Author Topic: finally disassembled a magneto  (Read 1783 times)

Online groily

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Re: finally disassembled a magneto
« Reply #15 on: 16.12. 2015 10:48 »
Using a gauss meter placed inside the mag bodies, one can typically see an increase of around 20% on a good post-war Lucas with a blast of magnetisation, often a lot more, but they almost always work 'as is'.  That using an internal magnetiser - which won't be as mega-powerful as the monster that B'macc has acquired probably! (But also not impeded by the 'air gap' of the alloy casing, because internal cores touch the poles on the inside, which helps a lot.)
It's hard to know when or whether truly full saturation has been reached - too much physics in there for me, so results are empirical. And then, the magnetism will drop off a bit and settle after the mag has been in operation.

The final result on a Lucas Alnico magnet may well be a bit stronger than on a similar-size BTH (eg KD1/KC1 or KC2 etc), but I can't measure the BTHs like-for-like from within, because the withdrawal of the armature or a keeper causes an immediate loss a great deal larger than on a Lucas (for the reasons I mentioned earlier in the thread). So external measurements like those cited are the best simple comparative guide to magnetic strength, backed up by dynamic tests to see what the magnetos actually do on test.

I can't say I have found the flip points on a typical BTH noticeably weaker than on a Lucas after a blast if things have been keepered. A well set-up K2F will deliver all sparks across three-point test gaps of 5.5mm (per Lucas spec) at around 120rpm of the armature, and so will a good BTH. (Lucas basic K series spec was I think 90% of all sparks at 130 across those gaps.) A Magdyno MO1 will do better as a rule (as the Lucas spec expects it to). But much depends on the exact 'recipe' used in a rewind and also on the air gap between armature and mag body. Some are pretty worn, after perhaps several rewinds and some truing up in lathes over the years, so their gap is larger than original and the outer extremes of their laminated parts may be smeared.

I just compared a rebuilt KC2 BTH against a rebuilt K2FC Lucas on the bench. Both delivered all sparks ahead of the Lucas spec. The flip points feel 'about the same' (with the contact breakers off to get rid of that element of friction), but without setting up a spring balance or something to measure the resistance to rotation at the flip, 'about the same' is all I can say. Plus the effect of oil seal drag and bearing grease comes into it too.

The differences in construction of the two types are also relevant to actual performance I think. The BTH (like Bosch) doesn't use a fully-laminated bobbin, relying on 4 pairs of screws to attach the end cheeks, and they are into 'solid' as they'd have to be. Lucas ones are all laminated, using through screws. Full lamination ought to reduce 'eddy currents' which impair efficiency  - the sort of thing KenF loved discussing. Mechanically the BTH fixing is bullet-proof and easier to get perfect alignment after separating the armature parts. But in terms of the respective magnets, there's no doubt the K series design is better at retaining strength with the armature withdrawn, because of the extended pole pieces with reduced air gap between the ends of them in the body. Often, a K2F mag body that has stood 'empty' for years has still got a lot of magnetism in it, and the same goes for Lucas Magdynos of the same vintage.

Bill

Offline jachenbach

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Re: finally disassembled a magneto
« Reply #16 on: 16.12. 2015 13:25 »
I checked 2 of my fully assembled mags (one on the bike, one on the bench) with a compass at a distance of 15 inches and got a 30 degree deflection. As I have such a very long way to go to have a running bike I'm going to wait and see if this is adequate.

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Re: finally disassembled a magneto
« Reply #17 on: 16.12. 2015 13:36 »
I'm sure Groily and I are on the same page which is a good thing as we both fix mags for a living! The point made about air gaps is a good one and perhaps why BTH have a better reputation than Lucas as their engineering seems to be to finer tolerances. The mags seem to run closer air gaps between the armature and magnetised laminates so the magnets have better effect. Anecdotally, I have never come across a Lucas K2F or MO1 which has lost so much magnetism it has ceased to function. However, It is clear they can be improved, some more than others, which will aid starting. Obviously I'm not saying they don't need remagnetising as I've just invested about two months' income on a remagnetiser with the grip of a gorilla!
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Online groily

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Re: finally disassembled a magneto
« Reply #18 on: 17.12. 2015 06:47 »
Reckon we are Andrew (B'macc).
And yes, I agree the tolerances often are finer on BTH.
I think they were also preferred by many for the longer condenser life owing to the use of mica after Lucas went to chemical paper, and for the better contact breaker construction where the pivot pin doesn't wear out at quite the same rate. The fit of manual camrings also seems often to be rather better, with more reliable firing intervals on twins as a result.
Anecdotally, completely agree about Lucas magnets having enough oomph at least to work. (Except in one case I had where the mag had been driving a Thorspark or similar set-up and had deliberately (I assume!) been demagnetised.) So I bet the weakest K2F you can find will probably work on a typical A10, but a bit of a boring job putting it on and taking it off again just to find out  . . .
It's funny how things go though - yesterday I was working on a BKH1 BTH off a 500cc Velocette. This model has a cast-in magnet and fairly extended pole pieces quite like the Lucas K series, with the outside of the laminates exposed on the sides - your Big Gorilla Grip would really be able to give that a Serious Blast! I'd say it had the strongest magnet I've ever seen on a small BTH, with a higher 'internal' gauss reading with the armature out of it BEFORE giving it a shot than I usually see on a remagnetised K2F: it was easily capable of a 6+mm spark with a flick of the wrist and produced all its sparks across the test gaps from 90rpm on test.  Very impressive I thought - and absolutely the job for a low-geared kickstart Velo.
Bill

Online Joolstacho

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Re: finally disassembled a magneto
« Reply #19 on: 17.12. 2015 08:45 »
Har, you got it... that low-geared Velo kickstart ratio, - killer! So yep you need good technique AND a decent mag.

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Re: finally disassembled a magneto
« Reply #20 on: 17.12. 2015 08:54 »
So I bet the weakest K2F you can find will probably work on a typical A10, but a bit of a boring job putting it on and taking it off again just to find out  . . .
I can cope with 'boring'! It's excitement I don't want at my age! Anyway I can do this almost blindfolded and the mag would stay on the bike and I'd sell the one that comes off as a guaranteed service exchange with about 2000 miles on it since refurbishing. It's the mag that's been to your house!
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Online groily

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Re: finally disassembled a magneto
« Reply #21 on: 17.12. 2015 14:53 »
Right Andrew! Well, that one (on the bike) seemed to work a treat, somebody will get a good deal there then. (By all means feel free to break down this side of the ditch if the sparks dwindle away far from your pet Gorilla - can probably find something to get you home!
And as for Velos Joolstacho . . . light (not like me), elderly (like most of us) people who haven't quite taken on board the technique for getting them going are a constant thorn in the side. One chap I know is thinking one of the new BT-H spark generators will solve the fact he can seldom start his wild pre-war steed on the kickstart - but I'm not so sure if there's anything that will help him apart from buying a house on a steep hill.
All for the want of a tooth or two  . . .
Bill

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Re: finally disassembled a magneto
« Reply #22 on: 17.12. 2015 21:47 »
Thing is of course, once you get a Velo going, all is forgiven! (I have been contemplating a portable paddock-starter though).