Tag Archives: Magneto Rebuilding

As The Magneto Spins – Part IV

This is part IV in our feature showing what is involved in rebuilding a Bosch ZR4 Two-Spark  magneto. The assembly is fairly straight forward with the most important part, being the setting of the contact point gap along with getting the gearing on the armature and rotor gear timed correctly.

The photo above shows the magneto spinning at engine speed on the lathe for testing. The second photo shows the spark that this magneto puts out at 1750 rpm. The second photo below shows the dual rotor and the distributor cap, the carbon brush, in the rotor nearest to the bronze gear, is in contact with the spring loaded carbon we showed you earlier in the rear housing. The other brush, is in contact with a round spring loaded carbon which you can see in the middle of the inside of the distributor cap. This brush is in connection with the other lead on the back of the mag by an external high tension wire which connects the two.

The third photo shows it installed on a Mercer Raceabout. Just behind the magneto is the “Holy Grail” of Mercer racing equipment. It is an aluminum Zenith racing carburetor complete with a velocity stack that we found and restored, but that will have to be another story in the future. The first photo at the bottom, shows the 0-1-2 switch which is in the shut off position. It is wired so the plugs over the intake valves fire in the 1-position and both plugs in the cylinder fire when in the 2-position. A wiring diagram follows the switch photo, so you can have a clear idea how it is wired.


One area that often gets over looked, is to be sure the point gap is the same on the two cams that these magnetos use. This unit was fine but we have found badly worn points housings which will keep the point gap from ever being consistent. We solve this by first inserting the inner points housing on the lathe in a special fixture to hold it (second photo below). The outside diameter of the housing then gets machined round and true and slightly undersized. The last photo shows the outer housing set up in the milling machine where the inside bore also gets re-machined until it is also round and true. Then we machine a sleeve in the lathe, which can be seen to the right of the housing which gets pressed into the outer housing. It then gets bored out .001-.002 thousands larger than the inner housing for a smooth sliding fit.


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As the Magneto Spins – Part III

This is the third post in a series showing the rebuild of a Bosch ZR4 Two-Spark magneto. In our last post of  the magneto rebuild that is being shown here, the armature was rebuilt and the magnets were recharged. This installment will give you a better idea of about how the rest of a magneto operates.

Above we see an armature winding between the two pole pieces that are arranged between the magnets and the base. At this point in the rotation of the assembly the combination of the influence magnets and the position of the windings, is were the maximum amount of current is generated as it spins. Generally this is also were the points are timed to open and trigger the release of current that has been generated out of the armature through the collector ring. The red wire connects to the ungrounded end of the condenser that is connected with the points and the black wire connects with the other end of the condenser to the armature. This in turn is grounded through another set of carbon brushes which ride on the brass area of the armature end we re-machined earlier.

This is a two-spark unit and contrary to popular belief, a two spark magneto only sends current to one of the two spark plugs in a cylinder. After it jumps the gap in one plug, the current enters the cylinder casting and then travels back to the magneto in a micro second, as it jumps the gap between the second spark plug ground electrode and then travels back through the center of the plug and through the second wire that leads back to the distributor cap.

When the points break and the armature releases the high voltage charge it has built up, it enters into the distribution part of its travel to the spark plug. The copper contacts on the slip-ring release their current to one of carbon brushes that are seen in the center of the bronze housing (above). The spring-loaded carbons in the photo are originals that are routinely replaced in a rebuild. The current then travels through the pickup assembly which is connected to the black half-round phenolic housing on the top. Once the current enters, it then is directed to another spring-loaded carbon that you can see pointed toward you. This carbon directs the charge to the rotor which spins inside of the distributor cap.

There is also another connection in the back of the phenolic housing which connects by an external wire to the front of the distributor cap and another carbon that connects with the second carbon brush on the front of the rotor.

If you look closely you will also notice two prongs coming out of the front of the housing on either side of the carbon at the top. They are aimed at smaller prongs pointing up from a brass plate which sits at between the phenolic housing and the bronze housing. These are the safety gaps which will allow the current to travel back to ground in case there is a break anywhere in the distribution system. Without it, if there were an opening and it was large enough, the spark could possibly jump and in doing so strain and eventually burn out the armature windings.

Above is the points assembly which attaches to the front of the armature. It then spins in side of the points housing in which you will notice a flat cam on either side. As the points rotate the red fiber pad on the end of the moveable half of the points travels over this cam which opens the point gap (this signals the armature to release its high voltage current through the distribution part of the magneto).

Below is another view of the points assembly on the end of the armature. The raised- headed bolt that you see in the middle connects the points which are insulated from the assembly by mica insulators to the condenser. The condenser or capacitor which they what they are also called in other forms of electronics, job is to keep the high voltage current for burning the points when they open. Think of it as a shock absorber. Below in diagrams this maybe easier to follow.

Illustrations of a Bosch ZR4 magneto are very hard to come by. If you study these two Bosch diagrams of an earlier single spark (above) and a later single spark (below) what we have shown you so far can be followed and understood better.

Below is a Stevens coil and condenser machine, another key piece of testing equipment which can do all tests necessary with a magneto or distributor. With it we can test coils and armature windings for output along with being able to test condensers. One more very important part of this tester is that it can generate its own high voltage current. With this source of current it can be used as seen below to test rotors, caps and any part of the high voltage half of the magneto for cracks, flaws, carbon tracking and any path to ground which will cause misfiring. Below you can see the double rotor and cap that a two-spark unit uses to distribute the high voltage current to and from the spark plugs. You can find Part Four in this series here.


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As The Magneto Spins – Part Two

This is the second in a series of posts about rebuilding a magneto. Last time we showed you what was involved in rebuilding the armature. This time we will show one more very important step which is recharging the magnets. Through time and use the magnets lose their strength which will result in a weak spark. By remagnetizing them they will help to produce a nice strong spark again.

We looked for a long time and were finally able to find one of the best rechargers which was made in the period by American Bosch which runs off of six or twelve volt battery. This charger is big one that is over a foot tall and weights in at about 60 pounds. Bosch specified that the magnets when recharged should be able to pull 25 pounds (see the article below to see how they are tested). With this charger we find that it actually gives us 45 pounds, which leaves a good margin of extra strength so they will stay adequately charged longer and put out a hotter spark.

Take the time to read the excellent article below by R. J. Everest which was in The Auto mobile magazine March 4, 1915, issue on the subject. Below the article in thumbnails are photos of some of the other rechargers that were on the market at the time. You can find Part Three in this series here.

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