but without the aroma of formaldehyde for the magneto). Since the basic thread is about a Bosch ZEV, I've included several images to illustrate the similarities between it and the Lucas despite a half-century of evolution (just as the reason for dissecting a fetal pig is because of its evolutionary similarities to human anatomy).

The Armature

The next photograph shows the armature of a post-WII Lucas K2F twin magneto on the left, and a c1915-20 Bosch twin magneto on the right. Other than the asymmetry of the steel in the Bosch armature, needed because it is for a V-twin with offset firing angles, the similarity is remarkable. I won't go into the details in this Appendix, but the asymmetry of the armature and of the magnet pole pieces in the housing (shown later in this Appendix) advances and retards the firing each revolution by half the angle of the engine's V. That is, since this Bosch KEV is configured to work with the 45-degree V of a Harley-Davidson engine, one firing pulse each revolution of the armature comes 11.25-deg. (22.5-deg. engine) earlier than it would for a vertical twin like a BSA 650, and the second comes 11.25-deg (22.5-deg. engine) later, for a total difference of 22.5-deg. (45-deg. engine).

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The next photo is repeated from an earlier post in this thread, and shows that the Bosch ZEV armature is made up of individual steel laminations. Early in the development of this type of magneto it was discovered that the eddy currents induced in solid-core armatures due to rotation through the magnetic field resulted in significant losses. Because of this, armatures are built up using laminated steel plates, each varnished to electrically isolate it from its neighbors to minimize eddy currents. The armatures of all Lucas (and Bosch, BTH, and all others) are similarly constructed using laminations.

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The next photograph shows the armature of a Lucas K2F after I sectioned it through the middle. The six rivets holding the laminations together can be seen, along with the two holes through which long screws attach the brass end caps to the central armature/coil section. Also seen are the ~200 windings of the primary (closest to the core) and the ~10,000 of the secondary, separated from the primary by a thick layer of insulation.

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The next photo shows the sectioned coil after removing it from the armature (it differs slightly from the previous one because it isn't the same coil, although they are nominally identical). Seen in this photo are that the coils of the primary are nearest the armature, and surrounded by the many turns of fine wire of the secondary. The horizontal wire exiting the coil carries the high voltage to the slip ring from the last turn of the secondary.

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On the left of the next photograph is a magnified view of the left half of the coil of the previous photograph, and on the right is the same coil under additional magnification. The largest wire is 0.045" in diameter, and the small ones making up the secondary are 0.003" in diameter (i.e. roughly the diameter of a human hair). The relatively large wire of the primary (0.030") connects to the 0.003" wire of the secondary, which in turn connects to the 0.045" wire that leads to the slip ring.

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The core of the armature is rectangular in cross section, as can be seen from the next photograph of coils cut from two armatures, one sectioned "horizontally" and one "vertically." As can be seen from the scale in the photograph, the core of the armature is ~1/2" x 3/4". However, the fact the core doesn't have a square cross section isn't particularly significant for operation.

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Earth Brush

There are a few more electrical parts to examine before turning to the magnet. An earth brush is needed to complete the primary circuit, and the Lucas K2F holds this carbon brush in a hollow "screw" inserted at the drive end of the housing. The next photograph is of the earth brush taken through a window milled into the housing (red paint outlines areas where I've milled away sections of the housing). The carbon brush extends less than 0.06" from its holder, so it can be seen from this that the clearance between the housing and the armature is quite small.

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This brush makes contact with the smooth brass endcap of the armature, completing the circuit (the brass on the armature in this photograph is rough, and would abrade the brush more rapidly than it should if used in this condition).


The voltage needs to get from the coil in order to make its way to the spark plugs, and this is done using a slip ring connected to the coil along with a carbon brush held in a nonconductive pickup. The next photograph shows the slip ring and pickup through a window milled into the housing. The output wire from the coil is inserted in the slip ring where it makes electrical contact with a brass arc molded into the base of the slip ring. This brass arc in turn makes periodic contact with the carbon brushes of the pickups located 180-deg. apart in the housing.

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The carbon brushes project ~0.08" from the end of the pickups, which is why the carbon itself has to be at least twice this long (or longer) to provide the necessary mechanical support for itself within the pickups.

Safety Gap

Under normal conditions the spark plugs fire at less than 5 kV, so the voltage experienced by the insulation in the coil doesn't exceed this. However, if the plug, plug wire, or pickup breaks the voltage would rise to a high enough value that it could break down the internal insulation. If that happens, permanent damage to the coil can occur. Because of this, a safety gap screw is located next to each pickup to keep the voltage within acceptable levels. The next photograph shows one of these screws viewed through the hole for one of the pickups (enlarged with an additional window milled into the housing).

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The brass contact in the base of the slip ring is long enough that a safety screw is always over it when the voltage is high. Normally that voltage is routed to the spark plug, but if for some reason that doesn't happen it will jump the ~0.25" gap to the safety screw if the voltage ever reaches ~20 kV. Also, it should be clear from this photograph why if the safety gap screws are not removed before attempting to withdraw the armature, the relatively thin and brittle lip of the slip ring will be broken.

The Magnet

Having finished with the electrical elements of the magneto, it's time to dissect the housing to examine the magnet. The next photograph shows the inside of a Lucas K2F housing before and after I sectioned it. I've color coded the parts as follows: the Alnico magnet is blue, the laminated steel pole pieces are red, the remnants of the brass cap that held the pole pieces together with the aid of rivets is gold, and the remainder is cast aluminum.

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The actual magnet is just the fairly small slug of Alnico at the top of the housing. However, the steel in direct contact with the Alnico is magnetized by it and "carries" the N and S poles to the right and left sides of the magneto, exactly as would be the case if everything shown in red were Alnico instead of steel. However, Alnico is such a strong (but expensive) magnetic material that inexpensive steel can be substituted for it in this way. It can be seen that the pole faces are symmetric since this is for a vertical twin engine with equal firing intervals, not for a V-twin.

From ZEV to K2F: A Half-Century of Evolution

On the left of the next photograph is the housing of the Lucas K2F and on the right is that of the Bosch ZEV (with just one of its two tungsten steel magnets in place). It can be seen that the magnets for both are in the shape of horseshoes (including the steel in the case of the Lucas), although a half-century of materials development allow that of the Lucas to be considerably smaller (the available energy from Alnico is ~9x greater than that of tungsten steel). The fact the Lucas directly evolved from the Bosch is readily apparent.

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The asymmetric pole pieces of the Bosch show that it is for a V-twin engine. Together with the asymmetric armature shown above, this design fires one cylinder earlier and the other later than the even 180-deg. firing interval of the Lucas K2F. Although I haven't shown any examples here, the internals of single cylinder rotating armature magnetos (e.g. Bosch ZE1, Lucas KNC, BTH KD1, etc.) are essentially identical to their twin-cylinder siblings, differing only in some details.

From a rebuilder's point of view, it should be clear that the most time consuming aspect of a complete restoration of any magneto is winding a new coil. Removing the old coil is relatively easy if it is the original one, but can be quite troublesome if it is a replacement that has epoxy firmly bonding it to the armature (n.b. if the replacement had been properly wound it would not have failed). As can be seen from the cross sectional views, once the old coil has been removed, winding a new one requires ~10,000 turns of very fine wire in a close-packed arrangement, with insulation layered in appropriate places, connecting 0.003", 0.030" and 0.045" wires without breaking them, and then vacuum impregnating the final assembly. However, although time consuming to wind, a correctly rewound coil will outlast anyone reading this post by many decades, so it's worth doing right.