Magneto Magnet strength ?

[Mharrisbarton]

I know that this subject can and has in the past caused some controversial issues, I make no claim to have anything but a basic understanding of the subject, so here goes! I have over the years had several magnetos refurbished by respected people in this field, no pun intended "nice Job at a reasonable price", but when it comes to the field strength this seems to be ignored, I can remember turning over by hand magnetoes that needed a strong flick to get them to turn over and they would spring back to their neutral position, none of the mags that I have had done are like this. On my 1938 VH I got fed up with the unpredictability of starting (good points,set correctly, everything clean,good earthing etc) that I invested in a BTH electronic mag with built in advance and retard, what a difference! so easy to start and at 73 this is beginning to be an important factor, before I bought theBTH I took the MO1mag back to the man who had rebuilt it some years before, he put it on his test rig ran it up to1000rpm and said there you are lovely spark nothing wrong with that, but I am pretty sure that I can't kick the bike over at 1000 rpm!! The cost of fitting electronic to all my bikes is financially prohibitive, and originality is important,so this has been a very long winded way to my question, Where can I get my magnetoes remagnotised? when I have contacted several mag rebuilders about this issue they have all been rather vague, thanks in anticipation Matt Barton.

[will_curry]

Matt,

The link from the Magneto Guys' site (https://www.themagnetoguys.co.uk/remagnetising-magnetos)
to a Lucas publication on remagetisation is worth following.

[nevhunter]

A lot there. Magnet strength and air gap (Don't turn your armature undersize to make it look shiny) Points should open quickly and at the right time in relation to the magnet flux, and a good condenser and you should be right for about 15 years. There's nothing wrong with a magneto when it's done right. It's not dependent on a battery. Start with a LITTLE bit of advance, use the flywheels inertia and don't open the throttle wide or you'll have a cylinder full of mixture to deal with instead of a part filled cylinder. Technique is a a big part of it. Nev

[cmfalco]

the man who had rebuilt it some years before, he put it on his test rig ran it up to1000rpm and said there you are lovely spark nothing wrong with that,

A Lucas manual says 300 rpm is the low end of kick-starting speeds, with 500 rpm normal. What this means is people with grey hair would be well advised to avoid any magneto restorer who does not have a large electromagnet in their shop to remagnetize the magneto after repairing it. A magneto with a partially-depleted magnet still will work fine at high rpm, but it won't supply sufficient voltage at "normal" kick-starting speeds. A remagnetized magneto will start an engine at 300 rpm, which is what any grey-haired motorcyclist should insist upon.

A magneto loses 20–30% of its magnetism the instant an armature is removed and the only way it can be returned to as-new performance is with an electromagnet capable of "saturating" the internal magnet. To do this for a post-war (alnico) magneto requires an electromagnet capable of 65,000-70,000 ampere-turns. Pre-war tungsten-steel magnets need only about half of this but, in either case, the electromagnets are large.

The electromagnet I built for myself weighs several hundred pounds and produces 83.7 kA-turns, which is comfortably above the values Lucas recommended for remagnetizing their Alnico-based magnetos. I should add that more A-turns than required is neither good nor bad, since the magnet "saturates," but having additional capability in reserve ensures that the magneto receives the full amount. In any case, hiring someone without an electromagnet to restore your magneto is like hiring a man without a pole to supply your fish for supper.

[nevhunter]

ALL true. My magnetiser is a very heavy professional one and you can't have too good a one unless you have a pacemaker. Air gap and magnetism both affect the spark at low revs and the instantaneous spark where the piston is before TDC can be quite slow. Leakage of the condenser affect slow spark also and will show up as points wear and sparking at the points. Nev

[Mharrisbarton]

Many thanks for the replies,I remember reading the "magneto" man's posts on the britbike forum some years ago very interesting, very technical and if I'm honest a lot of the information was outside my knowledge base, mechanical engineering is my best practical subject. so I assumed that the man I took the mags to would remagnetise them as a matter of course ,I was obviously wrong!he had a good reputation. My problem now of course is where can I get them remagnetised?many thanks Matthew Barton.

[cmfalco]

sparking at the points.

An excellent, "non-invasive" test of the condenser is to look at the points on a running engine. A good condenser will result in nothing more than tiny pinpricks of light between the points.

I should add that one of the magneto testers I have allows me to install a magneto armature without its capacitor, and then add a capacitor of my choice externally. Using a sensitive spot light meter with close-up lens, I measured the brightness ("luminance") at the points when a series of capacitors of measured "badness" (ESR -- electronic series resistance) were placed in the circuit. Without going into the details, ranging over a factor of ~100× of both, basically there was a linear relationship between the brightness of the sparks at the points, and the "badness" of the capacitor. What this experiment means is the brightness of the sparks at the points is an excellent substitute for an electrical measurement of the condenser, which only can be done by cutting the condenser from the circuit.

The following photograph shows what I mean by using the sparks to diagnose the condition of the condenser, although it is deceptively bright because it was a 3 sec. exposure showing a composite of ~75 sparks.

[nevhunter]

For clarification the air gap is between the armature and the pole pieces. In a Dixie its the armature end float. The magneto I have the Most difficulty with is the Lucas Magdyno because you can't get contact with the Magnet itself except on the earlier models where it's removable and you'd need a keeper there to assemble it in the best way Nev

[cmfalco]

The magneto I have the Most difficulty with is the Lucas Magdyno because you can't get contact with the Magnet itself except on the earlier models where it's removable and you'd need a keeper there to assemble it in the best way

I don't understand. You can't get make direct contact with the magnets on KNC, KVF, etc, either, because the magnets are encased in the aluminum housings. As the Lucas diagrams show, each of these requires shaped pole pieces for the electromagnet, to bring them as close as possible to the encased magnets in the magnetos. A keeper will do no good for any of these because the loss is nearly instantaneous when the armature is removed. No matter how quickly someone works, the horse will have been long gone from the barn before a keeper could be inserted. Or, if the bare housing is magnetized with a keeper in it, the same nearly-instantaneous loss will happen when the keeper is removed to swap it with the armature.

[nevhunter]

As I said the "earlier" model has removable magnets and these would need a keeper. while the Magnet was being moved. Alsowith the armature in place a lot of the magnetic lines of force end up going through the armature. Nev