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MG MGA - 1600 distributor

Help wanted. I have looked at 3 distributors that I have in the garage. The first is the original DM2 40510H with screw on vacuum with reading 7-14-10 54411301 marked around the edge. The second is a 25D 41288B with vacuum 5-13-10 54411985 markings and push on pipe connection. This one replaced the original when last on the road and finally another 25D 40897A vacuum 5-13-10 54411230. this has screw on connection like the DM2. Question can either work well in a 1600 Mk1 engine with 714 cam and is it possible to change the 25D screw on connection with the DM2 vacuum to maintain the original pipework, with different vacuum numbers.
Brian Paddon

Brian. You have several things you need to be aware of.

First, the DM2, the 25D4, and 45D4 Lucas distributors will all, physically, interchange if the drive attachment is set up for use with MG/BMC/BL engines. (Triumphs used the same basic distributors with a different form of drive gear attached and can, with the MG style drive gear, be used as a basis for a rebuilt distributor for MGs.)

Second, none of these distributors can be counted on to meet the specification stamped on them. They may have been rebuilt to the original, or some other, specification over the years. Additionally, the age of the distributors makes it highly probable that they are worn and the springs that control the rate of mechanical advance have weakened, allowing the mechanical advance to come on earlier than originally specified. The only way to actually know what curve your distributor currently has is by testing on a distributor test machine (most accurate) or a working engine (far less accurate, but indicative).

Third, even if the original distributor is in brand new condition and is capable of exactly replicating the original factory specified advance curve, that may not be what your engine needs today with the modifications made to the engine and the changes made to the formulation of modern fuel vs fuel available in the 1950s and early 1960s.

Each vacuum advance unit will have to be tested, on the test machine, to see of it is working--either properly or at all. A 50+ year old rubber diaphragm, if working when initially tested, will probably not continue working for long in regular driving use.

So, what to do? Get the engine running with either of the distributors you have used before, then, as soon as possible, take it to a good rolling road operated by someone who knows about MGs. (Peter Burgess would be my first choice, but there are others who people have used over the years with success.) Determine the best (optimal/most desirable) ignition settings at the various engine speed ranges, then get together with a competent distributor rebuilder who can build you a mechanical and vacuum advance system that will come as close as possible to the ideal curve your and the rolling road operator have determined.

At that point, you have accomplished several things of value and interest. You have established the ideal curve for your specific engine (and a starting place for other engines of similar specification), you have installed a rebuilt distributor having superior to the factory advance curves which should hold up for a number of years service life, and you will better understand what is needed for your engine which will help you, and help you help others, for many years.

As to exact type of distributor, each later model was some improvement over the earlier model. DM2 was not quite as good (in some ways) as the later 25D, and the 45D was introduced when the car producers needed to start using electronic triggering systems (vice points) to meet emissions requirements. For a show car, I would use a DM2. But, the 45D4 would be my first choice for any other use.

Les Bengtson

I could not agree more with Les's 3rd paragraph. I sent my original DM2 away to be reconditioned. When it came back, the car ran flat above 3000rpm. In short it was a load of rubbish. The restorer had put the wrong mechanical advance plate in the dizzy. It was only a 7 degree plate instead of around 12 needed for the MGA. (Double the mechanical advance, add the static and that is what you get at 3000rpm with the vacuum disconnected with a dynamic timing light). So I was getting barely 22 degrees advance instead of the necessary 32.

I ended up with a Bob West supplied Chinese version of the 45D4 in my 1800. By chance the manufacturer had hit the advance profile spot on and it has given me excellent performance for the past 6 years or so - and it only cost me about 50 ($75).

Les, just a thought. If the advance profile is wrong can it be a contributory factor to some owners overheating woes that we see so frequently?


Steve Gyles

Thanks for your comments. I have decided to clean the existing 25D and refit to get the car running. I have checked the manual and it states that 'the base plate components are assembled with a special lubricant and no further lubrication is necessary during the normal service life of the distributor' However if I clean all the moving parts I need to regrease. Can any one help with the type of grease I should use. Thanks
Brian Paddon

Alternatively, fit a box like the Aldon Amethyst which lets you set up any distributor profile by plugging in a laptop. Set your distributor with a static at TDC and lock out the mechanical advance.

I haven't one on the A but I have used one on a Frog for two years. It gives complete flexibility particularly when you are not sure what profile mechanical dizzy you should order.
Graeme Williams

This thread was discussed between 14/02/2016 and 27/02/2016

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