N1 Magneto, safety gap screw

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Joker_Bones
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N1 Magneto, safety gap screw

Post by Joker_Bones »

Suspecting the magneto for some running problems, I took it apart for investigation...

The safety gap screw was not present and its hole (and the other nearby small hole, the purpose of which I don't know) had been filled with weld and ground flush.
mag.jpg
I re-drilled the holes and made up a screw to fit.

Afterwards I thought, you wouldn't go the length of weld filling the holes and leaving out the safety gap screw without a reason but couldn't think of what that reason could be.

Anyone seen this before or have any ideas?
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Groily
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Re: N1 Magneto, safety gap screw

Post by Groily »

Weird JB! Maybe someone wanted 'waterproof' so blocked the ventilation hole and thought the safety screw also posed a threat? Anyway, best to have put it back. If you were able to calculate, the gap between the rounded or pointy nose of the safety screw's tip and the slipring ought to be of the order of 1/4inch, not less, or it might spark across there and not at the plug when under load.
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Taid
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Re: N1 Magneto, safety gap screw

Post by Taid »

Showing my ignorance .. nothing new there .. but willing to learn ..

What's the 'safety gap' all about?
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Greybeard
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Re: N1 Magneto, safety gap screw

Post by Greybeard »

It allows the magneto to discharge should there be a problem with the HT lead or plug - it saves the mag from damaging itself.
Would the other hole be a drain for any oil that found its way in - a mate of mine with a BSA RGS recently discovered that his mag was drenched in oil that couldnt get out because the drain hole was clogged with muck.

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Groily
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Re: N1 Magneto, safety gap screw

Post by Groily »

Just an escape path for the HT if there's a problem en route to the plug - the spark jumps to earth from the slipring to the pointy end of a screw positioned so it's within reach. With a big enough gap so the spark won't take that route as a matter of course.
This avoids stressing the HT winding were it unable to discharge after being charged up ready to fire.

Normally, absent problems, its presence or absence makes no odds, but it ought to be there and wouldn't have been designed in if thought unnecessary. Although there are older magnetos that don't have them, 'tis true.

When running in plain air most mags will create sparks capable of jumping quarter of an inch easily, so you might think the 'safety gap' would have to be really huge to avoid the spark taking the short cut all the time - but no, because the small gap at the plug is easier to jump. Even with quite high compression ratios - which directly affect the amount of energy required to fire the plug in an engine - it takes less oomph to fire the plug than jump the gap. As long as the gap isn't made too small, that is.

It is also why it is important to bench test magnetos with large gaps as specified by the makers when playing with them at atmospheric pressure on the bench. A standard 20 thou plug gap is no kind of test at all - Lucas said 5.5mm three point gaps should be used. For testing at kickstart speeds, a good wheeze is to use an old plug with the earth electrode ground off to provide something like a 4mm gap.

Hope this makes sense and apologies if it's grannies and eggs to many folk - and I see Greybeard beat me to it. Just as he says.
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Taid
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Re: N1 Magneto, safety gap screw

Post by Taid »

Thank you G and G, both ..

Never come across that before, but it makes sense ..

Question now is whether there's enough grey matter (grey pudding) to retain the info ..

Presumably there's something similar in all quality magnetos ...

Is there something similar in coil ignition systems?

Don't recall any mention, when we covered magnetos, when I was a Piston Engine Mechanic in the RAF too many years ago
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Joker_Bones
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Re: N1 Magneto, safety gap screw

Post by Joker_Bones »

Groily...
I set the gap, as best I could just using a probe to measure, to 7mm as the brightsparks website says it would be typically 6.5 to 7.5mm.

While there I saw brightsparks are not accepting any new work at the moment, to much to do?

I'm not sure what to do with this magneto. It was my intention to fit an easy cap condenser as a first port of call. I was assuming it still had the old lucas one fitted but when I opened it up it looks to have been refurbished already.
IMG_20220627_115905.jpg
I've done all the non specialist equipment tests, continuity and restance of high and low circuits , and all seems ok. The magneto will produce a spark that jumps a 6mm gap with a flick of the armature by hand. But when fitted to the bike I get a misfire under acceleration. I asked my friend's bike if I could borrow its magneto and when fitted... no misfire.
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Groily
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Re: N1 Magneto, safety gap screw

Post by Groily »

Ha, you were ahead of the game there! 7mm will be fine J-B.
Can't take on stuff commercially any more since Brexit, so nowadays I just help out my mates when I can because I like playing, prefer the shed to indoors and don't like 'idle'. We've been a bit hamstrung in recent times on the repairs side ever since my old colleague Ken died - but the online shop is still there for bits and bobs, and running along steadily.

Sounds as if the mag is faulty if substitution resolved the misfire, particularly if you used the same HT pick-up.
It clearly has been 'done' in recent times with some parts replaced - coil and condenser at least. Hard to see what's under the black goo - well, we can't! - but there's a condenser or possibly a pair of them in there somewhere.
Another risk area would be the slipring. Looks shiny but not sure if it's an old one cleaned up or a new one. They can (do) get leaky over time (especially hot) and it is usually wise just to replace them when the bearings are off frankly as they're not that pricey and they're a pain to do later.
Can't really tell from the pic how good yours is but I'm thinking I can see a blob of epoxy where the HT take-off spike from the winding passes through to get to the brass. (In line with the wire the other side.) That would suggest it's an old one cleaned up, as lots of them had a small hole through which you could see the HT wire just as it comes through the inner flange of the thing. If old, even if squeaky clean and unchipped as indeed it seems to be,, it might be your problem.

You're faced with the typical irritation of needing to make value judgments without being able to prove conclusively whether the expensive bit - the HT coil - is good or not. It probably is, but sometimes the gremlins strike early. Ideally, it needs testing thoroughly, hot and cold, with the condenser detached, to see if it does what Lucas designed it to do and to be quite sure there isn't an insulation leak. If it IS good (and your continuity test is a good starter), then the chances are that swapping the slipring and condenser will restore health. Both can be tested for leakiness to identify any fault, but not very easily sadly without some kit and without getting the bits out. If you lived up the road we could diagnose it in an hour or so . . . but unfortunately can't from here.

If you don't put a big premium on your time (!) you might want to have a crack at getting the armature apart and digging out the condenser from the goo, changing the slipring, reassembling and seeing what that does. It will PROBABLY sort it, for about thirty-five quid, but there's the risk it won't if there is hidden coil trouble . . . and they cost a lot more than that;

Sorry not to be able to say / do more - but Good Luck!
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Joker_Bones
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Re: N1 Magneto, safety gap screw

Post by Joker_Bones »

Thanks Groily...

The info about the slip ring was new to me, it all helps.

Yes... I used the same pick up, lead and cap so the only substituted parts were the mag and contact breaker. I couldn't use my CB because my mag is an NR1 with a ring cam and the one I substituted was a N1 with a face cam.

That blob on the slip ring is solder, attaching the HT to the slip ring?
mag2.jpg
I've cleaned the brushes, earth and slip rings and CB so I'll give it one more try on the bike and if the misfire is still there I will have a go dissembling the armature.

Alternatively, I may just wait till my friend's bike asks for its magneto back :)
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clive
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Re: N1 Magneto, safety gap screw

Post by clive »

Perhaps worth saying that the safety gap screw has to be removed before trying to get the coil out. Almost all the mags I have had apart have had a slip ring with a bite out of the plastic surround, glued back on again.
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