Getting The Best From Your Distributor

Details

Written by Ewen Korse

It is probably not news to anyone reading these notes that the Magnette engine used the Lucas DM2 distributor. However, it may be news that “DM2” only defines the body shape and the general lay-out of the components. Internally, a DM2 can use a variety of components to produce a range of different timing advance curves, so it is unwise to assume that “if it’s a DM2, it’s OK for the Magnette.” It is fair to say that any DM2 carburettor, with correct static timing, will probably allow the Magnette engine to fire up but it will not necessarily produce best performance throughout the rev range. In theory, the ideal distributor is the one designed for the engine to which it is fitted. It will have been designed to produce the right advance curves to match the engine’s characteristics in standard form. However, petrol has changed a lot since the Magnette was designed, so the factory data about timing the engine probably have little more than historic interest now. The Advance curve that the original distributor was designed to produce is unlikely to suit the combustion characteristics of modern fuel and any additional performance modifications carried out to the engine and its ancillary systems.

Engine performance modifications will change the engine’s ignition advance requirements, so if your engine has been modified away from standard, or if you want to be sure that your standard engine is set up for optimum performance it is best to seek the advice of an engine builder or better still, get one to set the car up on a dynamometer. This will give you data on performance throughout the rev range and will suggest any required changes to advance curve and fuel mixture.

The intended application of each DM2 unit can be identified from a part number stamped on the body. There were also two types of DM2: “early” and “late”. The later version is designated DM2P4 and indicates that the rotating plate on which the points are mounted takes the later points design that uses only one mounting screw. The mechanical advance components were also modified to stabilise timing at tick-over speed. If you are finding difficulty getting a steady reading with a stroboscopic lamp at tick-over, you probably have the early version. This number is stamped on the flat on the distributor body, along with “DM2”, an arrow indicating direction of cam rotation and a number which is a manufacture date code (e.g. 458 means fourth week of 1958). So if the date code on your distributor is later than your car’s manufacture date, you can be sure it is a later replacement.

The models fitted to our cars were: ZA - 40510A (later superseded by 40718A); ZB - 40587A

If yours does not carry one of the above model numbers, it is from a different car. Interestingly, the 40510A and its replacement 40718A were also specified for the MGA range.

To add complication, the vacuum advance unit was also designed for specific applications, so if you swap one from an unknown source, it may change the advance curve. Vacuum advance units are defined by three statistics:

 1. The vacuum strength at which it starts to move the timing.

  2. The vacuum strength at which maximum advance is achieved.

  3.  The maximum amount of timing advance that it will deliver.

These key measurements are stamped on the hexagonal end of the unit next to the threaded spigot where the vacuum pipe attaches. The vacuum strength is measured in inches of mercury, like air pressure in a barometer. So a unit stamped 5-12-6 starts to move at 5” and stops moving any further at 12”, by which point it is delivering 6? advance.

Using this information enables the engine tuner to set up a distributor to match the degree and purpose of tune (e.g. economy, fast road, full race).

Engines need timing advance because, although pistons move faster and faster as engine speed increases, the combustion process happens at the same speed regardless. Combustion is not a single instantaneous explosion but is a more progressive build-up to maximum “bang”. For correct timing, maximum bang needs to happen at the point when the piston is beginning its down-stroke (roughly 17?-20? ATDC), so that the power is delivered when it is most useful. If it is happening when the connecting rod is close to vertical, the bang will simply apply down- force to the big and little end bearings without converting enough of it into rotary force at the crank. This produces pinking (a.k.a “pinging” and “detonation”). The faster the piston is reciprocating, the earlier the combustion process needs to be started at the spark plug. The perfect moment for the spark to happen will be dictated by a number of factors such as fuel octane, engine compression ratio etc, which is why different engines need different distributors.

The mechanical advance is delivered by centrifugal bob-weights that spin inside the distributor body. If you hold the rotor arm and twist the bottom of the distributor spindle, you will feel the movement between them that is controlled by the bob-weights. When you strip your distributor down, do not be dismayed if you find that the bob-weight springs are not a matching pair. This is not a previous owners bodge. The two weights have different roles and are designed to be controlled by different springs. The bob-weights themselves are common to all DM2 units. Springs are defined by the material & thickness of the wire from which they are made, the number and diameter of coils and their length in the “at rest” position. This combination of data will produce a spring rating that a spring supplier can use to provide a spring of higher or lower rating for tuning purposes. The lower the rating, the earlier the advance will be delivered because the bob-weight can swing outwards more easily.

The primary spring is often smaller and tighter fitting on its mounts. It controls the early acceleration from tick-over. The secondary spring comes into play higher up the rev range and slows the rate at which advance is applied. It is a bit loose so that the bob-weight has some free movement before the spring control begins, while the primary spring is in control. Once the secondary spring comes into play, the rate of advance generally slows because of the cumulative force of the two springs. So if you represent on a graph the relationship of engine speed to degrees of advance, there will be a point at which it starts to level out. This is where the second bob-weight is having effect. Eventually the graph will level out because maximum advance has been reached and the cam is mechanically prevented from moving further. The shape of the plot on the graph is the source of the term “advance curve”.

The routine for stripping a distributor for maintenance is set out in the workshop manual. Once the baseplate has been removed by unhooking the vacuum unit and removing the two screws on the body edge, the cam can be removed by taking out the screw in the top of the spindle below the rotor arm. The bob-weights can then be lifted off their base-plate. Doing so will reveal a number stamped on the baseplate. (On later distributors like the 25D4, this number is stamped on the base of the cam, which has a different design). This shows the maximum amount of mechanical advance that this unit can deliver. However, this is expressed as degrees at the distributor and because the distributor rotates at half crankshaft speed, this figure is half the advance delivered as actual ignition timing at the crankshaft. So a unit that is stamped 12? will actually be capable of delivering up to 24? ignition timing. This may not sound like a lot, but this is added to the 10? or so that is input at the static timing stage, which is set with the engine at rest. The resulting total of 34? is much more like the advance needed by an engine turning at 4500rpm or more.

If you do strip down your distributor, keep a careful note of the position of the rotor arm in relation to the off-set segment on the bottom of the drive spindle. The reason is that it is very easy to assemble the spindle and the rotor carrier with their relative positions 180 degrees wrong. This leads to a baffling and frustrating unwillingness of the engine to start. It coughs and splutters but won't fire up. A recent bulletin board thread described just such circumstances and the owner could not understand his difficulties because he had carefully not changed the engine crank position while the distributor was out of its mounting, so as to preserve the timing. In his case he was the innocent party: it was the distributor rebuilder who had made the error and his rotor arm was pointing to the ignition lead diametrically opposite to the one that needed the spark! He solved it eventually by swapping the HT spark plug leads in the cap but he could have detached the rotor carrier from the springs and bob-weights and reassembled it after 180deg rotation.

If you are using the original points, it is best to set the gap by measuring the “dwell angle”. This is the proportion of a single cam rotation during which the points are closed. The points gap is merely a way of achieving the desired result so if you have the appropriate meter, you can check the actual angle being delivered. Sometimes the angle is expressed as a percentage rather than in degrees so if you only have the degrees, just divide that figure by 360 to get to the percentage. (e.g. 45? = 12½%). If the angle is too small, close the gap a little. If it is too large, open the gap up. Recommended figures normally quote a bit of latitude, e.g. ±3?, so with that latitude a nominal 60? is OK between 57? and 63?.

In judging whether a distributor is fit for use, you are primarily looking for wear in the following locations:

• the central spindle at its base, where it passes through the distributor body
• the top of the spindle that carries the cam rotor
• the bob-weight pivots and their sockets on the weights

Any wear in these areas is likely to make the unit imprecise in the timing that it delivers and it will undermine any other improvements that you are making. 

Check the effectiveness of the vacuum advance unit.  Suck on the threaded end. You should feel strong resistance but eventually the actuator connection will move. If the air sucks through unimpeded the diaphragm is punctured and the unit should be discarded.

Take care if fitting a non-standard distributor that has a vacuum advance. The vacuum take-off on the Magnette is on the carburettor body. On later MGBs and other models, this drilling on the carb was deleted to save manufacturing costs and the take off was moved to the inlet manifold where much less positional precision was needed. The vacuum characteristics are different in these two locations, so make sure you know which your replacement distributor  was designed for or you may get very confusing timing advance performance.

For a very detailed explanation of how to use the cam, the bob-weights and their springs to achieve the performance you want, click here.

Judging by problems reported on bulletin boards and in conversations with other owners, the non-Lucas consumable parts sold to fit our distributors can be of inferior quality. For instance, poor quality cam riders wear rapidly and the points gap reduces to nothing long before it is due for a service check. A recent batch of rotor arms was also infamous for shorting the HT into the spindle making a misfire extremely difficult to trace. It is surprising to learn that Bosch parts are available and bring with them a more reassuring track-record. The Bosch part numbers for any  DM2 distributor are: Points GL19, Condensor GL103 and Rotor Arm GL229.

The earlier models of MGB (and possibly the last of the MGA 1600 MkIIs) were fitted with a later design of distributor, the 25D4, which is reputed to be a better, more reliable design than the DM2. It is a direct DM2 replacement and will fit any of the B-series engines under consideration. With second-hand DM2s being a bit scarce, a 25D4 is a viable replacement and offers a better basis for a conversion to an electronic system. The challenge will be to set it up to match the DM2 being replaced. Examination of the cam bottom plate will give you the maximum advance and a figure in the region of 13? is a good start point with a 10? static advance. You can switch your original vacuum unit over. Thereafter it is probably best evaluated by test runs on the road or a dynamometer. To change the maximum advance, you either need to obtain a different cam or changes can be made by filing off metal from the end of the limiter leg (increase maximum) or welding more on (decrease the maximum).

If you want your distributor rebuilt professionally or you need some parts, talk to the Distributor Doctor.

Replacement non-Lucas after-market distributors are now also available and may offer a better solution than a reconditioned original. See the relevant Modifications section.