Author Topic: finally disassembled a magneto  (Read 1732 times)

Offline jachenbach

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finally disassembled a magneto
« on: 04.11. 2015 14:40 »
I've got 4 magnetos laying on my workbench in varying states. Yesterday, while waiting for primer to dry, I decided to disassemble one. Don't know what I was worried about, it was really quite simple. Anyway, the bearings seem very good. I have instructions for testing the armature. I'll certainly replace the seal and brushes (which are pretty well gone). My question has to do with the magnets. I'm assuming they're weak. Don't know of any way to test them, but I figure I may as well start with them in good shape. I've found instructions for building a remagnetizer. Too much trouble. Do I need a magneto specialist to do this for me or would an automotive electrical shop that works on generators do it? I haven't called around (too early, they're not open at this hour) but figured others on the forum have experienced this.

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Re: finally disassembled a magneto
« Reply #1 on: 04.11. 2015 15:59 »
You'd need a gauss meter to measure the strength - but then you'd need a reference point so you'd know what that meter would show if the magnet were up to strength on the type of mag in question. If all 4 are K2Fs, or indeed any post-war Lucas rotating coil jobs, chances are the magnetism won't have fallen off too badly, owing to the design of the laminate extensions in the bodies (which reduce the gap between the 'ends' compared to many makes). If they are earlier horseshoe types, or most BTH, then they'll have dropped off a good bit on withdrawal of the armature.
Not sure if an auto-shop would be able to help, as most of the things they play with, dc or ac, use a field these days, so applying a dose of magnetism isn't part of the usual day job. But you might be lucky!
On your opening point, I agree - spend half my life saying the same thing to people who have been mesmerised by folklore. The only hard mechanical job is getting the races off the shaft and out of the housing, especially the drive end. Effective, and quite easy tools to make, are among the clobber here: http://www.brightsparkmagnetos.com/workshop/BitsAndBobs/index.htm
On the armature, absent a device for testing it using a make-and-break with controlled current input through the primary 'tails', you want to see a simple resistance between the spike on the HT coil and either low tension wire of several thousand ohms (5K is typical on a K2F, bit more on a 'C' model often) - Open Line = dud although it might still work owing to a carbon trace bridging a small break. The low tension should be just over half an ohm, typically. The HT resistance should be identical with the slipring on as with it off. And when hot - hairdryer say, to about 50°C right through - it should remain pretty solid to the touch, with no insulation oozing out.
If you were to set up a contact breaker or electronic equivalent to feed the coil, it should spark continuously across a Lucas prescribed test gap of 5.5mm at (or under, better) 2 amps of applied pulses (with the condenser out of circuit) - and do it hot as well as cold.
Worst job of all can be getting the firing interval between sparks correct. A rotary table, suitable fixtures and a buzzer can help for 'static' measurement, but at speed things are often different. That's when a dynamic test set-up pays off.
More strength to your arm.
Bill

Offline RichardL

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Re: finally disassembled a magneto
« Reply #2 on: 04.11. 2015 19:26 »
Jach,

Here is a link to a post written to me by Groily's business partner, Ken (with a very sincere "RIP").

http://www.a7a10.net/forum/index.php?topic=5292.msg36128#msg36128

It describes an easy method for testing magnet strength without a gauss meter. After using this method and confirming that magnetization was one of my magneto's problems, I set out to find a local magnet manufacturer who could be pleaded with, convinced or cajoled into magnetizing my magneto. If you look through the whole thread you will get more story and see photos of the various magnetizers they had, the ones we tried and the one that worked.

Richard L.
Plan on signing up for the world-wide 2017 DISTINGUISHED GENTLEMAN'S RIDE on September 30, 2018. Watch website at https://www.gentlemansride.com for details.

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Re: finally disassembled a magneto
« Reply #3 on: 04.11. 2015 20:53 »
Well remembered Richard! I do recall that discussion (and playing with a compass with KenF to see how it played out) - and yup, it seemed to be pretty indicative. But I'd completely forgotten all about it until now.  Good to be reminded. 
Just goes to show that the Boy Scout in all of is Is Prepared if he has his compass in his heel (of the shoe with animal footprints moulded into the sole for easy identification of any passing wildlife), and the little pointy thing on his pocket-knife for getting stones out of horses' hooves. Never know when you're going to need this stuff  . . .
Bill

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Re: finally disassembled a magneto
« Reply #4 on: 04.11. 2015 22:22 »
A simple test is to see if the magnet alone will hold the armature in then body when held on end with all retaining fittings removed.  I'm rebuilding one right now and it still has great magnetic strength! Lucky? I'd guess not as I figure most of them are probably still quite strong even after all these years.
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Offline RichardL

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Re: finally disassembled a magneto
« Reply #5 on: 05.11. 2015 00:02 »
.... I figure most of them are probably still quite strong even after all these years.

Hmmmmmmmmmmmmmmmm, I wouldn't count on it.

Richard L.
Plan on signing up for the world-wide 2017 DISTINGUISHED GENTLEMAN'S RIDE on September 30, 2018. Watch website at https://www.gentlemansride.com for details.

Online Joolstacho

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Re: finally disassembled a magneto
« Reply #6 on: 05.11. 2015 03:41 »
I'm sure I've said this before, but my SR was always a hard starter. Pain.
Finally after YEARS I threw caution to the wind and sent the maggie off up to the guru in Sydney for a reco. Couple of hundred bucks, that hurt, until...

What then? first or second kick starter every time... what a JOY!!!
Can't tell you how many times I had rebuilt the carb, done the points, plugs, leads, all that, over and over, and yes, I had checked for a decent spark from the mag, and it always SEEMED fine. BUT!

Online olev

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Re: finally disassembled a magneto
« Reply #7 on: 14.12. 2015 12:17 »
just got back from a HMCCQ meeting.
One of the old diggers gave a talk on the restoration of his 1927 big valve 350 AJS.
He said his magneto was pretty ordinary until he stuck a super magnet (neo something) on the side.
He said he ties the magnet on to a piece of cotton and slowly offers it to the magneto so it finds its own position on the case.
He reckons it should work on a twin magneto also.
Anyone got any thoughts on this???
cheers

Online trevinoz

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Re: finally disassembled a magneto
« Reply #8 on: 14.12. 2015 20:09 »
Yeah, get the magy overhauled.

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Re: finally disassembled a magneto
« Reply #9 on: 15.12. 2015 08:34 »
Trevor,
Perhaps you should not dismiss the idea so lightly.
This gentleman is in his eighties and spent his entire life with motorcycles.
I believe he got the idea from an auto electrician.
the 1927 AJ is not a show pony. It will do 70mph and competes in trials up to 200k.
He has serious credibility in our club.
I'm not in a position to test his idea.
I was hoping someone with a wilting magnet might stick one on the top and see if there was any noticeable improvement.
If there is one chance in a hundred it improves magneto performance surely its worth looking at.
cheers

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Re: finally disassembled a magneto
« Reply #10 on: 15.12. 2015 09:30 »
I have just invested in a new remagnetiser at great expense and it is big! It is incredibly powerful, runs off two 12v batteries and takes two strong blokes to lift it. It has been built by a chap who understands this stuff and has built them before for other magneto restorers. He says this is the biggest one he has built and it is bigger and more powerful than the ones found in old Lucas service shops. I have only been using it for a few days so I am still experimenting with it. It will be a couple of months before I have fully investigated its capabilities and, when I have a complete understanding, I'll put some information on here. This is what I've found so far......... and its not very scientific so don't throw grenades just yet as I've lots more to do to form a scientific opinion!
I was repairing a BTH magneto for a pre-unit Triumph twin. Once completed, with the armature installed, the mag would throw a spark about 4mm when spun by hand. The spark seemed healthy enough but there was hardly any bump on the magnets as the armature was rotated. I tested the magnetism with my gauss meter and found the needle deflected to no.2 on the scale when the meter was about 10 cm away from the mag, I rotated the armature slowly to ensure I was achieving maximum deflection of the needle. I put the mag on the remagnetiser and achieved about 30% improvement in the magnetism, i.e. to achieve the same deflection the gauss meter was positioned about 13 cm away from the mag. The improvement in the bump against the magnets was quite significant and the spark gap could be extended to about 6mm and the spark sounded like a whip-crack! I have since grabbed a few old Lucas K2F's off the shelf and, without exception, they are all about 50% stronger than the BTH. It's a bit too early to draw any conclusions but, if the Triumph starts with an easy kick, I would have thought there was enough magnetism in a Lucas to do the same. It seems the Lucas magnets are bigger/better/ stronger than the BTH so shouldn't (unless something really horrible has happened) require remagnetising. Generally, once a magnet is saturated it cannot be improved any further so, if the Lucas magnets remain saturated (or close to saturated) over time no amount of remagnetising is going to improve them (much).
Priory Magnetos Ltd - A10 spares, magneto and dynamo refurbs. www.priorymagnetos.co.uk

Offline Greybeard

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Re: finally disassembled a magneto
« Reply #11 on: 15.12. 2015 09:52 »
I have just invested in a new remagnetiser at great expense and it is big! It is incredibly powerful, runs off two 12v batteries and takes two strong blokes to lift it.
Picture?

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Re: finally disassembled a magneto
« Reply #12 on: 16.12. 2015 03:39 »
.... I figure most of them are probably still quite strong even after all these years.

Hmmmmmmmmmmmmmmmm, I wouldn't count on it.

Richard L.

How many K2F magnetos have you had to "re-magnetize?"
1961 Super Rocket
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Offline RichardL

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Re: finally disassembled a magneto
« Reply #13 on: 16.12. 2015 04:32 »
.... I figure most of them are probably still quite strong even after all these years.

Hmmmmmmmmmmmmmmmm, I wouldn't count on it.

Richard L.

How many K2F magnetos have you had to "re-magnetize?"

One, so far, and the mag is now awesomely better. We're not talking about neodyium here. Heat, being beat around and being stored without keepers will lead to demagnitization. I would like to know how many mags here have been put through Ken F.'s (RIP) compass needle deflection test. My guess is few, and another guess is that many would be found wanting, maybe working, but wanting. Those who have done the test, please chime in.

Richard L.
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Re: finally disassembled a magneto
« Reply #14 on: 16.12. 2015 08:26 »

 Those who have done the test, please chime in.

Richard L.
[/quote]

A magneto needs sufficient magnetism to work throughout the rev range. The benefit of very good magnetism is felt at very low engine speeds, particularly kickstart speed. The function of the magnet is to energize the low tension circuit which, in turn, energizes the high tension circuit through the usual principles of an induction coil. A good magnet will saturate the soft iron laminates of the ignition coil core so that, as the coil turns through 180 degrees and its polarity changes, the primary windings are fully energised. If the magnet is weak it will only generate a low voltage in the primary circuit and, consequently a low voltage in the HT circuit. A magneto tends to work better at higher speeds (within reason!) in any case so, unless something is significantly wrong elsewhere in the magneto, the issue of magnetism is mainly a low revs issue. I read somewhere that you need 18000 volts to jump a 6mm gap in normal atmosphere (I may not have recalled this correctly and I'll try and find the article again). All K2F's, in decent condition, will throw a spark 6mm when spun at hand speed. Most of us run on 20 thou spark plug gaps and a compression ration of about 8:1 so the magneto is more than capable of doing the business. A couple of years ago I built a magneto, out of parts, for my own bike (K2F, automatic advance and retard for the A10) and did not remagnetize the magneto. At a petrol station in Leicester, returning from the Ace Café, I was turning the engine over on the kickstart to find compression and the bike started! This is anecdotal and proves nothing (except that maybe the ignition is a bit retarded!). We know from experience that if you don't kickstart a bike hard enough it will kick you back; this proves two things 1) The magneto is working! 2) insufficient momentum has been created in the flywheels to overcome compression (and maybe the ignition is overadvanced). If we're not careful, when discussing magnets in the context of magnetos, we can get into the realms of 'overegging the pudding'. There are two principles to observe, 1) If there is enough magnetism to start the bike at reasonable kickstart speed the magnet is fine. 2) If the magnet is already fully saturated no amount of remagnetizing is going to make it any stronger. But, of course, a fully saturated magnet is going to make the bike easier to start, or facilitate starting with a bigger plug gap, than a weak magnet.
With reference to previous comments about magnet strength I have been testing the magnetism of a pile of magnetos from the workshop shelf. I have a gauss meter which works on the same principle as the compass needle test previously described. What I have found is that the magnets on K2F's all deliver a strength of magnetism within quite a narrow range, let's say 8-11 cm to achieve the same needle deflection. BTH magnetos seem to be a bit weaker and fall in the range 6-10 cm. After remagnetizing, all the magnetos achieve maximum strength, or marginally more, which suggests the magnets are saturated.  I am prepared to bet that, although the measurements suggest some magnetos have lost over one-third of their magnetism, they are still capable of starting an A10 quite easily.As an experiment I intend to build a refurbished magneto from the weakest magneto (magnet) I can find just to see how it performs on my A10..... Any thoughts?!
One further point is to remember that magnets in K2F's are made out of a completely different material to the old soft-iron magnets you see on veteran bikes which are renowned for losing their magnetism and need to be recharged every few years. The magnets in K2F's are described as 'permanent' magnets for a reason.
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