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a magneto shown at B is practically the same, except that the retention member is a small knob which can be easily turned by the hand. At C the strap encircles the magneto completely and is held in place by a single nut under the bracket. A modification
of this method is depicted at D. The strap, in this instance, is just bent over the arch of the magnets and held in place by the long swinging bolt which is hinged at the bottom of the magneto.
One of the simplest methods of driving a magneto is that shown at Fig. 17 (Chap. I), which is a bottom view of the Ford engine
case. The stationary coils of the magneto are attached to the crank case, and the revolving magnets rotate with the fly wheel, which in turn is securely attached to the crank shaft. With this form of drive there can be no interruption in current generation and there are no gears, chains, or other connections to wear and produce noise or interfere with generation of current.
When the magneto was first introduced it was looked upon with suspicion by the motoring public. Therefore some designers compromise and furnish two separaté systems, one composed of a magneto, the other an auxiliary group comprising a battery, timer and coil, which supply the current to a set of spark plugs distinct from those supplied from the magneto. It was found difficult with some types of magnetos to start the engine directly from magneto current so the battery outfit was depended upon for starting the engine as well as emergency service. The parts of the modern high-tension magneto have been simplified and strengthened and as the various parts may be removed easily and replaced without trouble and special care taken so the adjustments and cleaning necessary may be easily understood by the layman, there is very little liability at the present time of a magneto giving out without warning.
When a magneto is installed some precautions are necessary relating to wiring and also the character of the spark plugs employed. The conductor should be of good quality, have ample insulation and be well protected from accumulations of oil which would tend to decompose rubber insulation. It is customary to protect the wiring by running it through the conduits of fiber or metal tubing lined with insulating material, as shown at Fig. 134. Multiple strand cables should be used for both primary and secondary wiring and the insulation should be of rubber at least 316 inch thick.
The spark plugs commonly used for battery and coil ignition cannot always be employed when a magneto is fitted. The current produced by the mechanical generator has a greater amperage and more heat value than that obtained from transformer coils excited by battery current. The greater heat may burn or fuse the slender points used on some battery plugs and heavier electrodes are needed
Fig. 132.—Typical Magneto Installations. A-Simms Magneto on Max.
well Motor. B-Bosch D U 4 on White "45'' Engine.
to resist the heating effect of the more intense arc. While the current has greater amperage it is not of as high potential or voltage as that commonly produced by the secondary winding of an induction coil, and it cannot overcome as much of a gap. Man. ufacturers of magneto plugs usually set the spark points about 464 of an inch apart. The most efficient magneto plug has a plurality of points so that when the distance between one set becomes too great the spark will take place between one of the other pairs of electrodes which are not separated by so great an air space.
Timing Magneto Ignition Systems.-An ideal method of magneto placing and one followed by a large number of manufacturers is shown at Fig. 132, B. In this the device is fitted to a four-cylinder engine, and as the armature must be driven at the same speed as the crankshaft, it is necessary to use but one extra gear, that being the same size as the engine shaft pinion and driven by the camshaft speed reduction gear. The sketch, Fig. 135, illustrates the best method of timing the magneto, which is one of the direct high-tension type. The position of the various parts is clearly shown. Having fixed the magneto to the engine crankcase, the driving pinion, or one of the members of a flange or Oldham coupling, is put loosely on the tapered end of the armature shaft, and the cover to the distributor and the dust cover of the contact breaker are removed to allow one to control the position of the armature. The motor is now turned over by hand so the piston in the first cylinder is at top center, which can be determined either by watching the crankshaft through a suitable opening in the engine base, by reading the marks on the flywheel rim, or by inserting a wire through a compression relief petcock or spark plug hole, if either of these is at the top of the cylinder.
The armature of the magneto is then brought to the position indicated in sketch, which represents the fitting of a magneto that is turning clockwise when viewed from the driving end. The distance between the end of the armature and the pole piece should be between 14 and 17 mm or between .5511 inch and .6692 inch. (See Fig. 95.) This represents an advance of about .5 inch on a motor with a five-inch stroke. A graphic chart, prepared by the Bosch Company and reproduced at Fig. 136, shows the relation