The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: RichardL on 19.02. 2012 00:15

Title: Magneto Magnet Strength
Post by: RichardL on 19.02. 2012 00:15
Hi All,

I am working on my magneto with plans to change slipring, add EasyCap from Brightspark Magnetos, replace brushes, check for eccentrcity (a la A10GF's "My Big Magneto Rebuild Project"), etc.  In checking the magnets, their strength seems less than impressive. I'd like to hear some descriptions of how to judge the correct maget strength without requiring a guassmeter or other test equipment that I don't have. Otherwise, maybe offer opinions of what adequate magnet strength should feel like.  As a test, I stuck a ½?-ratchet drive in the magneto body and there was enough strength (and more) to hold the weight of the drive by the square drive peg while horizontal. Does that make sense?

Richard L.
Title: Re: Magneto Magnet Strength
Post by: MG on 19.02. 2012 08:50
Sounds good to me, Richard. They are not really impressively strong as you say, by no means comparable to modern rare earth magnets.
I generally found the AlNiCo magnets used in the 50s/60s would hold their "charge" (as in magnetism) much better than the older ferritic things.

You got a new coil in your mag already? If not, this might be a good chance to get it rewound while you are at it...
From reliable sources I know that Brightspark have successfully tested their K2F remagnetizer just these days.  *grins*
Title: Re: Magneto Magnet Strength
Post by: LJ. on 19.02. 2012 09:24
I would have thought that some hand held luggage weighing scales attached to a bolt and pulled off giving some kind of lbs of reading would have been acceptable. This method although crude, would enable some of us to compare with each others spare magnetos. Just a though...

 Richard, I think your ratchet drive experiment would be more self explaining with pictures. I'm guessing this thread is going to need a lot of pictures!  *lol*
Title: Re: Magneto Magnet Strength
Post by: RichardL on 19.02. 2012 14:08

Thanks for the input. I was not expecting neodymium-like strength, but the ease with which a screw driver pulled off didn't seem quite right. Then, when I used the ratchet drive, there was a better sense of some strength. For one thing, the ratchet drive has more iron to affect, then, it is somewhat radiused, making it a fairly close fit to the radius of the magneto poles.

I think I am going to go ahead with the existing coil, for now. I was not completely without spark and, if a new coil is needed, a second dissassembly of the mag after the install of the EasyCap is but small trouble compared to replacing the coil. I will be using the "Quick Snip Condensectomy" per the Brightspark instructions.


OK, I came up with some photos to show the method and applied a scale similar to your suggestion. At this point, I can confirm that the end of the ratchet represents 0.9 Kg. It seems somewhat more tricky to come up with a standardized method to determine the ultimate release strenth. Torturing the idea, I see a rod with a contact lobe on it (all of known dimension and material specification) routed through the magneto and with weight hung from both ends (or a tension scale, like our photos) until it releases.

Richard L.

Title: Re: Magneto Magnet Strength
Post by: KenF on 19.02. 2012 21:52
Hi Richard. Thanks for purchasing one of our Brightspark EasyCaps.

I agree 100% with MG. If you have played with these new-fangled rare-earth neodymium magnets, then the alnico magnet in the Lucas K-series mag ain't too impressive, but the good thing is that they don't age quickly and they're fairly insensitive to the temperatures experienced in our mags (unless there is a complete catastrophe).

My pet method for checking the strength of a K-series mag's magnet without any specialised instruments, just a compass, is this:

1. Put the compass on the floor without any metalwork nearby, and line up the scale so that the needle's pointing North.

2. Draw a (pretend) line on the floor going due West for 15 inches from the compass.

3. Put the mag housing on the floor at the end of the (pretend) line, flange downwards, with the magnet in the housing (the bulgy side) facing South.

4. If the mag housing has skewed the magnetic field at the compass by 45 degrees or more, then the magnet is OK (i.e. the needle is now pointing  NW or more westerly than that, or NE or more easterly then that).

5. If the mag housing has skewed the magnetic field at the compass by 60 degrees, then the magnet is very, very good.

6. If the mag housing has skewed the magnetic field at the compass by a lot less than 45 degrees, then it's probably time to have the mag body remagnetised.

There are some limitations on this method. It works fine in Hampshire, UK, where the Earth's magnetic field is about  48 mTesla ( and dip is about 66 degrees down ( It's probably OK for the rest of the UK and Chicago. Elsewhere, people may need to use different limits.

If you'd like to try this method and let me know how much your magneto skews the Earth's magnetic field, I'll gladly give you an opinion about how lively your magnet is.

Ken (Brightspark Magnetos)
Title: Re: Magneto Magnet Strength
Post by: bsa-bill on 19.02. 2012 22:31
So Ken me lad it's you that's sending all those trucks up country lanes by upsetting their satnavs *lol*
Title: Re: Magneto Magnet Strength
Post by: RichardL on 20.02. 2012 00:57

Thanks for the advice. I will try it as soon as I get a chance.


If the magnet in my magneto could upset satellites, I could fire the spark plugs in your bike from here in Illinois (uuuh, maybe).

Richard L.
Title: Re: Magneto Magnet Strength
Post by: KenF on 20.02. 2012 08:57
By the way, I should have said that the compass test is done without the armature in the body.

With the armature in there, (1) the compass skew is a lot less beacuse less magnetism is escaping, and (2) the thing won't stand up on its drive end  *smile*

Ken (Brightspark)
Title: Re: Magneto Magnet Strength
Post by: groily on 20.02. 2012 10:07
As Ken says, got to get the armature out!
It's serendipity really that this Q came up just when we were starting work on our Brightspark measuring (and remagnetising) kit.
Having lost my boy scout shoes with a compass in the heel about 45 years ago, I just scoured the local town for a decent orienteering compass and I've run the identical tests on two K2F boxes, in northern France. Where I guess the magnetic pole is about as strong as in southern England (about 100 miles away).
The first body was a very old one, not used in years and stored with no armature in it (which makes no odds they say, but still). The second was my known-good spare K2FC in tip-top order and ready to roll. It was remagnetised about 3 years ago.
The unknown oldie showed 50° to 55° deflection at 15" distance (which says something for the retentive powers of those old magnets), and the Comp mag showed a good 60° deflection, maybe 62°.
On which purely empirical basis, I agree with Ken that anything much below 45° deflection  is probably a bit feeble, under the identical test.
But dunno at what point they cease to function, maybe we have to demagnetise one by stages to find out what the lower limit is!
Title: Re: Magneto Magnet Strength
Post by: MG on 20.02. 2012 10:11
Hi Ken!

Welcome to our happy bunch of Brit bike lunatics!
Now all left to do is get yourself a decent pre-unit twin from Birmingham.  ;D *whistle*

Best wishes, Markus
Title: Re: Magneto Magnet Strength
Post by: RichardL on 20.02. 2012 12:17
Bu' bu' but, will not the quality of the compass and/or the extent of magnetization of the needle have a big affect on the results of this method?
Title: Re: Magneto Magnet Strength
Post by: LJ. on 20.02. 2012 12:39
Welcome to our forum Ken and thank you for your description on how to test a magnetos magnet. Markus is quite right about the lunacy here as I just had to rush out to test my magnets!  *lol*

Richard beat me to compass quality issues etc and I wondered how exact the layout needs to be. In my pictures below I have drawn exactly 15 inches East to West and placed the compass and magneto housing exactly centred over each end of the line. Looking at the compass I read about 52 degrees. I presume this is the way to get a general idea of the magnetos magnetism?

The magneto I used here is a spare that's just cleaned up. It will likely need a new capacitor and its windings testing. No idea when it was last fitted to a bike. The magneto has always been stored as a complete unit.
Title: Re: Magneto Magnet Strength
Post by: wilko on 20.02. 2012 21:04
Too much spare time lads!
Title: Re: Magneto Magnet Strength
Post by: beezermacc on 20.02. 2012 22:02
Just looking at all this compass stuff makes me wonder whether a magneto would work at the north pole.....maybe that's why we've never seen a polar bear on a motorbike.

But seriously, I 've never come across a K2F or a MO1 where the magnetism had become so weak the mag wouldn't work after fitting with a rewound coil, new capacitor and proper assembly. Boosting the magnetism helps at very low rotational speed but even at normal kickstart speed there's sufficient rotational armature speed to achieve the necessary switch of polarity through the armature core to achieve enough voltage in the primary winding. Many of us have experienced the bike 'kicking back' when we are just trying to locate compression. This shows just how slowly it is possible to turn the mag and still get a spark, yet we have to give the bike a good kick to turn the engine over quickly enough for the flywheel momentum to take the piston over TDC whilst harnessing the power from the ignited fuel. What is equally important is the air gap between the armature and the magnet laminates; the build quality of Lucas mags unfortunately often leads to quite large air gaps, therefore poor magnetic induction through the armature core. On the other hand, BTH mags are usually very snug. A further issue is the fit of the alnico bar magnet between the laminates which again, on early Lucas magdynos (covered wagon type), was poor so the magnetism did not get conducted into the laminates because of their uneven fit against the magnet. In spite of this these magnetos still perform perfectly well.
Title: Re: Magneto Magnet Strength
Post by: KenF on 20.02. 2012 22:06
Evenin' all. Thanks for the 'welcomes' Marcus and LJ. Nah, don't need to get myself my own Brummy pre-unit twin cos (1) there's no room in the garage, and (2) luckily I have a mate who lets me ride his whenever I want ... a very very special (unique even) A10 which once had exhaust-assisted suspension ... but that's another story (hah, hah).

What you've shown in your excellent pics, LJ, is spot on.

A very good question, Richard and LJ. Any compass of the sort that you can get at the local camping shop will do. Within reason, it doesn't matter how strongly magnetised the compass needle is. All the needle is doing is aligning itself with the ambient magnetic field at the place where it's sitting on the floor. And the ambient magnetic field is the result of the Earth's magnetic field trying to make it point North-South, and your magneto body's magnetic field trying to make it point East-West. I chose the 15 inch distance because at that distance a K2F with a magnet I would consider borderline will twist the Earth's magnetic field in South Hampshire half way, to NE-SW or NW-SE

Having said that, you don't want a great big bar magnet as a compass needle, and you don't want something so feeble it can't overcome the friction of its bearing. Just a half-decent orienteering compass is fine, strongish or weakish doesn't matter.

Sorry if you find this a bit boring, Wilko. Was just trying to answer a sensible question and come up with something a bit more definitive than "Does the magnet grab the tip of screwdriver, voraciously, so so, or feebly?"


Title: Re: Magneto Magnet Strength
Post by: groily on 20.02. 2012 23:00
Some pix I took earlier are virtually identical to yours LJ!  I shan't redo them at lower resolution to get under the threshhold here.
As to 'Why mess about with compasses?', well, just a Q of trying to get a handle on things as useful reference points for 'the next time', and simply of knowing more. Curiosity is good, even if it killed a proverbial cat or two.
As Beezermacc rightly says and MG said earlier, Ks (and MO1 magdynos and most things Lucas with those same materials in) retain their oomph well, but I have seen quite recently one K series with no discernible magnetic pull at all. It certainly wouldn't - couldn't - be made to work as it was.
Remagnetising is a service most of the professional rebuilders seem to offer, so presumably there is some history/need - and of course the Lucas service sheets covered it too in some detail (see eg,%20Sec%20D-6,%20Issue%202.pdf).
Title: Re: Magneto Magnet Strength
Post by: RichardL on 26.02. 2012 23:30
Ken, Groily and, yes, all you poo-pooers,

Just to see how my mag compares with the examples given, and to make some people crazy, I've executed the experiment with the compass. My needle deflection is 41 degrees. However, the earth's magnetic field strength where I am is 54.4 mTesla. With no other way I could figure to draw a comparison between this and the 48.6 mTesla at Hampshire, I just applied a linear factor and come up with a virtual 46 degrees deflection. Hmmm? Not knowing exactly what to make of this, I am sending you my magneto case so you can do the test there. Naaaaah! Anyway, unless someone screams otherwise, I think I'll put it back together with the new Brightspark and the existing coil and magnet. I think I'll know right away if the spark is stronger, and disassembly, if needed, is not a big problem.

Just in case, any suggestions where to get magnetized (so many joke lines here) around Chicago, or in the U.S., for that matter.

Richard L.
Title: Re: Magneto Magnet Strength
Post by: KenF on 27.02. 2012 09:05
Ummm, Richard ... I stand to be corrected, but the following makes sense to me at least ...

What's making your compass needle point North-South is the component of the Earth's magnetic field in the horizontal plane, so you need to multiply by the cosine of the dip angle. So for you in Chicago, it's 54 cos 72 = about 16.7 uT. For me in Hampshire, it's 48 cos 66 = about 19.5 uT.

With the magneto body pulling the compass needle East-West, I'd have thought that there's a tangent relationship, i.e. flux density caused by the magneto = Earth's flux density in horizontal plane times the tan of the compass needle deflection. So, for you, that's 16.7 tan 41 = about 14.5 uT. For me, it's 19.5 tan 45 = 19.5 uT.

It's probably safe to assume that the flux density caused by the magneto 15 inches to its side is proportional to the flux density in the centre of the housing (where you want it) and that the constant of proportionality is the same for any K-series magneto. I know from my uncalibrated (but linear) gaussmeter that the flux density in the centre of my magneto that caused the 45 degree compass deflection was 156 KenFgauss. So, it would seem that the flux density at the centre of your magneto is 156 x 14.5/19.5 = 116 KenFgauss. I also know that if I remagnetise the magneto, it reads about 225 KenFgauss on the gaussmeter. So, that all seems to suggest that you mag has only got 116/225 = about 50% of its magnetism  *sad2*

From experiments I've done, 50% magnetism results in about 50% HT spark current, and more than double the speed required to get a spark in the first place.

Of the magneto people in the States, I've only ever heard good reports about Doug Wood in PA. Google "Doug Wood magneto".


Title: Re: Magneto Magnet Strength
Post by: RichardL on 28.02. 2012 02:30
Well,  while thinking about all that (applying the tangent to the needle deflection raises a hand that scratches a head), I have a question about magneto construction. On the humpy side,  There is curved piece of material between the laminated pole piecs.  Is that the alnico magnet or a wedged-in piece that holds the actual magnet in place?

Richard L.
Title: Re: Magneto Magnet Strength
Post by: beezermacc on 28.02. 2012 06:51
That's the magnet. The 'humpy bit' houses the magnet which is a bar magnet with its poles in contact with the laminated pole pieces (shaped like cheeks) which are mild steel. You get a better view of the magnet in an MO1 magdyno where the magnet is a bar magnet with its end exposed
Title: Re: Magneto Magnet Strength
Post by: RichardL on 28.02. 2012 11:56
Title: Re: Magneto Magnet Strength
Post by: RichardL on 07.03. 2012 23:04
Posting this in anticipation. I found a local commercial/industrial magnet supplier who has volunteered to re-magnetize my magneto on one of their big machines. I'll be doing this tomorrow, 44 miles round trip. I figure between the magneto and just seeing a cool place, it'll be worth it. Anyway, we have the interstate highway system that will make it pretty quick. I try to get some photos.

Richard L.

P.S. Don't forget to vote for best picture. Vote for whom you will, but be sure to vote.
Title: Re: Magneto Magnet Strength
Post by: RichardL on 08.03. 2012 21:04

First, I've decided I will be the unforgivable hound, and remind everyone, again, to vote for best picture of 2010-2011. Erling's efforts should be respected and justified.

On to magneto magnets...

I made it out to the magnet factory and it was quite intersting. They had some really big magnetizers, but the fellow helping me did not know the ampere-turns in them. Anyway , the upshot is that I was able to improve the compass deflection (using the method described in this topic) from 41 degrees to 53 degrees. I'd call that a great improvement. What do you think?

We tried three different magnetizers before getting a good result. The first one, with vertical flux, didn't get a close contact with the housing and I saw little or no improvement. The second one was a tunnel with flux running the lentgh of the tunnel. That one tried to fling the housing out the end of the tunnel and actually seemed to demagnetize a bit. The third was a twin-shoe type and did the trick, so much so that the large bearing race was pulled out from the insulating shim. I thought from the beginning that this would be the right one, but my host was magnetically attracted to the others. In fact, the twin-shoe was just like one I saw charging magnetos on YouTube.

Here are the photos, in order. The fourth photo is a magnetizer not tried. The fifth is the factory (sort'a).  

Title: Re: Magneto Magnet Strength
Post by: KenF on 13.03. 2012 08:21
Glad to see from the 'Brightspark condenser' thread that you're getting the mag sorted, Richard.

By comparison, I'm attaching some pics of our remagnetiser and magnetometer.

Brightspark Magnetos
Title: Re: Magneto Magnet Strength
Post by: RichardL on 13.03. 2012 10:30
Well, that's different. Does it use the laminated poles of the mag to route flux through the magnet, or does it have a circular field, or.......?

Richard L.
Title: Re: Magneto Magnet Strength
Post by: KenF on 13.03. 2012 11:05
The former, Richard. The remagnetising core is put into the mag body so that the pole faces of the core line up with the pole faces in the mag body (one way round or the other depending on where you want North), and then a big capacitor in the black box, charged up to 340V, dumps its charge into the remagnetising core. A lot of the flux gets routed by the laminated steel in the mag body through the magnet. It also has to be said that quite a bit of flux no doubt goes the other way and jumps the 1/8" aluminium-gap in the laminated steel directly opposite the magnet, but whichever way you remagnetise a K1F / K2F / KVF there's no getting away from that (unless there's some technique I don't know about for extracting the magnet out of the aluminium casting). Of course, some of the flux with find a leakage path through the air.

I drive the thing well into saturation, but the question is, 'What is getting saturated: the magnet; the magneto's steel, or the remagnetiser's core?' I have to admit that I don't know for sure. But what I do know is that, so far, I haven't come across a K-series mag that is more magnetised than I can get them, and that includes a K2FC that Groily had remagnetised the traditional way not long ago. So, I'm fairly confident I'm getting them as good as they can be got.

All good fun.

Brightspark Magnetos
Title: Re: Magneto Magnet Strength
Post by: RichardL on 13.03. 2012 15:20
It looks like it may be the best way to get in very close proximity to the flux path through the pole pieces. In the top photo I posted, the big cabinet in the background is the capacitor bank. The cables are shown plugged into the 800V output, the connections on the front of the cabinet are 1400V.

So, does the magnetizing plug work for all magnetos for which EasyCaps are available?

This stuff fascinates me to my ultimate demise or exposure of stupidity, or both.

Richard L.
Title: Re: Magneto Magnet Strength
Post by: KenF on 13.03. 2012 15:45
The remagnetising plugs we've made so far are designed for use with the K1F / K2F / KVF where the nominal armature diameter is 2". We haven't tried yet, but they would no doubt work with the MO1 magdyno, perhaps with some sheet steel packing to take account of the extra 175 thou (mil) in diameter. Haven't investigated BT-Hs yet.

We are thinking of offering a remagnetising service. Is that something people would be interested in, I wonder. The question is how does anybody tell whether their mag needs remagnetising. For that, were are currently experimenting with a magnetometer that can give you the answer while the mag is still on the bike. Early days for that yet, but if it proves successful, we may well offer it for purchase or hire.

Brightspark Magnetos
Title: Re: Magneto Magnet Strength
Post by: KenF on 23.03. 2012 13:20
Further to my earlier post, remagnetising is now available. £15 (if provided with the bare mag body) + return P&P. More details at:

Brightspark Magnetos