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A Discourse on Distributors 

Many of you who work on your own British cars have accumulated spare parts for them, possibly including some extra Lucas distributors. I happen to have an entire box of them in my garage, in various states of decrepitude. One of the apparent advantages to the Lucas distributor is that they appear to be almost all interchangeable. As long as you stay within the same number of cylinders (bumps on the distributor cam) or within either point type or electronic type, they all will fit, right? WRONG. Sure, they all have the same shaft and driving mechanism and will physically fit on the engine, but there are many detail differences that can significantly affect the performance of your car. OK, now how do we tell them apart and make sure we have the correct one for our engine?

If you look on the side of the distributor body, you will find a machined flat surface with some numbers on it. The top number is the Lucas part number, like 40897A (1966 MGB) or 40561A (1959 Morris Minor). This part number is the key to finding out which car the distributor fits. The bottom numbers show the month and year that the distributor was manufactured. Those of us that are real originality nuts have to make sure that this date is close to the production date of our cars! My research indicates that the suffix letter in the Lucas part number may not be significant, it may only indicate slight changes to the distributor during its production run.

But what are the differences in the distributors that are significant? Most of them have to do with the vacuum and centrifugal advance systems in the distributor, the systems that allow the spark to fire earlier in the cycle as we speed up the engine. First, the centrifugal advance uses two small springs and counterweights to turn the distributor cam slightly as the speed of rotation increases. Different distributors use different tension springs on the centrifugal advance, some have two equal tension springs and some have unequal springs. The distributor cam has a metal projection at the bottom that limits the total amount of movement of the cam in response to the counterweights, therefore limiting the amount of advance. The length of this metal projection can vary between distributors, from a maximum of 5 degrees advance up to around 20 degrees advance. The maximum degrees of advance for each cam is stamped onto the metal projection, you can see this if you remove the base plate from the distributor body ( the plate that holds the points and condenser, or the electronic trigger).

Vacuum advance units can differ as well. Outside of the obvious difference in vacuum line connection (early distributors have a screw-on fitting, later ones have a push-on one) the vacuum advance units can differ in several ways. The total movement of the vacuum advance spring, the vacuum at start of advance, and the vacuum at full advance can all differ between different units.

If you clean the rust and corrosion off the bell-shaped end of the vacuum advance unit, you will see two sets of numbers. One is an eight digit number like "54413568", this is the Lucas part number for the advance unit. The other set of numbers has 3 numbers separated by slashes or dashes, like "5-7-4". These are the critical numbers that tell you the characteristics of the unit. The first number represents the vacuum point (in inches of mercury) at which the advance unit begins to operate. The second number gives the vacuum point at which the unit is at its full advance. The final number gives the total advance (in crankshaft degrees) for the unit.

If you need to put in another vacuum advance unit, you should try to find one that is close to the characteristics of your original one. The same is true for your distributor cam and centrifugal advance springs. But how do you find out what parts are supposed to fit your car?

I have Lucas parts listings that cover the years 1948 through 1976, these will tell us the distributor part number, the vacuum advance part number, and part numbers for the distributor cam and springs for most popular British cars. Sometimes Haynes workshop manuals also provide the Lucas distributor part number for each car, most original workshop manuals or parts books for MG, Morris, etc. will not give the Lucas part number, only a BMC or Leyland equivalent number. I also have an interchange listing from BMC/Leyland to Lucas, but it only goes up to about 1974.

I also have an old paperback manual from Speedsport entitled "Tuning Lucas Ignition Systems". This book has listings of most Lucas distributor numbers at the back, along with advance system specifications. We can use both the Lucas parts books and this book to try to nail down what was original for your car.

The only gray area concerns the distributor cam and springs. Since these parts are not stamped with their Lucas part numbers, the only way we can match them to a particular distributor is if we know that your distributor has not been modified, and we compare these parts visually. Again, if anyone has any old parts books or literature that lists part numbers and characteristics for advance springs and distributor cams, please call me. I will also be happy to verify if what you are running in your engine is the correct distributor. My number is (916) 961-3060.

Paul Kile