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The Marelli Magneto for the Lambda

Mike Benwell mike@mikebenwell.com and Jonathan Reeve casarokardo@btinternet.com With additional notes from Kees Jan Boosman keesjan@lanciaservice.nl

With the advent of the Third series Lambda, when Lancia switched from the early Bosch duplex magneto-dynamo unit to the Marelli at a time when Bosch were having difficulty filling orders in a timely fashion during the German hyperinflation, it seems likely that he had three technical advantages in mind: The Marelli is lighter, which benefits the longevity of the water jacket to which it is attached; it does not come nearly so close to the exhaust manifold and so is subjected to much less heat (the intensity of which is inversely proportional to the SQUARE of the distance from the heat source); and it generates 80 watts of electricity compared with the earliest Bosch offering of 60, which made long night drives with the 25 watt Zeiss headlight bulbs practical. We have previously described the rather conventional circuitry employed by Marelli and Lancia for the third to sixth series Lambda. In this article we shall describe the Marelli magneto, with its mechanical drive and constituent parts. We advise against relying on one of these magnetos without it being professionally re-wound, since aging of the 1920’s shellac used by Marelli for the secondary windings seems always to lead to a breakdown of insulation as soon as the unit gets warm. The symptoms are that the Magneto can be made to work quite well on test; but after 5 or more miles when the heat of the engine has warmed the magneto, the insulation breaks down and the magneto starts to emit weaker and weaker sparks leading to a roadside breakdown. General Principles in dealing with an unrestored, non-functioning, or poorly functioning Marelli magneto Re-winding a magneto is not a job for an amateur; it requires specialist equipment and generally speaking auto-electricians familiar with our vintage equipment have access to professional magneto rewinding. Alternatively, if you are going to mechanically service your own magneto, there are professional re-winders that advertise in Journals such as “The Automobile”. Either way, the Lambda owner needs to understand his magneto to a sufficient degree so that he can provide all the information the professional re-builder or (s)he him/her-self needs, so a lot of time is not wasted finding out by trial and error how to get good performance out of a newly rebuilt magneto. The Appendix describes how the mechanical duplex coupling of the dynamo, which is through the automatic advance and retard to the magneto, can be safely taken apart. There are many books from the 1920s onwards that describe the way the magneto works and the reader is referred to them if he or she wants to understand the electrical principles underlying the generation of the spark at the plug. However, in simple terms, a magneto uses a magnet with its associated lines of force to induce electrons to race round within coils of insulated wire that are wound round an armature. To achieve this movement of electrons, either the magnet or the armature must rotate. When the circuit is broken by the opening of the contact breaker, the fast moving electrons are diverted via the slip ring into the distributor, which directs them into one or other of the high tension leads connected to a spark plug. Such

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is their number and velocity that a spark composed of many electrons then jumps across the gap in the spark plug, providing it is earthed. If there is no earthed connection the spark is earthed instead through the magneto’s own internal earth, which is located inside the distributor’s rotor arm. For our purpose it is necessary to understand that the Marelli is a rotating armature type of magneto with an armature that rotates CLOCKWISE as viewed from the driving side, also often designated as a right-handed magneto (Destro abbreviated D in Italian). The cap over the oiler on the top front of the Marelli magneto should show the original direction of rotation – beware if you have a magneto adapted from a Fiat or other make because it might have originally been an anti-clockwise (Sinistro, S) magneto. Mechanical characteristics of the Marelli MP4 magneto The magneto armature in a 4-cylinder car needs to rotate at engine speed and therefore is coupled to the rear of the dynamo through the automatic advance mechanism that will be described later. Duplex magnetos are always coupled to a driven dynamo, rather than the other way round, because magnetos consume very little power, unlike dynamos, which can consume much power. With 2 sparks per revolution of the engine, and 4 spark plugs to feed, the distributor is geared at half engine speed so that the distributor brush rotates once every two revolutions of the engine.

Fig 1: Cross-sectional diagram of the Marelli MPL4 Magneto, as used when not coupled to a dynamo. The text refers to the modifications made so that this magneto can be used in the Marelli duplex Magneto-dynamo.

Figure 1 shows the Marelli MPL4 magneto in cross section in its original, non-duplex form. To serve Lancia’s needs for a compact duplex unit, Marelli designed a thinner, flattened distributor cap compared to the Fiat version and re-designed the dynamo to make it shorter,

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by removing its rear bearing, so that at its non-driven end the un-tapered keyed spindle fits directly into the automatic advance and retard mechanism which is securely attached to the magneto armature’s spindle. Therefore once dismantled from the magneto, the dynamo armature is un-supported at its rearward end. The advance and retard mechanism depends for its action on a type of governor arrangement. As with other automatic advance mechanisms, hinged weights fly centrifugally against the restraining force of springs in response to increasing revolutions. This Automatic Advance is in turn mounted on the taper and key-way on the front end of the magneto drive. In consequence, the accurate rotation of both magneto and dynamo are dependent on the ball races used to locate the magneto armature. Figure 2 shows the various parts of the Marelli Mag-dyno after partial dismantling.

Fig 2: A partially dismantled Marelli duplex magneto-dynamo showing the main components.

In the Marelli magneto, the distributor gear sits vertically above its pinion, which is located at the front of the magneto, between the front ball-race and the armature. The pinion on the armature shaft driving the distributor gear has two adjacent teeth marked as shown in Fig 3. Because Marelli also supplied other manufacturers who required an anti-clockwise Magneto (eg Fiat) the driven gear on the front of the distributor is marked with both a “D” and an “S” - D for destro (right) and S for sinistro (left) presumably so that Marelli could use the same

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Fig 3: Close-up view of the gears at the driven end of the magneto to show the correct meshing of the large distributor gear with its pinion. Note that the distributor gear is usually marked with an “S” as well as a “D”; for the Lambda, because it is a “clockwise” (Destro) magneto the distributor gear tooth marked “D” should mesh between the two marked pinion teeth.

parts in clockwise and anti-clockwise magnetos. In the case of the Lambda the tooth on the driven gear marked D should always mesh between the two marked teeth on the driving gear in order to ensure that the distributor brush is on a segment connecting to a spark plug when, at full retard, the low tension contacts are just separating. If the gears driving the distributor are not set up correctly, for example by placing the “S” tooth between the marked teeth on the driving gear, then an attempt may be made to deliver the spark to a segment of bakelite between the brass segments on the distributor cap and the charge will instead of arriving at a spark plug be diverted to the safety gap connected inside the magneto to the distributor. The magneto is timed internally so that the pulse of electrons that creates the spark is at its maximum amplitude when the contact breaker opens. This amplitude varies greatly around the 360 degrees of arc that comprise one complete rotation of the armature. To ensure that the spark at the plug is optimized, the slip ring and contact breaker are designed so that together with the correct meshing of the distributor gears the largest possible spark is delivered to the plug. Evidently, although many parts of Destro and Sinistro Marelli Magnetos are common to both, the contact breaker has to be set up appropriately for the magneto’s direction of rotation. Marelli made Destro and Sinistro contact breakers so it is important that the correct Destro contact breaker is used (Fig 4).

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Fig 4: The Marelli Destro (MD) contact breaker seen from front and back. Note that the key locating the contact breaker onto the armature spindle is “built-in” thereby locating the contact breaker correctly for clockwise rotation. Anti-clockwise or Sinistro Marelli magnetos require a different contact breaker in which the points open differently in their angular relation to the keyway on the armature spindle.

Marelli also appear to have changed the design of both Destro and Sinistro contact breakers, so as to use more than one spring arrangement to keep the fibre cam follower applied to the cam track, the later one using a straight spring supported by a coil spring beneath it, the previous version using only a curved spring. Some contact breakers appear to be dated (month, year) but not all are dated and most are stamped “Magneti Marelli”. Most have stamped on them MD or MS referring we think to the Destro and Sinistro contact breakers respectively. What I suspect is my newest contact breaker (of 4) is stamped with the date 11 25 approaching the date at which Lancia switched back to Bosch (just as Bosch’s German market was collapsing`); and has the serial number 191591. None of my other contact breakers have serial numbers. To ensure that the contact breaker opens at the correct angle of rotation of the armature shaft, it is fabricated with a captive key and sits on a taper, being held on the spindle of the armature by a special machine screw so that its cam-follower tracks sequentially over the two segmental cams mounted on the rotatable cam-cage. The Lambda cam cage was originally limited to 20 degrees of rotation, consistent with the Lambda’s specification, which provides for 20 degrees of automatic advance and a further 20 degrees of manual advance. Some Lambda magneto cam rings have been modified however, perhaps in response to seizing of the automatic advance mechanism through lack of lubrication over the decades, and had metal cut away from the cam ring to permit a full 40 degrees of manual advance. Readers should be cautioned that the remainder of the manual advance mechanism is poorly adapted to providing such a wide range of advance and retard; and there is a risk of either mis-timing the magneto so that it runs retarded at engine speeds over 1000 rpm; or if better timed for fast running they might experience severe kick back on starting the engine on the starting handle, endangering their forearm and hand, through not achieving full retard with the advance and retard lever on the steering wheel.

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The condenser in the Marelli magneto (like the earlier non-Lambda Bosch FF4) is located in the forward part of the armature and is inaccessible from the contact breaker end of the machine. However, magneto condensers rarely give trouble because their manufacturers take great care that they are robust and of ample capacity. However in the rare event that the Marelli condenser is faulty, there will be a great deal of sparking at the points with sooty deposits very evident. In the absence of such evidence, the condenser is probably best left well alone. Servicing the Marelli Duplex Magneto-Dynamo Marelli magnetos and dynamos like those of other manufacturers need lubrication at intervals of 1,000-2,000 miles (1,500-3,000 Km) but lubrication should be done sparingly. The Magneto requires only 2-3 drops of light machine oil to lubricate the distributor gears, which is done through lifting the lubricator cover with the arrow on it at the top of the driven end of the magneto. It is probably best to remove the Duplex unit from the car after carefully marking the coupling to make re-assembly without loss of the magneto timing easy. Then the dynamo’s commutator can be examined after carefully lifting out the dynamo brushes, since a dirty dynamo commutator can lead to starting troubles. The automatic Advance and Retard mechanism is inaccessible without full dismantling; but since many are seized or partially seized, if it has not yet been restored to full function the reader is referred to the Appendix so that it can be attended to. The contact breaker itself can be removed if desired by removing the long centre screw, but be very careful that the key registers with the key-way on re-assembly. The contact breaker itself requires adjustment to within 0.025 mm either side of 0.4mm as specified by Marelli. The points of the contact breaker are we think made of a platinum-iridium alloy that resists erosion and corrosion over winter lay-ups; but dirt will get past the contact breaker cover and find its way in. The contacts may be cleaned by using a special purpose magneto file, or if not available passing a piece of doubled over blue backed finest grain (“FF”) emery paper between the contacts having first ensured that the fibre heel is free of the cams allowing the contacts to be separated. As the emery paper is drawn between the points, gently hold them together with your fingers. One drop of light oil should be placed on the cams every 1,500 km to lubricate the fibre heel; on no account should any other oil be allowed near the contact breaker since it will abort the action of the magneto. Attention to the Brushes There are four carbon brushes in each Marelli magneto (Fig 5). Fig 1 shows them schematically. There are three associated with the distributor cap. These should be checked regularly (eg every 1000 miles/1500 Km). Made from 7 mm dia carbon rod , they are machined at one end to 5 mm so a very fine coil spring can be attached pushed over its 5 mm end, before the brush is inserted into its housing spring first. The spring then pushes the brush gently against its contacts.

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Fig 5: The Marelli distributor cap and brushes, labeled A to D. Brush A is located in the rotor and brushes B and C in the distributor cap. The fourth brush earths the armature.

These springs are very fine, almost invisible in poor light, quite springy and easily lost! Ideally dismantle your distributor cap on your kitchen table or bench not on a gravel road! The carbon can wear down and the springs can become coil bound with clumsy assembly, either of which can make a good magneto

Fig 6: Left Magneto failure due to a worn brush B, or a broken or coil bound brush spring. Note the very poor projection of brush B from the distributor cap; about half the brush should be projecting. Right: rotating the distributor rotor into the dotted position to achieve full advance with a working Advance and Retard (A&R) mechanism – courtesy the Lancia Lambda handbook.

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produce a weak or non-existent spark. Fig 6 shows a worn down central distributor brush “B” that in the author’s Lambda led to magneto failure. The fourth brush is on the side of the mag and earths the armature. Timing the Magneto (traditional method) It is first necessary to know the firing order of the Lambda (which is 1,2,3,4) and how the four cylinders are numbered. Number 1 cylinder is on the driver’s or right hand side nearest the driver. Number 2 is on the same side, nearer the radiator. Number 3 is also at the front of the engine on the passenger’s side and number 4 behind it and ahead of the exhaust manifold and horn. In other words the cylinders are numbered anticlockwise (looking at the engine from above) starting with the cylinder nearest the driver. It is best first to remove the bonnet. The Flywheel is marked with two lines indicating top dead centre (tdc), for cylinders 1 & 3 (1 3) and 2 & 4 (2 4) respectively. Forty degrees ahead of each tdc line (ie to the right looking at the flywheel over the top of the radiator) is a line marked AA. These AA lines indicate where the points of the magneto should be just separating at normal cruising speed. Now remove the spark plugs and rotate the engine on the handle to line up the pointer on the back of the block with the 1 3 line. Then lift the valve cover to find out which of your two possible cylinders has just fired 1 or 3: it will NOT be the one where the inlet valve is just opening and the exhaust valve is just closing but the other one, where both inlet and exhaust valves have been shut to compress the charge of petrol vapour during the compression stroke of the cylinder that is firing (be aware that the camshaft rotates in the opposite direction to the crankshaft!). Next, turn the handle so that on the line 1 3 the valves for No 3 cylinder show that the exhaust valve is just closing and the inlet valve is just starting to open. Cylinder No 1 is now at TDC and on compression. Next turn the flywheel back a bit past the AA line (by hand – the starting handle only turns it in the opposite direction!) and forward again to the AA line (to take out play). The engine is now in the position it should receive a spark at maximum advance on no 1 cylinder, including manual advance and automatic advance (if functioning – see appendix for how this may be serviced). The first thing to do is put the hand advance in the full advance position (rotate the lever anti-clockwise). Then check how the hand advance rotates the Magneto Cam Cage, disconnect the rod connecting the cam cage to the hand advance and ensure that from now on the cam cage remains in its fully advanced position. Next, fit the magneto with the distributor arm pointing to plug lead no 1 on the inside of the distributor cap (Fig 5). Let’s start by assuming that you have a functioning automatic Advance and Retard mechanism. To check if this is likely, twist the rotor of the distributor against the resistance of the mag-dyno drive with or without the duplex unit disconnected and see if you can find about 20 degrees of movement that is resisted by the springs inside the advance and retard (Fig 6 right). If there is no movement, the Advance and Retard is locked up so skip first to the section below on Seized Advance and Retard; but if the A & R appears to be working, continue as follows. One of the main perceived disadvantages of the Marelli is that you need a small mirror on an extensible stick and a light to examine by eye the opening of the points when the magneto is in its running position. Kees Jan Boosman overcomes this difficulty by instead using a traditional cigarette paper (the non-smokers among our readers are advised that these

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papers are usually available at tobacconists and most other types of paper are far too thick for this purpose). If you use his method, at a convenient moment you slip a single layer of cigarette paper between the contact-breaker points. You will have to do this several times. When the timing is exactly right in full automatic plus hand advance the points will be just opening and the cigarette paper will be released. This is unlikely to be the case the first time you try it.

You are now going to accurately time the magneto one of two ways: either with the help of a vernier coupling between the dynamo and the engine; or with the help of an adjustable rod connection between the magneto and the hand advance. The flywheel remains aligned with the AA line. If you are using a vernier, hold the front of the dynamo drive fixed while you rotate the distributor contact arm clockwise looking towards the front of the car, against the resistance of the automatic advance mechanism, and engage the mag-dyno vernier drive with the middle engine gears so that the points are just opening when the vernier coupling is in its operational position, with the clamp holding the mag-dyno unit against the side of the engine tightened up. For accuracy, you will repeat this by rotating the vernier coupling backwards and forwards a few degrees in either direction until you are satisfied that the points will just be opening as the AA line passes the pointer. If you are not using a vernier, adjustment is done by rotating the mag-dyno inside its clamp AFTER disconnecting the rod connecting the cam cage to the hand advance mechanism on the steering column. You must also ensure that full hand advance is preserved by holding the cam cage in its fully advanced position (or full retard if that is your chosen method). Some connecting rods are conveniently provided with a miniature adjuster similar to the one provided on the rear brake rods. You should adjust the length of the hand timing connecting rod by whatever method AFTER the magneto is timed so that the timing is preserved. This is the Factory method and is illustrated in the Handbook. In the view of some it is easier; but one of the authors has had trouble with the early type sprung factory connection of the dynamo to the middle gears, with the layers of spring steel breaking up leading to backlash developing. Timing the magneto with the “ Magneto Static Timing light “

This device has been tested by Mike Benwell with excellent results. It has two bulldog connectors that clip on each side of the points, As soon the points open the red light comes on (see Fig 7). According to Mike it makes the whole job of timing the ignition a lot less fiddly. The“ Magneto Static Timing light “ may be obtained from The Magneto Guys: www.themagnetoguys.co.uk

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Fig 7: The magneto Static Timing Light in operation (see text).

Seized Advance and Retard If you have a seized advance and retard the above procedure will leave you with insufficient retard at low engine speeds; but you will have correct ignition timing at running speeds. It is inadvisable to start the Lambda on the handle with inadequate retard because of the risk of backfire and consequent injury to the wrist or hand. Overcoming difficulties with manual dexterity If you find difficulty in timing your magneto as described above, an easier compromise requires that you first paint a line on the flywheel 20 degrees before tdc (ie half way between the AA and the 1 3 lines) for use instead of the factory AA line. Next, leave the automatic advance in retard by NOT twisting the distributor rotor while you time the magneto. This method has the disadvantage that you are making assumptions about the accuracy of the Marelli automatic advance after more than 90 years of probable neglect. Your Cam cage has been modified to give 40 degrees of “hand” advance A previous owner with a seized A&R mechanism might have modified your cam cage to rotate 40 instead of 20 degrees on the hand advance. Then you might think it worth trying to time the ignition in full hand (and automatic) retard so that firing occurs at tdc when the engine is just ticking over at 300 rpm. But with this method there is no guarantee that you will get the ignition timing right at running speed, due to the geometry of the hand advance mechanism, which was not designed to cope with 40 degrees of hand advance. If on checking you cannot achieve 40 degrees of hand advance, we recommend that for adequate engine efficiency you consider time the magneto at maximum achievable advance using the AA line and the instructions given above.

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APPENDIX Notes on Dismantling the Marelli mag-dyno unit. The dynamo and magneto are maintained in the correct geometrical relationship by both being attached to an aluminium casting that encloses the advance and retard mechanism and houses the forward bearings of the armature shaft and distributor shaft of the magneto. The dynamo case is screwed to this casting by 5 metric countersunk machine screws. IT IS ESSENTIAL THAT NO INAPPROPRIATE THREADS ARE USED ON THESE SCREWS ( UK B.A. threaded screws have been found here with dire consequences for self dismantling), if the owner wishes his mag-dyno unit to carry him very far down the road after a rebuild. The other thing to be wary of is the very delicate nature of the casting in the region of the front bearings of the armature and distributor shafts. Some castings that have come down to us have been repaired after a clumsy attempt to remove a worn bearing. This creates major difficulties and unsatisfactory mechanical compromises in view of the lack of adequate space to insert a plate to repair the housing. For the complete dismantling of a magneto, rather than minor servicing, it is desirable to remove the magnet. This is held on by four screws and it is essential that an iron or steel “keep” is placed across its open “jaws” or the magnetism will be very quickly lost. In any event, after a major rebuild it is highly desirable to re-magnetise the magneto. Auto electricians generally are able to perform this task in a few minutes. The automatic advance and retard (A&R) mechanism is enclosed within a thin, spun steel housing that is screwed onto its base by a very fine thread. It is very unlikely that you will be able to undo this housing after 90 undisturbed years without damage due to the changes that occur with use and advanced age, yet many of these A&R mechanisms are seized internally so that the advance mechanism no longer works, presumably through loss of lubrication. If this is your situation, you will need to decide whether to try to free off the seized internal joints and bearings or to accept that you have permanently lost 20 degrees of advance. This has implications (including for safety if you need to start your Lambda on the starting handle) and is discussed in the main article. Here we will assume that you wish to have the advantage of a functioning A&R mechanism. The un-tapered and keyed rear end of the dynamo armature shaft is clamped into the driven end of the A&R mechanism (Fig. 8). First remove the front dynamo cover and lift out the dynamo brushes without disconnecting them, following which the dynamo casing should be free to be removed over the driven end of the dynamo armature after unscrewing the 5 or sometimes 6 machine screws attaching the dynamo casing to the aluminium casting that joins it to the body of the magneto. It is then necessary to unclamp the rear of the dynamo armature spindle from its mounting in the clamp attached to the A&R mechanism. There are two slotted hex-headed machine bolts that need to be undone to free the dynamo armature from the A&R mechanism. Once the

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Fig 8: The clamp that grips the rear end of the dynamo spindle is shown attached to the front of the Advance and Retard mechanism. The Dynamo spindle can be withdrawn following release of the two clamping machine screws.

dynamo is removed, it will be found that the clamp for the dynamo shaft is screwed to the A&R mechanism by 4 machine screws that are wired together to prevent them coming undone. If these screws need replacing, they have to be long enough to fully engage their female threads, but not so long as to obstruct the centrifugal action of the bob-weights by protruding inside the A&R mechanism. After these screws are removed, the A&R mechanism will be revealed, attached to the front of the magneto armature by a unique round nut that requires the making up of a special tool to assist its removal (Fig 9). The A&R mechanism is fitted to the magneto by a Woodruff key and taper. The removal of the special nut holding the A&R mechanism onto the magneto taper will permit various options for using a puller to draw the A&R mechanism off the armature taper. Kees Jan made a special puller to avoid damage (fig 10). For lubrication purposes, the interior of the A&R mechanism (shown in Fig 11) can be accessed through the four threaded holes use to attach the dynamo mounting clamp. However it seems inadvisable to attempt lubrication of the A&R mechanism without first removing it from the magneto. In any case to rewind a magneto that has not had attention for some decades, you will need to remove its armature from the magneto case. If the A&R mechanism is lubricated and re-installed, it is essential that every effort is made to ensure that none of the lubricant

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Fig 9: The rear dynamo spindle clamp removed from the Advance and Retard mechanism by the unscrewing of 4 machine screws that are wired together. To remove the Advance and Retard mechanism from the front taper on the magneto spindle, it is first necessary to make up a tool (shown) to grip the special nut that holds it onto its taper.

Fig 10: Kees Jan Boosman’s special puller and method for removing the A & R mechanism after the nut is removed.

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Fig 11: a partial view of the internal weights and springs that actuate the automatic Advance and Retard mechanism. It is to be hoped that for lubrication it is un-necessary to remove the cover that conceals this mechanism because to do so usually results in destruction of the cover.

Fig 12: To run the dynamo for test or repair purposes independently of the magneto, it is necessary to make up a temporary rear cover with a rear bearing that is geometrically accurate and can be screwed to the dynamo case.

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will escape into the dynamo or magneto. Fortunately the interior of the A&R mechanism is effectively sealed by re-inserting the four machine screws holding the clamp for the rear of the dynamo armature. NB: If it is desired later to run the dynamo up on test without the magneto attached, it is necessary to make up a flat plate containing a ball bearing in which the rear of the dynamo shaft can run, which can then be screwed onto the dynamo case by the 5 previously mentioned retaining screws (see Fig 12). Re-assembly In general reassembly of the duplex unit is dismantling done in reverse order. However adjusting the play on the front bearing of the dynamo is important. Kees advises as follows: First fit the armature extending forward (ie in the direction of the front of the car) of where you think it should be positioned. Next, tighten the clamping bolts (Fig 5) loosely. Then fit the dynamo body, in the process sliding the armature backwards part way. The clamping screws are tightened a little more and with a plastic hammer then tap the armature backwards until the smallest amount of play can be felt at the front ball race. Pull forward the dynamo cover so you can tighten the clamping screws and then perform final assembly in this position and re-locate the dynamo brushes. Version V4 March 2018