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How To: Magneto rebuild #2. Capacitors/Condensers

July 12, 2010












update Feb 2012:  See more info here and here.

The very first How To article on OcchioLungo was about rebuilding magnetos. here is the link.  It covered much of the mechanical items, but skipped over the steps of winding an armature and replacing the capacitor (aka condensor)  Since then my friend Sten vW sent me the following information about capacitor/condensor replacement that he has gleaned while rebuilding mags.  Much of this was also shared on the Velocette Technical Forum.

Sten’s notes on replacing capacitors:

Paper Flowers

-Paper flowers should hold bearings and provide isolation.

-Old reused paper flowers will either not grip the bearing or break when inserted.

-Modern paper flowers are much stronger than old.

-Paper flowers came in the past in varying thicknesses so that grip could be optimized.

Nowadays this seems not to be the case, possibly ok with BTH but not with sloppy tolerance Lucas units.

-My frustration over old and new paper flowers have had me starting to make paper-mache “flowers”. Printer paper saturated with white wood glue stuck onto bearing outer is my substitute.  It is almost as difficult to fit as the original paper flowers. Insert “wet”, slight press fit can be achieved. Drop rotor in and assemble so that the rotor can align the very slightly mushy paper-mache. After drying, a drop or two of bearing lock (not bearing fit!) on the paper-mache completes this bodge!

-Windings, primary about 1Ohm, secondary usually around 6kOhm’s

-Mains voltage and small neon lamps can be used to diagnose shorted caps, disconnect cap leads from windings or windings might get damaged.

-BTH good, Lucas bad, at least mid 30’s onwards, don’t have any experience with 20’s Lucas mags.  (Sten’s opinion!  your mileage may vary –Pete)

Guidelines for replacement capacitors:

0.22 microfarad

AC Voltage rating: 400V continuous
Operating temperature: 80 degrees C.
Dv/dt: 1000 or higher
Dissipation factor: 0.05% or less.

Preferably teflon/foil cap

or polypropylene/foil.

try to avoid metalized polypropylene

Sources for capacitors/condensers

elfa below is “” a Swedish component similar to RS.
The cap I ended up using was slightly too big (thick!) too fit the available space in the K1F. The cap was clear potted so that the internal structures where visible, I walked over to the band sander and hey presto fit like a glove!

RS: 190-8400, Epcos B32652A6224K
9×17,5x18mm RM=15mm

RS: 334-934, Epcos B32522C6224J
7×12,5x18mm RM=15mm

RS: 210-522 evox rifa PME271M622K
11,3×16,5x24mm RM=20,3mm

farnell: 9717358, RS: 206-7746 evox rifa PME271E622K
12x22x27mm RM=22,5mm

elfa 65-578-21 evox rifa PHE426KB6220J
7,5×14,5×18,0mm RM=15mm

elfa 65-580-01 evox rifa PHE426MB6220J
9.5×17.5×18.0mm RM=15mm

Evox Rifa PME271E capacitor has been used by some UK mag rebuilders with success

Replacing Capacitors/Condensors

It’s a bit involved, but here is a try to illustrate my method.

HT and LT checked with voltmeter before commencing, t’is good to know before going any further. HT at around 6 Kohm’s +/-1 Kohm and LT at around 1 Ohm is normal.
The wires to the cap and ground point are soldered in and these are preferably tackled before anything else.

If HT/LT isolation is to be tested, this is the point to do it, my winder uses 230VAC in series with a small neon lamp (limits current) connected between windings and body (ground wire disconnected). If the neon lights up then current is passed=dud isolaton=re-wind.
Most rotors (i’ve played with BTH and K1F/K2F)need the slip-ring taken off before access to the rotor screws is had.

To get the slipring off the bearing inner track needs to be taken off by a special puller or (special clamp and standard puller) that grips the roller track.

Some sliprings are tightish on the shaft and needs heating gently to loosen. Sliprings are brittle!

The exposed HT lead that is exposed when the slipring is off is EXTREMELY delicate if pulled/pushed or tweaked it can break the HT connection inside the rotor. My dad (vintage 1933) squeezed one in his hand and cost me 100GBP’s worth of re-wind, and this was after i cautioned him to be careful!

Screws next Lucas from one side, BTH from both I seem to remember.

Occasionally corroded and stuck screws need decapitation and new DIY screws made up (BA2 i believe).

After screws are out delicate knocking axially on the end brass to dislodge ends. Ends are pegged so needs to be separated alternate knocking from side to side about until about 1/16″ open before loose.

Lucas K1F: The cap is integral with the nut that the breakerplate screw is screwed into. This nut is isolated from the armature.

I tend to carefully destroy the old cap so that i can get the nut with enough isolation left to re-use with a sliver of brass shimstock soldered on to act as a terminal for wiring.

Space is at a premium here so the nut/isolation/terminal might need to be almost flush with the floor of the brass endpiece to give enough of space for the cap.

The ones i ground down to fit where Farnell #971-7358
RIFA PME271E 220nF 300Vac.
The grinding is done to reduce the thickness so there is some room for the
adjacent windings.  An extremely tight fit but can be done!
My reason for using this cap is that it’s the highest spec pulse cap i could
find that had a chance of fitting.

PS/ the pegged interface of the ends to the central bobbin is sometimes not
enough to define how they came apart, mark ajacent parts so that things can go
together as they came apart/DS

PPS/ Wilfried the bruf-man has some pictures from a re-wind, re-cap he did. Not
very step by step (and his cap also looks small) but identifies parts nicely

12 Comments leave one →
  1. July 13, 2010 4:33 am

    agree with your capacitor choice – though the actual measured rise ( by me on a K1FC) is about 200v in 6uS, so anything significantly better than 40v/uS should work- though obviously the higher spec the better. I’d always remagnetise a mag after stripping – easy if you’ve got the kit – I built mine years ago and published the design in FT.

    • Anonymous permalink
      March 24, 2014 12:16 pm

      CAN YOU DIRECT A NEWBY TO THAT KIT? i NEED TO remag my mag,thank you,B.B.B

      • March 24, 2014 1:17 pm

        There are a few places to buy capacitors/condensors online. You can try Digikey or Alltronics, or maybe Mouser. Good luck.

  2. Nick permalink
    July 13, 2010 9:04 am

    I have really enjoyed your blog and then this??

    “-BTH good, Lucas bad, at least mid 30′s onwards, don’t have any experience with 20′s Lucas mags.”

    Why not strike that sentence as it serves no purpose but further inflame the anti- LUCAS sentiment that is so common on the web. They are not bad magnetos.

    The bearing shims are available from either Lindsley or Independent in a more modern material.

    Cheers, Nick

    • Pete Young permalink*
      July 13, 2010 9:22 am

      Hi Nick. Sten wrote that article, I suppose it is his opinion about Lucas mags. I’ve been very happy with the Lucas mag/dyno on Kim’s 1936 Ariel. I wasn’t aware of an “anti-Lucas sentiment on the web” but maybe there is a reason for it? 🙂 cheers, Pete

      • Nick permalink
        July 13, 2010 3:20 pm

        Thanks Pete, I see that now. Keep up the good work!

  3. Dev Dhurvas permalink
    September 26, 2010 12:22 am

    Thanks for this helpful site. Im tryingto recondition my Lucas magdyno. I had the coil rewound–Ht 9k ohms Lt about 1 ohm. Could you please let me know which of the 2 Lt wires goes to earth and which to the points–how do I identify it? Also please let me know what type and value capacitor I should buy. How should a Lucas MO1 be placed in the magnetiser for bes results? I live in New Zealand. Thanks In advance. Dev

    • Pete Young permalink*
      September 28, 2010 8:16 am

      Hi Dev. Any of the capacitors listed in that article should work, but I haven’t personally used them yet. (I’ve always done the mechanical rebuilds and sent out the armature for new windings). On the magnetiser, the mag shop will know how to align the mag. Just make sure that the magnetiser is a strong one. Putting a good magneto on a weak magnetiser will do more harm than good.

  4. Shiva permalink
    November 4, 2010 3:02 am

    Hi Nick

    I have a Lucas mag for my 1941 Matchless

    Can you advice on the capacitor that i can use ,

    I have go the Armature rewound but the spark is very thin


    • Pete Young permalink*
      November 4, 2010 8:00 am

      I’d try a search for vendors where you live that carry equivalent parts to the Elfa or RS parts listed in the post. Or you can buy capacitors from many of the magneto shops such as Independent Ignition, Mark’s Magneto’s, etc.

      • Shiva permalink
        November 10, 2010 4:32 am

        Hi Pete

        Managed to find Advanced capacitor (Tubular 5mm )
        Rated 0.22uF 650dc

        Scooped by Lucas N1 capacitor and inserted the Capacitor

        Wow,Fat blue spark

        Need advice on the Thin make capacitor which can use

        Thanks any way

  5. Skip Brolund permalink
    April 14, 2012 4:31 am

    I highly recommend bypassing the old capacitor & use the Brightspark unit, it fits on the points plate, way nicer than the old arrangement & excellent instructions on their website:

    Skip Brolund

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