Hi Tone, i agree in everything you wrote here, this is exactly what i mean. The capacitors work is to “eat up” the spark in the points, and help with a rapid breakdown of the magnetic field.
I maybe explained it wrong?
Well, there are some differences in capacitor capacity between 1 to more cylinder engines, but that is not something one would notice (myself i often use car capacitors, or whatever i find for small engines, even chainsaws, no difference)
But, there are some old science behind this: if the breaker points get a “crater” hollow in the grounded/ negative point, you need bigger capacity, if the crater appears on the positive/coil point- the opposite. OBSERVE- this could be backwards-the opposite! I often remember things wrong 
This old way to see it is ofcourse depending on Everything, the revs the engine is working on most time, which coil used, and so on.
This is mostly/only of use when tuning/hunting for horsepower, or someone that Really cares about the life-span of his breaker points.
So, i think a capacitor rated for atleast 300 volts, bipolar for a magneto, where it’s hard to tell the field polarity, and, i don’t remember which “uF” we need, but i have a box of modern “universal” somewhere i can check
I think this will clear it up but to summarize.
- capacitance. usually in NF (nanofarads) they are saying 100-250nf
- working voltage should be several hundred, they like 600v.
- dV/dT or the change in voltage with respect to time. they say several hundred volts per micro second.
The first two should be marked on the current capacitor. The last one will be found on the datasheet.
You wrote well Goran, in fact the whole process unfolds when the contact of the switch begins to open and interrupt a relatively large current, if the capacitor is too small, an arc still occurs, but if it is a bit too large, the discharge of electricity from the capacitor causes another arc between contacts, since the contacts move apart very slowly, compared to the speed of changes in the electrical circuit. Somewhat more modern systems therefore began to replace the switch with a transistor, well, the transistor is another level of speed and capacitors are no longer needed, the size of the coils can therefore be smaller, the oscillation of the magnetic field is also enabled - more consecutive sparks upon ignition.
Well, capacitors from 1920- 1940 doesn’t have any markings, if there was, they are unreadable, in magnetos with rotating magnet, it’s a big, stationary, sturdy capacitor, often with markings-sometimes just a part number.
On magnetos with stationary magnets, and rotating coil it’s another story, in that case the capacitor could be baked in the rotating windings in worst case, in normal it’s located close to points in a brass “can”.
I’ve found out that broken capacitors (not shorted) often could be tested with a capacitance meter, almost everytime it reads a value that seems correct. These capacitors mostly dies from the dielectric “paper” drying out.
And about markings, there could ofcourse be a lot better markings on American made magnetos, im not so experienced on these, only some Wico, IH and old Wizard.
Edit: oh i forgot, you’re right about the voltage, 600 volts is a lot better, it’s just me that cheats a little: 300-400 volts work, but say heat, high revs and to wide points gap maybe peak them? Or shorten their life…
Thanks Tone, but you explains this much better than me.
A fact from the woodgas era is there was some tune-up shops that worked on ignition systems (Bosch) a common thing to do was replacing the coil with a more powerful one, this affected the balance coil-capacitor leading to a “ringing” in the coil (a wave like in a tesla coil) which could lead to inducing a non-visible spark in the next sparkplug in the firing order, hydrogen rich gas fired easily on this (glimljus translation?) Which lead to intake backfiring. A correctly sized capacitor often solved this problem.
On my old volvo i replaced the ignition system whit, if i remember, the distributor innards from a Opel Manta? Some Opel with 4 cylinder anyway… it was Bosch old electronic “stupid” system, hall effect sensor, and transistorized control of the coil. I really felt it run better, and smoother.
All in my head? I don’t know
yeah I remember the Allis used to have a -huge- one and they replaced it with a tiny one. I was thinking that couldn’t be right, but it was.
I would google for it for a while, and if I couldn’t find the value, I would just guess.
If you are off a bit, it usually doesn’t matter. But they are only like 2 dollars. This one is close to what the site I posted sells
PME271M622KR30 KEMET | Capacitors | DigiKey?
my son has finished the magnetizer…we tested today the polarity with a little 12v hand cranced generator(from a windshield wiper)
seems work well …
the old really weak horseshoe magnet, used for the test, becomes again stronger just with this little current…
polarity works fine…
next test will be with a battery…
a question about the capacitors…can they be replaced also with a modern block capacitor?
or only with electrolyt capacitors?
I assume you mean like the film capacitor like they use from the link I posted. Also double check the size from another reference, they stated nano farads, and capacitors are typically micro or pico farads so they may have stated it incorrectly. There are several sizes of the 271M series.
It depends on the application. The biggest difference is Aluminum electrolytic capacitors are polarized, meaning they have a +/- side like a battery, and electric cannot go backwards through it. The film capacitor is not polarized and is used in AC applications. Some circuit designs take advantage of the polarization. Adding a diode would prevent current coming backwards to give the same effect as a polarized capacitor. I do not know if that makes any difference in your application.
Other then that it is just the characteristics of the capacity, discharge rate and temperature range. Film capacitors typically have a faster charge/discharge (dV/dT or ΔV/ΔT). Whether that actually makes any difference in your application I don’t know. The only issues I could think of is the spark time could be shorter, or the ignition coils heat up.
Since we have a number of SHTF or DIY types in this forum.
Capacitors in general are a surface area game because the electrons sit on the surface of the material, then wait to be discharged similar to static electricity. There is a formula for calculating it.
This is the best overview of how to make modern one, that I have seen.
There is a formula for it, but it is essentially, the surface area, and the distance between the positive and negative plates and the conductivity of the separator.
The material science improved drastically over the last 100 years. The very cheap household aluminum foil is now .15mm (min) and they are saying they use .1mm (max) for the commercial process. And parchment or tracing paper is available in rolls, so there is a chance it is possible to make a 1920s era capacitor at home, although it would cost more and be time consuming. Not worth it unless it is SHTF or there is an interest in learning about the process. I think the film capacitors skip the electrolyte and just use a plastic film similar to saran wrap instead of paper, then they probably have to melt it a bit to get the distance shorter, but I couldn’t find as good of information on it.
I think part of the reason for the capacitor in igniion systems is to set up a reasonance with the inductance in the coil or magneto, Ringing you might call it. The energy sloshes from voltage in the capacitor to current in the inductor/coil. It’s roughly a decaying sine wave. This gives a longer, hotter spark. Many sparks in sequence really. Some protection for the points is a bonus. An electrolytic capacitor would probably kill this reasonance. A diode, unless it failed as a short, would probably kill it also.
I could be wrong. I have a history of being wrong 
.
This thread is pretty detailed and you may have already found it.
I can’t find =anything= on the Marelli MCR2
thanks sean for this lot of good information,
especially this , so i will post this link here…
our magneto is from 1950-53, and the first modell where marelli used a rotating magnet…
Giorgio, the capacitor and coil are supposed to be in some sort of “balance” , well , there is a lot of explanation above about the operation and function of each part , Sean really provided a lot of useful links and provided good information , .
… well, it’s good to have a lot of different capacitors on hand (as they are very cheap), so we can find out the optimal value by testing different ones. It is certain, however, that an electrolytic capacitor is not suitable for this purpose, here it must be a “normal” foil capacitor.
Hi Giorgio, i agree with Sean and Tone, a film capacitor is the way to go, no electrolytic. If you don’t find a electrolytic special made for ignition applications.
Film/foil capacitors are much more sturdy for this applications.
I forgot to mention, that capacitors connected in parallel are additive because it is about surface area. A .22 uF cap + .11 uF cap connected to the same positive/neg leads in the circuit would result in .33uF of capacitance. The reason why I am mentioning this is because you can attach temporary leads outside the magneto, use a bunch of small caps in parallel to determine what works the best, then order the right size cap then put that one in the magneto in a more permanent fashion. And even with several hundred in test equipment, you still have to do it.
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Here this is useful for trying to help determine ‘proper’ size, it is for outboards, but it is applicable to magnetos in general. In a nutshell
“SO………for a given engine, you can DECREASE the condenser size by half without much effect (but not much more than that!)……and you can INCREASE the condenser size by slightly more than 10x too large before completely killing your engine’s ignition. So, the conclusion here is that the condenser size is FAR from critical; anything large enough to quench the arcing across your points is all you need. Putting in a condenser that’s slightly too big is the safer way to go!”
In general electronics repair, if you can’t find the exact replacement, then going slightly larger is the general rule of thumb. There are some exceptions like a ‘tank circuit’.
And he also mentions that even with a half dead capacitor, you can get a ballpark capacitance from a capacitor tester. An electronics repair, or maybe auto/small engine/airport mechanic or even a hobbyist may test it for you. I think the out of circuit ones off like aliexpress are like 20-30 dollars. In circuit ones are more expensive but a different type of tester.
Then at the bottom he has a link to even more information.
here a foto with the original capacitor, 0,32 microfarad, 6 volt…
if i must replace the original capacitor, is the blue one the right kind - foil capacitor?
does it matter when the blue one keeps till 250 volt?
thanks giorgio
Giorgio,
Either capacitor will work, if there is space to install them! 
Edit: An “ignition” style capacitor has a much sturdier mounting, and grounding of the case, of course. That may lead to failure of a general-purpose capacitor due to engine vibration. I would look for a long-term solution in a closer replacement. Keep the old part for reference. You may also find the old part still functional.
Hi Giorgio, the blue one is a good option, but it’s voltage rating is a little on the low side, i had luck with 300-400volts but as Sean mention a rating up to 700 volts would be better.
I found some of my spares marked Rifa, Evox Rifa and Mkp.
Maybe some magnetic ignition systems have really high voltage, but I think this one works with primary winding voltage up to 6V, because you also have a 6V dynamo installed on the tractor, which produces electricity for the lights, and there is also a possibility that the ignition system would work. to this voltage. You can use the blue capacitor (0.33yF) without fear, well, it is interesting to see the lower white capacitor in your hand, which is manufactured by the company Iskra, which is located here about 2.5 km from our home.
By the way, you can install the capacitor under the dashboard and connect it to the engine stop button.
tone, göran, mike,sean thanks for informations…the capacitor signed with 6 volt is the original one…the iskra capacitor is from old electric stuff, what my son collected from old television apparates…when he was exploring around with the bicycle it was always completely full with scrap when he returned…
reading the informations from sean about self build magnetizer, i found that there are shown 2 methods to connect the coils, in series, as mine is connected i think , and parallel (left design) …the parallel one produces more amps, is stronger…
can i make this also with my coils, each has 5 meters wire on…? or is there a ostacle with my short wire?
design of the built guide …
Short wire=more amp’s, i recomend you giving as much amp’s you dare to, next to short circuit.
The modern magnets are harder to magnetisize.
I should look after a youtube videos i watched, showing a industrial re-magnetizer, big capacitors discharge in to the coils, eye and hearing protection needed