How To: More Magneto Rebuild Tips.
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.
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!
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.
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.
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.