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by ‘Wilfred’

The majority of cyclemotors and autocycles of the 1950s relied on a flywheel magneto to provide the ignition (Vincent and Lohmann being two of the exceptions).  Wico-Pacy, Miller and others made units suitable for our machines and both made versions with and without lighting coils.  The construction of all of these was very similar but here I will cover one of the most common types, the Wipac ‘Bantamag’.

This is perhaps one of the simplest magnetos ever made.  It consisted of a back-plate supporting a double wound HT/LT ignition coil, contact points and a brass cased cylindrical condenser; a shaft-mounted cam; and a Mazak rotor containing cast-in magnets.  There are no lighting coils and the Bantamag is very reliable.  However, its spark is not too strong so a close eye must be kept on its adjustment.


It is possible to adjust the contact breaker through the holes in the rotor.  Turn the rotor in the direction of your engine rotation until either the hole marked ‘CCW rotation’ or ‘CW rotation’ is in front of the contact set, as appropriate for the engine.  There is no adjusting screw so loosen the contact set fixing screws slightly and open or close the points by moving the contact set mounting plate with a screwdriver.  They should be set to 0.018" (0.45mm).  When they are set correctly, tighten the screws and re-check the gap.  While you are checking the gap it is a good time to re-lubricate the felt cam pad (when did you last look at yours?)  Either use grease (if the magneto rotor is off) or ordinary engine oil will do.  Place the nozzle of an oil can hard up against the pad and gently pump in oil until the pad is just saturated.

The timing of the Bantamag is achieved by slackening the two screws holding the back-plate onto the engine casing and rotating the back-plate.  Turn it in the direction of engine rotation to retard the ignition (hotter running, better slow speed performance, better starting) or opposite to the direction of engine rotation to advance the ignition (cooler running, better high(?) speed performance).  When the desired timing is achieved tighten the two screws hard.  If they come loose they can wreck a magneto in seconds (I know, I’ve done it!)

All of these adjustments can be carried out with the rotor on or off, but the operations are easier with the rotor removed.  To remove the rotor, undo the central fixing nut and remove the washers.  Then carefully screw on the extraction tool so that its fixing screws project just below the surface of the rotor.  Finally tighten the extractor and, when there is some load on the screw, give it a smart tap with a hammer.  When refitting ensure that the Woodruffe key is properly in place and has not fallen out.  If the rotor continually breaks keys when in use, gently lap the taper of the rotor onto the shaft with fine valve grinding paste, clean all the parts, and sprinkle some French chalk onto the taper before refitting it.

Problems and Remedies

There are no major problems with the Bantamag.  As with most magnetos, it is very gap-sensitive on its contact breaker and it is worth the trouble to ensure that the contacts are clean and the gap correct.

As all Bantamags are now very old a condenser fault is not unheard of.  Replacements are not now available but a modern electronic capacitor can easily be fitted into the old brass casing and will give a good performance.  Use a capacitor of 0.1 to 0.2 microfarad, of a non-electrolytic type.

Coil failure is very rare without mechanical damage.  The coil can be checked by measuring its resistance.  The high tension (HT) coil should measure between 3,000 and 4,000 ohms from the plug lead to the magneto back-plate.  The low tension (LT) coil should only measure about 3 to 5 ohms, so a continuity check should suffice.  This is measured across the contact breaker when the points are open.

If the rotor is kept away from the magneto, or is subjected to severe shock or heat, it can become demagnetised.  At present [this was written a long time ago] I know of no cure for this with this type of rotor, and another one should be found.


The use of a modern capacitor in the condenser can greatly increase the strength of the spark available from one of these units and has no known detrimental effects.  The only other improvement easily achieved is to increase the available timing adjustment range; this is very small on the Bantamag, and often a better engine performance can be achieved with a more advanced timing than this unit can give.  Carefully file out the mounting screw slots to provide the extra movement.  Usually only about 1/16" or 1/8" is required.  The very adventurous can also advance the ignition by opening the contact gap to around 0.025" (0.64mm) but this is not recommended for the uninitiated, as it can produce complications.

Many years have passed since ‘Wilfred’ wrote the article above … it was added to this archive in 1999 but I think it was written some years before that.  Magnet technology, in particular, has moved on …

Thanks to this e-mail from Akhilesh Agarwal, we can bring the article up-to-date.

Bantamag update

by Akhilesh Agarwal

Dear Mr Andrew Pattle,

I am a retired mechanical engineer living in Lucknow, India, and have a lifelong interest in engines, vintage cars, clocks, and cycle motors.  Over the years, I have acquired and restored a rare, hard to find Lohmann cycle motor to perfect running condition, as well as a Berini M13 cyclemotor.  They are both quite rare and the ones I have may be the only ones of their kind in India.

The Berini I restored recently has a Wico Pacy Series 90 (Bantamag) magneto, which had some basic problems due to which it gave a weak spark, preventing the engine from starting.  The major problem was weak flywheel magnets.  I refer to Wilfred’s article on Wico Pacy Bantamag, which says there is no known solution to this problem.  Since I have found a way around this problem, I would like to share it with you as it may be of use and interest to others.  I am giving below a copy of a letter I wrote to my friend David, who lives in the UK, about my restoration work:

Dear David,

It is very kind of you to say that ‘failure’ is not in my vocabulary, but the story of the Berini is not without failures!  I think I wrote that I had purchased it second-hand from a mechanic about 45 years back when I was doing my engineering.  I fitted it on my bicycle and tried very hard to get it going, which it did on rare occasions, but I could not diagnose the faults correctly due to lack of knowledge and experience.  The Berini was lost for 45 long years and rediscovered recently when I demolished our old house.  The taste of my past failure was still fresh in my mind and rankled, so I wanted to restore it now to erase the stigma.

It proved to be easier said than done.  It was not in bad shape mechanically.  I replaced the bearings.  Then I found the crankcase oil seal was damaged, and so was the rotary valve plate.  Crankcase compression is vital in two-stroke engines because primary compression takes place in the crankcase before transfer to the cylinder.  I purchased the required oil seals during a visit to, of all places, Singapore, and carefully lapped the valve plate/seat, thus resolving the crankcase compression problem.

The major problem was the magneto which gave a weak spark.  Although a Dutch product (designed by German ex-DKW engineers), the magneto is made by Wipac, Bletchley, UK.  I wrote to them but did not get any response.  The magneto had multiple problems, making it hard to diagnose.  Firstly, the magnets had become weak and no recharging facility is available.  So I got around it by drilling holes from the outside and fitting Chinese Neodymium magnets.  They are very strong and recharge the original magnets by induction.  But there is a polarity issue.  The original pole layout was unknown.  Much research on the internet and study of magneto design and function did not clear the issue, I used my own logic.  I followed unipole alternate polarity rather that dipole or tripole polarity more commonly used.  The Neodymium magnets I used were of 10mm diameter × 10mmmm length.  Round magnets are better because you can drill holes from the outside of the flywheel just up to the outer surface of the original magnets, fit the Neodymium magnets so they are in contact and fix them in place with Cyano Acrylate.  The glue should be used only after getting the polarity right, once glued the magnets are impossible to remove!  It does not matter if the magnets protrude above the flywheel.

Then there was an ignition coil problem.  The original coil showed low resistance in the primary but OK in secondary, showing shorting in the primary.  I took it apart but found it was not shorted, I suspect actually the secondary was shorted, but not detected by the multimeter!  New coils were available in the UK for an unaffordable £50(the Berini was originally purchased for £2.50!).  Secondary coils are very hard to rewind at home due to large number of turns of very fine wire, so I decided to use an external ignition coil.  But that needs a power coil in the magneto to power the external ignition coil primary.  Conventional wisdom says that the voltage induced is proportional to the number of turns, so I wound a home made coil of 450 turns compared to only 120 in the original, but it didn’t work well.  I increased the turns to 600, there was slight improvement in the spark, not still not good enough!  I had some wire left over, so I wound another coil of just 150 turns, based on a different design—two coils in parallel, the first power coil driving the primary of the external coil.  This reduced the resistance from 1.5 to 0.5 Ohms.  In this arrangement, called energy transfer system, power transfer is maximised.  It is a slight modification to the original design, but it worked!  The rest is history!

Here’s a video of the Berini running.

Please let me know if I can be any further service to you.

Best regards
Akhilesh Agarwal

2 March 2017

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