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ZR4 ZR6 Bosch Magneto Instruction Manual

This is a later American Bosch ZR Series mag with an aluminum base plate in lieu of brass. Very similar otherwise. The cover that encloses the brushholder is often potmetal ready ro crumble and is held on with two 4.5MMx.75 screws (do not lose them!). They have slightly wider magnets than the DU4 et al and are used in heavy duty installations like early Caterpillar gas powered crawlers and some Rumely Oil Pulls.  They are an excellent hot mag. Here are some images from a well used catalog of the day to share with some tech tips to follow.

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Mind the witness marks on the armature and distributor rotor shaft gear. There will be two marks on the rotor gear. Using one or the other depends on the rotation of the mag. If the drive shaft runs clockwise, index the armature gear witness mark with the distributor mark labeled "CLW" ,"R", or "C". Counterclockwise marks may be denoted by "ACLW", "L", or "CCW". Sometimes these marks are well defined, other times they are nearly impossible to see. If yours are not well defined, note the position of the rotor carbon brush and the armature breaker assembly keyway.  This is a critical step. If the mag fires and the rotor is not pointed at a path to a plug it will find its own way to ground as a carbon track. This also applies to the DU4 mag.

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Some rotor gear witness marks are nearly impossible to see.

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The condenser or capacitor lives in the base of the armature and can be a source of trouble. The early devices were made of mica and are robust but can still accumulate moisture . The later versions used paper as an insulator and are often found to be leaky. Always test for leakage first, then capacity and internal resistance. If there is leakage, replace it. Here is one way to retrofit a replacement for the stock condenser. The center  brass sleeve has 4MMx.7 internal threads to accept the breaker assembly mounting screw and is insulated from ground with nomex paper.

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This is a Sprague/Vishay 0.22MFD 400 Voly Orange drop capacitor. We found these at Tubes n More formerly Antique Electronic Supply of Tempe, Az. These are epoxy hermetically sealed and made to handle spike currents and robust. From here mud in some Bondo to secure leaving a loop of the grounded pigtail visible to connect to the coil primary ground. This should add some immortality.

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