V Twin Magnetos

V Twin Magnetos

In a conventional rotating armature magneto, for each revolution of the armature, there are two positions where an optimum spark can be achieved. These positions, where the armature core leaves the pole piece, are exactly 180° apart and are usually referred to as the ‘break’ points. One of these break points is used for single cylinder machines where just one ‘bump’ on the cam ring causes the points to open and the spark plug to fire. Both break points are used with a two bump cam ring for twin cylinder engines. This is perfect if the twin cylinder engine configuration requires two plugs to fire at 180° intervals such as in parallel and opposed cylinder engine layouts.

However, V twin configurations are a special case. The cam ring for a V twin magneto has two bumps positioned to open the points at the unequal intervals required to suit the particular V twin engine. V twin magnetos always rotate at half engine speed so the angles between the cam ring bumps are given by the following equations:

180° – (Ɵ/2) °  and  180° + (Ɵ/2) °

where Ɵ is the V angle of the engine in question measured as an angle on the crankshaft.

As an example, a Vincent Rapide series B has a V angle of 50° so the angles between the bumps on the cam ring need to be:
180° – (50/2)° = 155°  and  180° + (50/2)° = 205°
These angles are easily checked with a protractor disc attached to the armature shaft and our Magneto Static Timing Light used to indicate exactly when the points open. The cam ring can be reground to correct any discrepancies. (This check and, if necessary, adjustment is standard procedure when we overhaul both normal and V twin magnetos).
Apart from the cam ring, the internals of the Vincent’s Lucas KVF magneto are more or less identical to the K2F magneto as fitted to BSA and Triumph twins. These magnetos all follow the pattern of a conventional rotating armature design and so have two break points 180° apart. However, only one of the bumps on the Vincent cam ring can be made to coincide with a break point. As a result, only one cylinder receives a good spark whereas the other cylinder receives a much weaker spark because the points open long after the break point by which time the magnetic field has been collapsing for an appreciable time. A situation which is far from ideal and is, perhaps, one reason why starting engines fitted with Lucas KVF magnetos is not always easy.......

Other magneto manufacturers sought to alter their designs to cater for the needs of V twin engines. What was required was to change the 180° angle between the two break points. A number of designs were tried but the most common and probably the best solution was to alter the shape of the pole pieces and the armature core.
In this EIC Type AV2 magneto, each pole piece is staggered by an amount determined by the required V angle - in this case, 50°....
....and the armature core staggered to follow the contour of the pole piece:
This configuration enables the two break points to be the required 205° and 155° apart as shown here:
In addition to this, the cam ring is made with it's two bumps at the required 205°/155° spacings.

Engines with other V angles were catered for by  altering the widths of the staggered portions on the armature/pole pieces and modifying the cam ring accordingly so most manufacturers offered their V twin magnetos with a number of variations. For example, period literature from The Bosch Magneto Company states: The type "ZEV" magneto can be furnished for two cylinder engines having cylinders set at 20°, 30°, 35°, 40°, 45°, 50° apart. Here is a table of just some of the various angles used over the years by different manufacturer's.
A great deal of care is needed when re-assembling V twin magnetos and also when refitting them to the engine in order to ensure that the right spark occurs in the right cylinder at the right time! There are plenty of variations out there....
Look carefully at these two Bosch ZEV end covers. The engraved markings show that they are both from 50° magnetos but the one on the left has pickup number I on the right whereas the one on the right has pickup number I on the left.  There is nothing else on these covers that makes one 'right' and the other 'wrong' for a particular magneto so they may have been changed sometime in the past without affecting the actual working of the magneto....
On this Bosch ZEV cam ring, the individual cams are numbered I and II. They can be removed and swapped over..... Which is what you should do if reversing the direction of rotation of the magneto.....

So..... Don't believe all that you see - these magnetos are decades, maybe even a hundred years old so someone is bound to have worked on them in the past.......
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