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How To: Magneto rebuild #2. Capacitors/Condensers

February 2, 2012


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 condenser)  Since then my friend Sten vW sent me the following information about capacitor/condenser replacement that he has gleaned while rebuilding mags.  Much of this was also shared on the Velocette Technical Forum.

Update February 2012: Charles Falco wrote in the last AMCA club journal that he has experimented with a lot of replacement condensers and found great results by using a pair of them soldered together in parallel.  He used Panasonic .082 microfarad polypropylene-film caps, from Digi-Key ( for $0.74 each. The part number is ECQ-P4823JU.  More info in the Winter 2011 issue of The Antique Motorcycle, or from Charles here: charlesfalco at   (replace the “at” with @).

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

7 Comments leave one →
  1. Jim Abbott permalink
    February 2, 2012 10:07 am

    I’m AWFULLY glad I sent mine off to be done. I don’t know a uF from a uS.
    Jim A., Tucson, AZ

    • February 2, 2012 10:35 am

      just be careful WHO you send it to. There are some really great rebuilders out there, but it seems that there are plenty of unsatisfied customers. Based on what I hear, it runs about 50% of people have a bad story about their mag shop, and 50% are happy. I’ll share what I can, and try to show y’all what to look for in a good rebuild.

      • Jim Abbott permalink
        February 2, 2012 11:56 am

        I sent mine off to Doug Wood in PA at John Stanley’s recommendation. It works perfectly and looks brand new. He even polished the cases and repainted the Lucas label.

  2. February 2, 2012 11:38 pm

    Be sure to specify if you want it left original in finish. Also if you have a mag shipped in from Ebay ask them to take off the bakelite terminals. I just got a mag and a mint terminal had been smashed to bits in transit because of poor packing and being dropped. Thanks, Paul

  3. Anonymous permalink
    January 22, 2013 10:53 pm

    The Panasonic capacitor has been obsolete since 2008. Odd that he would test such a part. I have lots of them as I could not buy just a few but had to take all from the vendor I deal with. So if any one needs some I will be selling off excess stock.

    Bruce Farren

  4. Dave permalink
    August 30, 2017 1:17 pm

    Hi – was trying to figure out why my recently rebuilt 1923 Bosch mag had no spark. I’ve got spark now, but do not remember seeing the condenser when I had it apart. I’ve seen the round condensors on more recent points motos, but could you show me what a condenser looks like on these old magnetos?
    Thank you,

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