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Thread: Bosch ZEV ID parts

  1. #1
    Join Date
    May 2014

    Default Bosch ZEV ID parts


    I needed a 50 deg ZEV for a 13 Thor engine and I bought one marked 50 on the side plate - but the cam ring is marked 42C. when I took it apart - the side covers , base and magnet are all stamped 527 - I am guessing that they were all stamped at the same time and belong to the 50.

    a few questions:

    are the base, rotor and pole block pieces the same for the different degree magneto's and if not how do I identify a 50 ?

    I was told that the only difference was the cam ring and rotation - but that doesn't seem right to me given the angle between the rotor and the pole block needs to match the cam - right?

    am going to send this out for a full rebuild - but want to make sure I have all the correct pieces

    at the very least I need the 50 deg cam ring

    I know you guys have a lot of knowledge and I need some help - thanks

    some of the pics didn't upload correctly - ill try again laster

    Attached Images Attached Images

  2. #2
    Join Date
    Nov 2009



    Bosch was quite good about making precise fitting parts to fit each engine type. There are a number of pieces that are specific to each degree engine arrangement, I'll list them below. The numbers stamped in your body pieces are simply an assembly number, meaning those parts were all originally together. That number does not tell you the application. Of course, the end plate showing 50 degree is correct, really just for identification purposes as any end cover will work. Think about this: front cylinder fires, then the crank must rotate 360 degrees, minus 50 and then the rear cylinder fires. After the rear fires, the crank must rotate 360 degrees plus 50 and the front fires. again. Think about using a 42 degree mag on a 50 degree engine, the timing will be off quite a bit somewhere!

    Remember that in order to get the hottest spark from a magneto, two things must happen simultaneously: The rotating pole assembly (in the Bosch design, that is the coil assembly) must break the magnetic field at exactly the same time the points open. With this in mind, it then makes sense that the following parts should be specific to match the application.

    1. The rotating pole assembly which houses your coil should be stamped 50 deg. This is located on the iron portion of the rotating assembly at one end. The edges of the poles are cut to make the field break at just the right time. Also the indexing mark for the points is in the exact correct position.

    2. The iron pole shoes which are mounted to the aluminum base and hold the magnet in place are also stamped 50 deg, at least one is anyway. This stamping is located on the top of one of the iron poles. If only one is stamped and the assembly numbers match, they should both be the same.

    3. The points housing, or interrupter housing as Bosch calls it, must be stamped 50 deg also. In addition, it must have the correct rotation for the application. Magneto rotation is always referenced when looking at the drive end of the mag. Not sure about your application.... A clockwise rotating mag would have the degree # followed by the letter C stamped on the advance arm, sometimes on the rear of the arm. Counter clockwise rotation is denoted with the letter A for anticlockwise. And, to add to the difficulty, the advance arm must be pointed in the correct war, forwards or backwards to align with whatever mechanism was used to advance or retard the timing. The housing you have is marked 42 C, meaning for an Indian V twin with Clockwise rotating mag and the arm pointing forward as used on early powerplus and Hendee motors.

    4. The points assembly must be correct for the rotation as well. On the back of the brass base they are stamped with the model of the magneto, so in this case yours should show ZE I believe. To my knowledge there is no rotation reference stamped on them. However, simply looking at the front of the points block one can tell. Just remember that the bakelite block that actually touches the cam when the points open should always trail the rotation and drag across the cam, and never lead into the cam and hit it head on.

    So, a tough find to get the correct stuff for a 50 degree mag. However, getting it right will make all the difference in starting, idling and power. With that said, a 45 or even a 42 degree mag could be made to work, but it wouldn't be perfect.

    Now for all you heavy thinkers....... after reading the second paragraph above you're wondering how these two things can happen at exactly the same time when the timing is advanced or retarded!!!!????? Most magnetos are designed such that the poles still break at the same position relative to the crankshaft but the points open earlier or later????!! So in theory that exact perfect point only happens at one spot of the advance lever. Anything before or after that the internal timing of the mag is off......... That exact point is usually about 1/2 way between full advance and retard, maybe closer to retard. This makes starting easier and the faster the mag spins, the hotter the spark generated so it does fine at idle and above, if all is in order. So, it's a compromise. Dixie mags are the only mag I'm aware of that solved this problem, by attaching the entire rotating field, pole and magnet assembly, points and cam to the spark advance lever making the entire assembly rotate together. So, that perfect internal timing between points opening and magnetic field breaking never changed. It was a brilliant idea, unfortunately they used the cheapest Zinc die castings ever and they wore out and distorted so badly that things fell apart, drug or shorted out. They developed a nasty reputation for poor dependability. George Yarocki told me that his father always said "If you own a Dixie magneto and it rains in the next county over, your magneto will quit working"

    Time for another cup of coffee...

    Chris I hope that helps!

    Gene Harper

  3. #3
    Join Date
    May 2014



    I tried to respond - but not sure where it went

    Thanks for the detailed information - you deserve that second cup of coffee.

    I looked at the pole block and your right its marked 45 deg - I don't get how all these parts are stamped 527 - like they were assembled at the same time and the side cover is a 50 and the pole blocks are 45

    I am also not sure about the points block - its stamped Z on the front of the ride arm and 44 and 88 on the base plate. I am going to post a few more pictures of the markings

    thanks again for your help


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