Classic Harley Tech: Final Article On How To Rebuild a Bosch ZEV Magneto

This is the final article in this series on rebuilding a Bosch ZEV motorcycle magneto. We hope this has been helpful to those of us interested in such matters. With the author’s permission, we have identified him at the end of this article. We thank him for taking the time to follow this process and share it with us.

EPILOG

As far as I can tell, at the time of this writing (Fall 2012) this is by far the most detailed description of the restoration of a magneto on the web or in print. Because of this, some additional observations might be useful for people on this Forum as well as those who find their way here thanks to Google.

Basically, there are three audiences for the information in the previous posts in this thread: people who plan to rebuild their own magnetos; people who want their magnetos rebuilt for them; and people who rebuild magnetos for profit. This Epilog is primarily for the second group: people who want help identifying someone who can properly rebuild their magneto.

The Major Problem Areas in Restored Magnetos

Briefly, when magnetos are improperly rebuilt, the subsequent problem(s) they develop most likely result from one or more of the following factors, all of which I have addressed in my previous posts in this thread:

– Inappropriate condenser
– Improperly rewound coil
– Improper (or no) remagnetization
– Aftermarket brushes that are either too hard or too soft

There can be issues other than these, but these four account for most of the failures I have seen in magnetos that were professionally rebuilt. Because of this, my first recommendation is that you make sure whoever you hire to restore your magneto at least has the tools required to correctly deal with these four issues at a minimum.

Tools

I wrote in the initial installment that my goal in restoring this Bosch ZEV rotating armature magneto was simply to return it to the condition it had when it left the factory. In my experience, to do less than I described in these posts would have resulted in a magneto that was not as reliable, had a shorter life, and/or produced a lower output than it did when it left the factory nearly 90 years ago. The major tools and test instruments shown in this thread that were required to accomplish this were: 

–Mechanical–
Mill with digital readout
Lathe with continuously variable speed
1/2 h.p. long-term tester
Modified distributor tester
General Radio Strobotac
Talyrond roundness tester
Bench center with four dial test indicators
Surface roughness tester
Contact breaker pressure gauge
Precision scale 

–Electrical–
84,000 A-turn electromagnet
Gaussmeter
500/1000/2500 V megohmmeter
LRC meter
Milliohmmeter
Merc-o-tronic magneto tester
Eisemann magneto tester
[coil winder & vacuum pump-- not used, but would have been had there been more time before the deadline]

–Optical–
Centering microscope
Stereo microscope
Traveling microscope
Metallurgical microscope 

Although I have no way of knowing for sure, I seriously doubt even the most heavily-equipped professional magneto rebuilder has the range of equipment and facilities I do for diagnosing and restoring magnetos (not all of which were needed for this Bosch ZEV). But, not all of this equipment would be needed for many rebuilds. So, which of the above tools would someone not need if they only planned to “restore” a magneto as “efficiently” as possible? That is, what is the minimum set of major tools that a rebuilder would need to return the majority of faulty magnetos to customers in a condition where they appeared to function properly when removed from the shipping box? 

If the screws in the armature of this Bosch ZEV had not been broken, microscopes and a mill wouldn’t have been needed. Although I used the lathe and mill to help remove the epoxy, if someone just wanted to get the stuff out they could have used an acetylene torch instead (I’m definitely not recommending this, but I have seen armatures where the restorer had done this). Anyway, under these conditions, the only tool from the above list actually required would have been an electromagnet. Further, if someone only worked on post-1930s magnetos containing Alnico magnets, even that wouldn’t be essential in order for the “restored” magneto to spark when turned with an electric drill (albeit, with a significantly weaker spark than it should have). The point being, there is a huge range of equipment, expertise, and time required between being able to claim to “restore” magnetos, and actually being able to restore them to the reliability and performance they had when they left the factory.

As a recommendation, since even the most straightforward rebuild will require the following, you should not consider sending your magneto to anyone who does not, at the very minimum, have a: 

– 65,000+ A-turn electromagnet (can be less powerful for older, pre-Alnico, magnetos)
– 2500 V megohmmeter
– Merc-o-tronic, Eisemann, or equivalent magneto coil tester

I can think of five people who posted to another on-line motorcycle forum over the last six months who said they do magneto repairs as part of their business. There was enough information in the posts of four of them for me to see that they do not meet even these minimal requirements, illustrating that by no means is everyone who claims to be able to repair magnetos actually able to do a proper job of it. It also illustrates why such a large number of “professionally repaired” magnetos fail. 

Magneto Repairers

There is no accreditation board for magneto repairers, so anyone can claim to be an expert. One YouTube video shows a magneto being turned with an electric drill and the “expert” declaring — quite incorrectly — that the intermittent spark it develops at higher rpm indicates it needs to be remagnetized (the problem almost certainly is due to a bad condenser). Neither does a Consumer Protection Agency evaluate advertising claims for veracity. For a number of years one well-known supplier sold replacement condensers with the claim “Modern substitute, very high specification, zero failure.” Despite this claim, many failed in service. Ignoring self-proclaimed statements of expertise or reliability, if you are looking for someone to restore your magneto, at a minimum you should determine if they have the necessary equipment to do a proper job (described in the previous section). That alone will eliminate a significant number of possible rebuilders from consideration.

Unfortunately, even if someone owns equipment more advanced than an electric drill, it still may not be obvious whether or not that equipment is appropriate. For example, a well-known magneto rebuilder has a web page showing the equipment he used to rebuild a post-WWII magneto, with one photograph showing it being remagnetized. The commercial magnetizer shown being used for this does not have the necessary field strength to fully magnetize Alnico, which means that magneto was returned to the customer with sub-standard performance. The same is the case for a small “internal magnetizer” another firm inserts in place of the armature. Irrespective of what field that magnetizer is able to produce, the moment it is removed from the magneto the reluctance of the circuit changes significantly. For reasons explained in references cited in earlier posts, this change in reluctance forces the working point on the Alnico’s B-H curve to shift, resulting in permanent partial demagnetization, in turn resulting in sub-standard performance of the magneto (i.e. 20-30% reduced output). 

The component that is responsible for most failures of rebuilt magnetos is the condenser. Given the countless magnetos that have failed because rebuilders used inappropriate condensers, coupled with the dismal history of false claims from suppliers like “Modern substitute, very high specification, zero failure,”, I strongly suggest you determine what condenser the rebuilder uses. As I wrote in an earlier post, the condenser I used in this Bosch ZEV was a pair of Panasonic polypropylene film/foil capacitors. I used these capacitors because the manufacturer rates them for high pulsed currents as they will experience in a magneto, they passed extensive environmental and electrical stress tests described in a two-part article in the Fall and Winter 2011 issues of ‘The Antique Motorcycle’, and they are the only replacement capacitors I am aware of that have passed such stress tests. However, if it were not possible for the rebuilder to get these particular ones, other film/foil capacitors (retail cost ~80 cents) have similar electrical specifications, so alternatives exist for installation in the original location in the condenser cavity. In any case, I recommend that you do not send your magneto to any rebuilder who uses a ceramic chip capacitor (retail cost less than 10 cents) that is packaged by one supplier for use in the points housing. 

Armature Winders

If your magneto needs to have its armature rewound, chances are the person doing the restoration will arrange to have that work done by someone else. This is perfectly fine, except… 

Although various people offer this rewinding service for ~$150, I spent considerably more money than that to buy my own coil winder, pump, etc. in order to wind and vacuum impregnate armatures myself. As just one of many examples for why I did this, a rewound armature supplied by a well known rebuilder repeatedly seized in the 2010 Cannonball Run due to him having used improper resin to encapsulate it, which continued to ooze for several days. No doubt some rewound coils are made to proper standards, but many are not, and there is no way to look inside the coil of a completed armature to see if it was wound with appropriate insulation and has correct encapsulating resin in the right locations.

Despite the rewound armature that came in this Bosch ZEV having passed all my tests, there is no way to test for possible slow abrasion of the insulation due to relative motion of the wires that could happen if the coil had not been properly vacuum impregnated. Although I judged the chances of failure of the coil as not large, this is the only aspect of this rebuild where I had any uncertainty. As a result of it, I breathed a sigh of relief when my friend called me from the finish line in San Francisco. 

Coil winding is such a fiddly job that I would gladly pay someone $250 to do it for me rather than doing it myself. Unfortunately, after having seen a number of rewound coils, I concluded that winding them myself was the only way I could be sure the magnetos I restore will be at least as reliable as they were the day they left the factory. It takes me at least one long day without interruption to set up the equipment, remove the old windings from an armature, wind new ones, vacuum impregnate the coil, and put everything away again. However, if winding armatures were my only job, I can imagine the continual practice would allow me to wind them in maybe two hours (or even less) rather than a full day.

Magneto armatures have been around for more than a century so you would think that by now it should be very well established how to wind them correctly. As late as the 1950s magnetos were produced by the thousands every year just to supply British factories, so techniques of mass production resulted in excellent reproducibility and reliability. Today, though, the winding of magneto armatures is a cottage industry, done one at a time by people working alone and without any independent quality control. To repeat something I wrote in an earlier post, “there is a difference between hand made and home made.” This alone explains why many rewound armatures fail in service. Also, it is clear from what I have seen that many armature winders simply don’t understand that there is a lot more to winding a reliable armature than putting many turns of thin wire on top of fewer turns of heavier wire.

The relative number of turns in the primary and secondary determine the output voltage, but the windings ratio is just one aspect of a proper armature. Other essential aspects include the total number of turns of each coil, not just the relative number; thickness of the wires; type and thickness of insulation on them (the insulation on “magnet wire” sold today is available in four thicknesses and at least seven different classes of material, not all of which are appropriate for use in a magneto, with breakdown voltages differing by over a factor of 4 — and there’s no guarantee someone isn’t winding their coils using 50-year old spools that has obsolete and age-degraded insulation they bought cheaply on eBay); type and thickness of insulation wrapped over the core and between the layers; type of resin used to encapsulate it (i.e. not just that it has enough viscosity not to ooze out, but that it completely fills all the voids and fully hardens to eliminate all movement of the wires); and whether it was properly vacuum impregnated (which itself requires more than just applying a vacuum to the armature).

Despite my experience with the poor quality of some of the rewound coils being produced, in trying to prepare for possible eventualities I might face I contacted one well-known firm to ask them their turn-around time for rewinding a Bosch ZEV armature. Since I knew it would be difficult to find the time to wind a new one myself if the magneto arrived with a bad coil, I wanted to know if there was another option. When I contacted the firm I also wrote that I would test their rewound coil for a number of hours at 120 oF on my Wiedenhoff magneto tester before installing it and, since I would be working to a deadline, I wanted to know their policy on refunding money if the armature failed (rather than offering to rewind it again, since there wouldn’t be time for that). I don’t know if it was because I mentioned having my own coil tester, but I never received a response.

Unfortunately, the possible need for a rewound armature makes the restoration problem two levels deep. Not only do you have to find someone with the tools and expertise to properly rebuild your magneto, that person has to have the expertise to identify someone else who properly rewinds armatures. Of course, both good and bad magneto restorers will assure you that the person who rewinds the armatures for him does an excellent job. Maybe they do, but odds are they do not. I realize few people reading this will have an interest in winding their own coils, and I wish I could offer a constructive suggestion here, but all I can do is offer this information on coil winders as an observation.

How Much Should it Cost to Have Your Magneto Rebuilt?

So, how much should it cost to have someone properly rebuild a magneto? The good news is your magneto again can be as reliable as it was when it left the factory. The bad news is, it can’t be done for $150. 

I didn’t keep track of the hours I spent on this Bosch ZEV because I was doing it for a good friend and had no intention of charging him for anything. Also, I had never worked on a ZEV before, so I had to spend additional time researching obscure screw threads, and machining a fixture and pulley specifically for it which, if I were doing this as a business, already would be in hand. Also, if I could have worked on it full time from start to finish it certainly would have been more efficient than piecing together a few hours at a time between trips. With this in mind, how long would it take for a fairly straightforward rebuild of an ailing rotating armature magneto that had been “professionally” rebuilt before, but whose armature hadn’t suffered at the hands of the rebuilder as much as this one had? 

No matter what, count on having to spend time removing the globs of epoxy that almost certainly would be holding in place the inappropriate condenser the rebuilder had used, in order to make room for a proper replacement. Doing that, skimming and preparing the surface for the earth brush, truing the slip ring, adjusting the end float, etc. all take time beyond just replacing brushes, greasing bearings, and bead blasting the body (even if what’s inside is a bodge, magneto restorers always make sure the outside looks pretty, because that’s how 95% of their customers will judge whether or not it has been properly restored). Assuming the coil did not need rewinding, my guess is a magneto in reasonable condition would take me the better part of a full day to rebuild to the same “as-new” standards as this Bosch ZEV. This includes conducting the necessary measurements and tests described in these posts (but not counting the hours chugging along by itself on my long term tester). 

It should be clear from the posts that doing such work properly is skilled labor, and it requires specialized tools and instruments as well as expertise. One yardstick for cost might be the $85/hour that a machine shop in my town charges. At that rate, the labor alone to properly rebuild a magneto would be ~$700. Although this may seem high, I would have to charge at least that much if I were willing to take on the work. Other than perhaps a few Brough-Superior and Vincent owners, I doubt there would be many motorcyclists ready to pay that (owners of expensive classic automobiles might be more plentiful). Even if you think I am wildly off on my time estimate, and that it actually only would take four hours, that still would be $350 plus parts. But, having said that, I don’t believe anyone could restore a magneto to as-new operation in only four hours, no matter how efficiently they were able to work. Even if someone who was skilled enough to command higher wages were willing to work for only $40/hour (which also has to cover health and business insurance, retirement savings, rental of space, and repair and replacement of equipment), and even if they could do a professional job in only six hours, that’s ~$250 for labor. Again, these figures all assume a magneto that was in reasonably decent condition to begin with.

Cosmetics 

I restored this Bosch ZEV for use in a motorcycle rally, so I spent no time on its external appearance. Had cosmetics been important, just soda blasting the alloy and painting the magnets would have added at least an hour. However, a full concours restoration would have required sending fasteners out for replating (after spending time polishing them), polishing brass and alloy, and finding better-looking HT pickups (or spending time making the current ones look much better). To do this would take more than a few hours and could double the estimates given in the previous paragraph. Again, this is just for the labor. 

Final Comments

Even with the proper equipment a rebuilder still needs expertise, but that’s harder to verify, and recommendations mean almost nothing in this area. There are a lot of unqualified people doing poor jobs rebuilding magnetos, but who get good recommendations despite magnetos that routinely fail. As an aside, I’m amazed at the number of times I’ve heard people recommend the person who rebuilt a magneto for them that subsequently failed. I’m not a psychologist, but this seems to be some form of the Stockholm Syndrome. To paraphrase Bruce Springsteen’s ‘Magic’ one final time, as far as magneto rebuilders are concerned “trust none of what you hear, and less of what they claim.” 

Although I am sure qualified people do exist, I would have to personally know someone’s work before I could recommend them as being able to properly restore your magneto. However, in your search for that person keep in mind that it is not that you get what you pay for, it’s that you very seldom get more than you pay for. Even though a $150 quote for labor is highly unlikely to get you a $750-level rebuild to as-new performance, paying $750 still may only get you a bead-blasted housing that disguises a $150 repair of dubious quality. To end this thread on a positive note, when looking for a magneto rebuilder, Trust, but verify. Send questions or comments to classic.vehicle.electrics@gmail.com.

About the Author

Although he wrote these posts anonymously, I can now tell you, with his permission, the author is Dr. Charles Falco. The detailed insights into the physics of magnetos should not be surprising given that he has published over 250 scientific manuscripts, has 7 U.S. patents, and holds the rank of Fellow in four scientific and engineering societies, including the Institute of Electrical and Electronics Engineers (IEEE).  He also is a life-long motorcyclist, was co-curator of the Guggenheim’s record-breaking “The Art of the Motorcycle” exhibition, and has lectured internationally on magnetos and other motorcycle subjects. 

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One Response to Classic Harley Tech: Final Article On How To Rebuild a Bosch ZEV Magneto

  1. Paul says:

    Many thanks for posting this series of rebuilding Bosch magnetos! Tremendous amount of valuable information and tips!