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. Manufacturers of magneto plugs usually set the spark points about 64 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

Fig. 133. Simple Methods of Holding Magnetos in Place on Engine Base to Permit of Easy Removal of Apparatus when Desired.

between piston travel and crankshaft movement for engines of different strokes very clearly. The armature is uncovered by removing the flat casing cover lying between the horseshoe magnets, this often carrying the safety spark gap, and normally serving as a lid. If earlier timing be desired for any special purpose the gap may be widened a trifle, if it be thought the timing is too

far advanced, the gap may be lessened. The contact breaker is fully advanced at this time and the contact points are just about to separate. Having placed everything in position as described, tighten the coupling on the taper shaft and ream out for a small taper pin.

The connections to the various cylinders must be made in the order they fire (see following tabulation). When the cover to the distributor is off, see at which segment the brush is contacting.

Fig. 135.-Simplified Diagram Explaining Method of Timing Magneto

Ignition System.

The wire to the spark plug in the first cylinder is then led to the terminal corresponding to this segment. Then the plug in the cylinder that is next to fire is coupled to the next segment, and so on. The numbers on the distributor show the order in which the various contacts are brought in contact with the rotating distributing brush, and not that in which the cylinders fire. In the sketch the cylinders fire 1-2-4-3. Therefore, the segment num