Flywheel magnetos

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robint
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Flywheel magnetos

Can anyone tell me if flywheel magnetos become de magnetised with age? To the extent they wont produce a proper spark.

Is there a way to rejuvenate them?

Can they be drilled out and have modern neodymium magnets installed for example

I am sceptical about claims that they can be remagnetised using an electrical current.  Magnetism is normally impressed when the metal is being cast.

wristpin
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They do demagnetise with age

They do demagnetise with age and they can be remagnetised with the correct coils etc. It is also possible to reverse the magnetism if the job is not done correctly. One of the old Briggs and Stratton manuals covering pre WW2 engines actually includes instructions for making the coils to do the job.  Small portable generators that are not used for a couple of years can loose their residual magnetism and refuse to generate. That is usually cured by "flashing" the field coils.

As for re-engineering flywheels with modern magnetic materials, I'm guessing that while theoretically possible it is unlikely to be a practical proposition.

I can safely say that in a lifetime of messing about  with small flywheel magneto engines I have never found one that has lost its spark through loss of magnetism. It's usually due to an old cotton/shellac insulated coil where where the copper wire has corroded and either gone open circuit or shorted out. Condensers do also suffer age related failures and when overhauling / reconditioning engines of "unknown provenance",  I always test them  using a proper condenser tester that stresses them enough to identify the ones that are likely to be unreliable.

 

robint
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Thanx WP I am suitably

Thanx WP I am suitably illuminated. I havent got a spark yet on my 515 but will clean the points soonest.

I got one of those neon testers, very useful couple quid as I found you couldnt always see the spark in daylight

I tested the coil with an AVO and it read ca 10k  seems a bit high, isnt 5k more usual?  But also the points arent opening so that needs sorting

 

Reading about magnets - its almost certain that the magnet is an AlNiCo type which is relatively weak compared to neodymium magnet - which are dangerous. Alnico degrade about 3% strength per 11 years - 50 years 15%

BUT big WARNING these magnets can be demagnetised if subjected to another strong magnet in the repulsion mode

 

LEARN SOMETHING EVERY DAY

wristpin
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That figures, workshop

That figures, workshop manuals often say that spare flywheels should not be "nested" together . If I'm scrapping an engine but saving the flywheel I always place the stator inside it.

If you've ever dismantled a 'puter hard drive you will have experienced some ultra strong finger pinching magnets!

robint
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Quite so.  These neos are

Quite so.  These neos are quite cheap and I had a couple dia ca 2p piece and ca 1/2" long.  One would pull the other from more than 6" apart.  The scary thing is if left unchecked they would crash together in an eye blink and probably shatter as they are brittle as a tea cup. If you were dumb enough to get your finger in the way it would crush it severely - really quite dangerous and NOT for kids.

 

I went and checked my spare rotor and it had plenty of pull on a steel bolt (when tested on the inside which is next to the stator)  Did the manuals ever mention using a keeper bar across the inside faces when in long storage?