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How To: More Magneto Rebuild Tips.

February 2, 2012

Earlier articles on magneto rebuilding are here and here and here



Three magneto armatures, from 1910’s Bosch units.  The left one is the original Bosch parts.  The middle is a very good modern rebuild, the right unit is a GREAT modern rebuild.  Knowing the differences between these three is difficult, but will help the rider to understand the quality of the work.

#1 Is a serviceable unit, and still works ok.  But the capacitor/condenser is slowly dying.  They all are, but this one still has some life left in it.  The shellac type insulation is still in place, and prevents arcing.  Windings have not broken down, bearings are ok, etc.

#2 Was rebuilt a few years ago.  It has a new cap that should last a long time, new windings, new insulation, etc.  The taper on the drive end to mount the timing gear could be a bit smoother.  There is no safety gap at the cap mounting (see below).  The insulation material was well chosen, and didn’t get hot and melt.  A lot of rebuilt magnetos on the Motorcycle Cannonball had an insulator material that melted and locked up the armature when it stuck to the inside of the mag housing.  It was not a happy time for the riders, nor for the rebuilder who had the wrong batch of material supplied to a more than a dozen mags on the ride.  The only happy folks were the Fedex shippers, as mags were sent overnight from the road to the shop, then back again the next day, then back to the shop for another rebuild, then back on the road a 3rd time, over and over again.  I understand that the material was a bad batch, and subsequent rebuilds have been fine.  But be careful.

#3 Was rebuilt last year by a retired magneto wizard in Oregon.  The driver taper is clean, the capacitor has been tested for thousands of miles on a test bench.  The insulation material is an epoxy that is used to make bowling balls.  It sounds odd, but it works.  It has been tested at high temps and for thousands of miles on a test bench, and for 30 years on various bikes that the rebuilder has supplied to.  The original design for the safety gap is used (see below).  The slip ring has been rebuilt with new flanges and a new track without any grooves.



Safety Gap:  A safety gap is included in the design of most mags.  If a break occurs in a high tension lead, the spark will jump the safety gap and prevent the breakdown of the magneto insulation.  In some Lucas units, it is designed as a screw that fits down into the slip ring, but doesn’t quite touch the track.  In Bosch mags, it is more subtle, a gap between the armature end and the capacitor mount.  When replacing the cap, some rebuilders don’t notice this and simply solder a wire to the lead of the cap.  But the careful technician will replicate the gap, as shown the photos below.

IMG_7812 - Copy


IMG_7813 - Copy

IMG_7814 - Copy


Bearing Race Puller:  If you decide to open up your magneto armature to replace the windings or the capacitor or the slip ring, one hurdle is that you must remove the bearing race.  Do not attempt to pry it off with a flat blade screwdriver, or to fit a 3 fingered puller behind it.  That will just break the slip ring.  But a simple puller can be fabricated from a round bar of steel and a few screws.  These photos show the general idea.  Bore the steel bar to clear the drive end of the armature, then cross drill and tap three holes.  The three screws should have rounded ends so they fit down into the groove of the bearing race.   The end of the puller is drilled and tapped for a large screw that is used to push against the end of the armature.  A screw that is as wide as the armature is a good idea, and/or create a cap piece to prevent any damage to the armature.  Thanks to Dave at Moathouse Magnetos for the puller idea!bearing race puller

puller on Bosch armature


How To Make Ignition Points:  An old magneto rebuilder has been giving me some tips on how he has been able to service mags for the last 30-40 years.  One item was that it can be difficult to find points.  He now makes points, and it isn’t too hard.  First, find a set of hex head screws and nuts that fit the points plate.  They are likely to be metric, but may be British BA or even Unified? on later units.  Then get a TIG electrode from a welding shop, made from tungsten.  These are made in various diameters, pick the one to fit your screw head.  Then part off two thin slices, say .050” thick.   Silver solder them to the heads of the screws.  A bit of work with a fine file may be needed to clean up the surfaces.  And don’t forget that one point is fixed, it will need a shorter screw than the adjustable point.points


Setting The Timing:  This can be done on the side of the road with a bit of solder or a stick and some paper. 


The old books always recommended using a piece of cigarette paper between the points, but if you don’t roll your own smokes, you can use a receipt from an ATM.  It is thin paper and works just as well.  The bent rod is a piece of solder that I’ve notched with a pair of dykes to show the piston at TDC, and also another notch a bit before TDC.  How much before depends on your bike, but it might be 7/16” or so.

  timing the excelsior magneto

The procedure is fairly simple.  Put the bike in top gear, place the paper between the points (mounted inside the mag of course), then roll the back tire forward slowly and watch when the paper becomes loose.  Just use your left hand to gently pull on the paper and you’ll feel it slide.  Your right hand can rotate the back tire.  The place the solder or rod or a tree branch or whatever down the hole and see if the piston is at the right place.  If not, loosen the mag drive, move the piston to the right place, and re-attach the magneto drive.  Be sure to check it twice when you are done.  It never seems to be perfect on the first try.


magneto schematic

18 Comments leave one →
  1. Tom permalink
    February 2, 2012 12:39 pm

    This is outstanding tech info.
    Thank you!!

  2. Anonymous permalink
    February 2, 2012 9:13 pm

    thanks Pete ,, it was amazing how many mags went bad on the cannonball,, fortunately my rebuilt mag went fine,, real good article

  3. Vince permalink
    February 2, 2012 9:20 pm

    Great stuff Pete thanks Vince

  4. Chris Potempa permalink
    February 6, 2012 8:19 pm

    Pete —

    The use of tungsten for points in lieu of platinum contradicts what Steve Blancard has to say on the subject via his website. There was a reason platinum was used for magnetos and tungsten for battery ignition systems and the two aren’t neccessarily interchangeable — for very long.

    Second, for those concerned about riding with marginal magnetos I’ll share a piece of advice Francis Clifford (R.I.P.) told me that should also give others some piece of mind: carry a spare 6 volt car-type ignition coil in your tool box. If the mag craps out you simply wire the coil in series with the points of the mag and battery making the set-up into a battery-coil ignition system until repairs can be made. This works great for singles but for those with twins you’ll have to get creative with tying in the high tension wiring to make use of the magneto distributor as well.

    Finally, as ammo for those in defense of magnetos over battery-ignition set-ups it’s helpful to remember that all piston-powered aircraft still use the former under the cowling as opposed to anything else.

    • February 7, 2012 4:12 pm

      Hi Chris. Good to hear from you. Some of the mag guys that I’ve talked to said that platinum was only used in the very early days of magnetos (stopping around 1910). But I haven’t read much about that in the motoring press. But you & Steve might be right!

      I just read Steve’s website and he does say not to use tungsten. And Independent Ignition Supplies will sell you set of platinum magneto points for about $110 plus shipping… (custom made only. made to order, not stocked on the shelf) I wonder how many guys are really using platinum and how many are buying tungsten points? hmmm. I’d have to study my mags a bit closer to know myself. All the points I’ve bought in the past were $10 a set and lasted for years…

      Good tip from Clifford. He knew a few things! I’ve never tried it, but I’ve heard that works well and is easy to do if your mag has a cutout switch on the end cover (to attach the car coil to).

      take care. see you on some rides this summer?

      • Chris Potempa permalink
        February 7, 2012 6:14 pm

        Good lord, $110 for a set of platinum points?!? Okay, I can now see the merits of tungsten in a pinch….

        No plans to come back north for at least another year and I’m working on the purchase of a vintage sloop so that alone will keep me busy too.


      • tom permalink
        February 16, 2012 7:00 am

        Good tip for most mags but not the Bosch’s.There is no wire to the points as they are spinning on rotor.

  5. charlie 101 permalink
    February 10, 2012 11:38 pm

    You can try BMZ shop for new made points and paper flowers in different thickness and sizes. They have new coal brushes and all sort of stuff as well. Klick “ersatzteile”

  6. charlie 101 permalink
    October 26, 2012 1:16 pm

    Regarding the safety gap. A tecnician, a lot smarter than me, pointed out that the metal tounge visible in the picture is a fusing point for the condenser, breaker contact and both the primary and secondary coil. The sec.coil has a resistance of about 0.5 ohm, and the other end is connected to ground. In reality the potential can’t rise at that point as it is practically grounded, so that metal tounge will never work as a safety gap.

    The Bosch doesn’t have a safety gap. When the Hi tension lead to the sparkplug is broken the voltage in the secondary coil rises and it is up to the insulating material to withstand the stress. Later or on other magnetos the safety gap was introduced to avoid the stress on the coil and insulating material.

    The wiring diagram is wrong for the Bosch in question. The secondary coil is grounded via the primary at the fuse point coil/points/capacitor.

  7. diego permalink
    January 11, 2013 12:17 pm

    hello try winding a bosch magneto ff2a 1922 and not turn ratio leads to the primary and secondary. isolate nor as proper activation. spool when I function well, but as da spark catching loses spark revolutions and quality of work. if anyone can give me advice would greatly appreciate it. carefully diego of Spain

    • January 11, 2013 4:59 pm

      Hello Diego, I have never would a magneto armature, but maybe one of the other people who reads this page has. good luck. Pete

  8. April 19, 2014 2:48 am

    Hallo ich würde gerne wissen, wer diese Spulen baut BOSCH Dank

    • April 19, 2014 9:12 am

      Some are from Mark’s Magnetos in Connecticut, and from Moathouse Magnetos in England and also from Lane Plottner in Oregon. Regards, Pete.

  9. cesar permalink
    August 10, 2014 5:09 pm

    hello, I would make you a consultation. I have a magnet lucas of a Royal Enfield. My problem is that the magnet will not pull spark to the spark plug but strip the platin. I wonder if you could give me an indication that could be. thank you very much

    • August 11, 2014 10:59 am

      Hello Cesar. Are the adjustable points on the magneto eroding or being stripped? If so, that indicates an issue with the capacitor/condenser that is located in the magneto armature. Replacing the capacitor requires some effort and some soldering, but can be done at home if you are careful. Good luck!

  10. Alex permalink
    December 30, 2014 1:37 am

    Hi Pete,

    Read the article with interest, have to check on the safety gap on my rewound Bosch armatures.

    Have a better bearing puller then pictured above, see pictures here;

    You can see how it works, the big ring slides over the body, and contracts the 4 pieces over the bearing groove, then turn the lever, and the bearing will come off.

    There are even much better pullers, but very, very expensive, and only big magneto rebuilders use them, have seen one in action at a rebuilder in the UK, he told me they were 5000 pounds to buy, and were made by Lucas at the time, will take pictures next time I see him.

    Keep up the good work,


    ps, email me and I send the pictures direct.

  11. March 6, 2017 8:27 am

    I know this is very old, but care to share who the, “retired magneto wizard in Oregon” was?


    • March 6, 2017 10:31 am

      Lane Plottner. He rebuilt mags for decades, and they still work well.

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