View Full Version : Need suggestions for machining magneto part

07-01-2011, 07:44 AM
I got another DOA Bosch DU4 mag to play with. I don't know why I like this model so much, I now have 4 of them but only one engine they fit. I've rebuilt the others, but never came up with this problem before. Some early mechanic made the high tension pick up spool out of wood. I guess it must have worked for a while.
The remains of the spool can be seen behind the tapered shaft. Along side is a stock spool from another magneto. Not at all uncommon to find a broken spool or to break one trying to remove it from the shaft, they are fragile bakelite but can usually be repaired with epoxy.
This stock spool I repaired with epoxy, but belongs to another DU4 I'm rebuilding, so I need to either find a replacement, oooor..., try to build one.
The lug on the coil side of the spool fits in a hole in the coil mount. It has a small hole to accept the coils high tension lead which contacts the brass commutator.
I would like to hear suggestions as to what material the spool can be made from, as it needs to insulate the high spark plug voltage, and construction methods.
Ideally the brass commutator ring would be cast in, but that may be a tad impracticle for me to attempt. I'm sure the ring could be added after trhe spool is machined, either soldered or epoxied in place.
Comments and ideas are very welcome.
Forgot to mention that the coil in the armature shown on the first picture is a new replacement coil and belongs to the other magneto I'm working on.

Weston Bye
07-01-2011, 08:12 AM
The best way would be to use the same type of material and method: molded from bakelite. I could do it, but would have to do several hundred to make the effort worth while....or economical.

Molding, or casting, from some lesser material, some sort of epoxy might be possible, but the epoxy would have to withstand the voltage and probably heat. You would still have to make the mold - four parts would be required.

The biggest issue would be voltage - those big flanges are there to prevent stray arcs. I'm surprised that wood worked for more than just a short time. Maybe it was well varnished.

Turning two flanges to fit on a hub carrying the slip ring might be a choice, but a built-up assembly might allow gaps for a stray arc, and once an arc has passed, it usually leaves a carbon trace allowing the next arc to pass more easily. Eventually, pyrolysis of the surrounding material occurs, further degrading the insulating qualities.

07-01-2011, 12:04 PM
I don't know how they make FRP (fiber reinforced plastic) but I wouldn't be afraid to experiment with a project like yours. Think I'd mix epoxy resin with all of the fiberglass mat it would hold and see what happens.

07-01-2011, 01:15 PM
I think Weston is on the right track. If it were me I'd make the hub with a couterbore on either side and a mating ring on the flanges. Assemble with insulating varnish (GLP is the one that comes to mind). It should hold ok.


07-01-2011, 04:16 PM
Fibrglass and epoxy are both excellent insulators, and pretty much any epoxy will withstand heat up to 300F or so, sometimes much higher temps. You could make a two sided mold from wood, wax it up using floor wax or even a proper resist material, then lay the glass cloth and epoxy it. Go a few layers for strength, add some extra epoxy in the center hole so you can machine it for fit, then break the wood mold off. It's going to be better than bakelite, which is an old and highly touted material, but prone to breakage, and only reasonably resistant to high voltages. It was the material of choice in years past, but it doesn't surpass epoxy/fiberglass in toughness or voltage standoff value.

You're basically making a spool. The brass ring would be inserted into the center of the wood mold halves, and the glass epoxy work would be done on the outsides of that, with fibers passing through the center hole. A couple of coat hanger wires sticking into the wood gives you a handle to hold it. Pretty easy work really.

07-01-2011, 04:59 PM
How about using some fibreglas circuit board. You could even use recycled circuit boards from defunct/obsolete/broken radio, vcr, computer motherboard, burned up power supply etc

John Stevenson
07-01-2011, 05:17 PM
Made plenty of these in the past when I was racing. Two top hatted washers made out of Tufnol, I think you call it Phenolic ? and a brass ring.

All held together with Araldite and left to set with a big weight on top, then finished machined in the lathe.

Don't think I have any left but somewhere I have the extractor for getting the bearing inners off without doing any damage to the bearing ring which is what used to break most slip rings, prizing against the ring to get the race off.

Did plenty of magneto modifications, never any rewinds or overalls just alterations to get them to look period and similar.

Managed to buy a load of BTH single cylinder magneto's once that came off those road thumpers, what we used to call Irish motorbikes on account they had no wheels :o

These were the same as the bike mags but has an impulse driver arrangement and fixed spark.
The impulse driver was no problem as it just bolted on but I got some bronze end castings done and converted the cam end to manual advance and retard by cable which was period for the Manx Nortons.

07-03-2011, 07:41 AM
Hi guys, thanks for the suggestions.
I'm quite leaning torward the two half spools epoxied together. As I stated, casting would be impracticle for me.
If I recall, I have a few phenolic aircraft pulleys stashed somewhere, a possible jumping of point. And now I'm thinking I have some pieces of phenolic sheet. Got to dig a little and see what I come up with. The wheels are turning.

Many Thanks


07-03-2011, 07:52 AM
G7 (http://k-mac-plastics.com/g7.htm)

07-03-2011, 08:29 AM
Try here for parts;


07-03-2011, 11:19 AM
1) Make it in 3 parts.
2) Part one is the metal ring, maybe a piece of brass tube if you can find the size.
3) Part two is a thick flange of the proper diameter (so you can machine the protrusion as well as turn the flange. Turn a length of it to fit part one and half the length of part one.
4) Part 3 is a flange and the other part to fit in part one.

I would start out by making an arbor that matched the bore of the original part and had a shoulder and threads so you could mount the part blanks both to turn them to diameter and hold them in position when you glued/epoxied all three parts together. You could bore/ream Part two and three before making the arbor so you dont have to dial in the arbor.

07-03-2011, 11:29 AM
Make it from a scrap of PEEK (Polyether ether ketone) or glass reinforced PEEK from a local machine shop (one that machines for medical or areospace). Tough stuff, takes high temps and voltage.