Classic Harley Tech – Rebuilding a Motorcycle Magneto – Reassembly, Remagnetizing & More

This series of articles is reported here with the permission of the author who as asked to remain anonymous.

Finally, this Bosch ZEV magneto is ready to go back together. After cleaning off the unknown grease that the rebuilder had used on the bearings, I packed them with Sta-Lube high temperature disc brake bearing grease, applied a small amount of Lubricam to the cam and its pivot, and reassembled the magneto. I also put a yellow tag with a note on one of the oil cups telling my friend not to use oil, since it would only wash the grease away.End Float

BTH and Lucas armatures have shims for both the armature shaft and for the housing to adjust end float, so just the fact these shims exist indicates it is something that needs to be checked. Especially with magnetos rebuilt by someone else it almost always needs to be adjusted. Since this magneto has a new bearings it means both races were removed and reinstalled by the previous restorer, and the position of the races determines the end float. Anyway, given how many problems I had found with this magneto, checking the end float was especially important. Inserting 0.005″ shim stock between the end cap and the main housing and then measuring the end float of the armature let me determine that the end float would be 0.000″ with no shim at all. I don’t know if Bosch ever published specifications for the end float for this magneto but, even if they did, I don’t have them. However, the only reason to use shims to get a positive end float would be if the thermal expansion of the armature was significantly larger than that of the housing, resulting in undue pressure on the bearings at operating temperature. Several years ago I made careful measurement of the thermal expansion of a Lucas armature and housing constructed of similar materials as used by this Bosch. Those measurements, plus the fact Lucas called for zero end float (0.005″ max.) for their magnetos, makes me confident in using zero for this Bosch ZEV.

Contact Spring Pressure

An important measurement is of the spring pressure on the contacts. If it is too small the points will float at high rpm, and if it is too large the rubbing block will wear too rapidly. Because removing the cam leaves the rubbing block exposed on this magneto it is particularly easy to measure, as is shown in the next photograph taken using my ZE1.

Lacking specifications for this pressure from Bosch required deciding on a reasonable value for comparison. Literature from various magneto manufacturers has at the low end of the recommended range for tungsten points 10-18 oz, and at the high end 15-30 oz. Consistent with this, I measured my Vincent’s Lucas KVF to be 24 oz, although its spring needs to keep the rubbing block in contact at nearly twice the rpm as does the 1923 Harley-Davidson’s magneto. Complicating this further is that when I removed the ZEV’s moving point arm from the assembly I found its balance point was slightly towards the contact end. Because of this, centripetal force will tend to open the points against the action of the spring, and this force increases as the square of the rpm.

With the above as background, I measured both the Bosch ZEV and the ZE1 at 13-14 oz. Although it is encouraging that they are the same, unfortunately, it may only mean their springs have weakened with age by the same amount. However, since 14 oz. is a reasonable value, although toward the low end, I decided to proceed with testing. I made a note that cannibalizing the spring from my ZE1 to use to double the pressure on the ZEV is an option, depending on what I find from my dynamic tests of the magneto (although this would move the pressure from near the low end of recommendations to near the high end).


It’s impossible to properly restore a magneto without being able to remagnetize it, so several years ago I made an appropriate electromagnet. I based its design on requirements given in the 1953 Lucas Workshop Instructions booklet Remagnetisation of Magnetos, after also checking specifications for magnetizing Alnico in several texts. The Lucas booklet calls for an electromagnet with a core winding value of 65,000-70,000 A-turns in order to magnetize their post-WWII Alnico-based magnetos.

Incorporating construction principles detailed in Laboratory Magnets by D.J. Kroon (Philips Technical Library, 1968), the electromagnet I built weighs several hundred pounds and consists of ~4500 turns of 14 AWG wire of total resistance 12.9 Ohms wound on a yoke made of Armco magnet iron. Since its inductance stores a serious amount of energy at full current, and such a DC current is difficult to interrupt without arcing, I use a 20 Amp Variac to ramp the current up and back down over a few seconds. Also, having an electromagnet whose field can be varied continuously, rather than only operate on/off, has other advantages for experimentation on magnetos. In any case, applying the full 240 V of rectified AC from the wall results in 18.6 Amps, and therefore 83,721 A-turns, which is comfortably above the values Lucas recommended for remagnetizing their Alnico-based magnetos. I use a clamp-on ammeter during operation to verify the applied current, and also used this ammeter along with a Bell digital gaussmeter to determine the full field vs. current curve of the electromagnet.

Interchangeable Armco iron pole pieces have faces that are shaped to closely conform to a variety of Lucas and BTH rotating armature and rotating magnet magnetos to minimize flux leakage. Further, I have pole pieces to remagnetize Lucas rotors from later motorcycles. Actually, the operating fundamentals of magnetos don’t leave a lot of room for unique designs, so these pole pieces also work on Fairbanks-Morse (and A.R.D., Joe Hunt, and Morris), Splitdorf, Wico, etc. If I ever needed, I also have blanks to machine into whatever shape is required. However, simple flat pole pieces are all that are needed for this Bosch magneto.

Because the magnets on the Bosch ZEV are exposed they can be placed in direct contact with the pole pieces, so the amount of flux lost to “leakage” is significantly less than with a post-WWII magneto where the Alnico is encased beneath a shell of aluminum. Even if this were not the case, the ~84,000 A-turns of my electromagnetic is over 2x higher than that needed to fully magnetize the tungsten steel used for the ZEV’s magnets (as well as higher than that needed for the later cobalt steel that preceded the Alnicos), so there is no question this electromagnet is able to fully magnetize this magneto. The next photograph shows the magneto ready to be magnetized. I already have attached the aluminum pulley that I normally use to drive magnetos at 2000 rpm on my long-term tester (more about this in the next installment).

For the magnet to be left with the maximum remnance the armature must be oriented correctly when in the electromagnet. However, the precise orientation isn’t too critical, so this is easy to do by rotating the armature “backwards” by ~90-deg. from the position where the points are about to open (i.e. turn forward until maximum resistance is felt, then back by ~90-deg.). Although it only requires one cycle up to the full magnetic field to magnetize a magneto, I ran the magnet up a second time for good luck. After magnetizing the Bosch ZEV I attached the two HT cables and found that just a gentle flick of the armature gave an impressive spark.

Note, though, that while it is comforting to see a spark, such a “flick test” is often incorrectly used to claim a magneto is functioning properly. First, if a small gap is used for this test (e.g. the ~0.02″ of a spark plug), a much smaller voltage is required to create a spark than will be necessary at the ~150 psi cylinder pressure during actual operation. Second, the instantaneous rpm when the armature is flicked through the position where the points open and the magnetic flux reverses typically is higher than it will experience at tickover speeds, again deceptively making it appear that the magneto is performing better than it actually is. Only a test using an appropriate gap (~0.2″) and under steady-state operation can properly determine if the magneto is functioning as it should.

———– Sidebar About Magneto “Chargers” ———–
Since it seems to be a common misunderstanding by many people who restore magnetos, it is worthwhile explaining why an electromagnet designed for earlier magnetos will not fully magnetize a post-WWII Alnico-based magneto. Just the fact the magnetic energy stored in Alnico is over twice that of the previous generation of Co steel magnets indicates a higher field electromagnet is required. Also, post-WWII magnetos have their Alnico magnets encased within an Al housing so the pole faces of the electromagnet cannot be brought into direct contact with them as they previously could be with the older horseshoe magnets. Because of this, some of the magnetic flux from the electromagnet “leaks” away through the Al housing without reaching the Alnico and as a result an even larger electromagnet is required for these “modern” magnetos than otherwise would be needed.

As an aside, the field produced is determined by the current and number of windings, which in turn determines the wire diameter, operating voltage, and overall size of an electromagnet. Because of this, someone who is familiar with the design of electromagnets can tell just by looking at one if it is capable of magnetizing Alnico. Just as someone who is familiar with engines can tell just by looking at a BSA Bantam and a Vincent Black Shadow that one is capable of 120 mph and one is not.

An electromagnet must be capable of driving the magnet into full saturation in order that it be left with the maximum remnant field once the magneto is removed from the electromagnet. The 2672 Oe of my electromagnet is ~20% more than that needed to fully magnetize a magneto containing anything from the Alnico family (Alnico, Ticonal, Alcomax, etc.), and it is 2.3x more than needed for older steel magnets. Stated differently, the field from an old magnet charger is a factor of ~2x too low for Alnico. This leaves an Alnico-based magneto less than fully magnetized (however, magnetic properties are nonlinear, so the performance is degraded by less than 2x). Although the field from earlier chargers is enough to magnetize an Alnico-based magneto sufficiently to spark an engine when it is kicked over at a higher speed, the magneto will have significantly degraded performance because the Alnico is only partially magnetized. The effects of this will be most pronounced in the form of harder starting and missing under load at low speeds.

Despite these scientific facts, some restorers insist their old chargers work “just fine” with Alnico, and that their customers are happy with the results. If you would accept as “just fine” the performance of your car if returned from servicing with a plug wire disconnected, you can accept the use of an old magneto charger on your post-WWII magneto. Otherwise, not.

Another important point is that a magneto has to be magnetized after it is fully assembled. The negative consequences on performance of magnetizing it with the armature removed (or removing and replacing the armature after magnetizing it) are amply documented in a various authoritative texts on electromagnetic devices. For example, in their chapter ‘Magnetising and Timing a Magneto’ in ‘Automobile Electrical Equipment’ (Iliffe, 1958), A.P. Young and L. Griffiths write that if the armature is removed and replaced “The flux density instead of being of the order of 10,000 lines per sq. cm., would be more nearly approximate to 7,000 lines per sq. cm…,” i.e. the output of the magneto would be reduced by approximately 30%. In ‘Permanent Magnets’ (Pitman, 1949), F.G. Spreadbury shows that the output from a magneto with a Ticonol (“Alnico”) magnet is reduced by 23% in actual operation if the armature is withdrawn and then replaced after magnetization. Even the Lucas shop manual on ‘Remagnetisation of Magnetos’ says “… it is necessary to remagnetise them, particularly after an armature or rotor has been removed from a magneto for repair or examination.” My own measurements are consistent with the 20-30% reduction documented by these authors.

Despite the information in the previous paragraph, one company offers a service that magnetizes magnetos with the armatures removed from them. Further, they don’t merely say that it is “just fine” to do it this way, but actually claim their measurements show it is just as good as magnetizing them when fully assembled. To paraphrase a line from a Marx brothers movie, “Who are you going to believe, this company, or all those lying books?”

Like the use of older magnetizers, people who have their magnetos remagnetized with the armatures outside them are not getting what they pay for. As the references cited above show, these magnetos have 20-30% degraded performance, depending on which member of the Alnico family is inside. By the early 1950s Lucas had used Alnico, Alcomax, Ticonal E, Ticonal G, and Alcomax 2, and my guess is that by the end of the decade they also had used Alcomax 3 and Alcomax 4. Although there is no reasonable way to determine just which member of the Alnico family is inside a given magneto, the B-H loops of all are similar enough that the figure of 20-30%” degradation should cover all possibilities.

Once again paraphrasing Bruce Springsteen’s ‘Magic’, as far as magnetizing a magneto is concerned, “Trust none of what you hear, and less of what is claimed.” Only a large, 65,000+ Ampere-turn electromagnet used on a completely assembled post-WWII magneto will fully magnetize it. Further, if you remove the armature for any reason from one of these magnetos (such as to replace a faulty condenser), it will have to be remagnetized after it is reassembled or it will have 20-30% lower output than it should have. It is even worse for a pre-WWII magneto, which won’t effectively function at all until it is remagnetized.

A few final comments about the magnets: An often repeated piece of advice when restoring magnetos is to immediately place a steel “keeper” across the poles of the magnet as soon as the armature is removed to keep the magnet from losing its strength. Unfortunately, this advice is wrong. No matter how fast you are, a keeper will do you no good at all because the magnetic domains rearrange themselves nearly instantaneously (less than a millisecond). However, enough strength will be left that the magneto will still spark across a spark plug at atmospheric pressure, so you might think things are fine. They are not. A few other wrong, but harmless, pieces of advice you might run across for achieving full magnetization include charging the magneto 4-5 times (once is enough), holding the electromagnet at full field for a number of seconds (a fraction of a second is plenty), and tapping the magneto with a brass hammer while the field is applied (harmless, but pointless).
———– End Sidebar About Magneto “Chargers” ———–

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