Bruce D Skivington writes:
Most of the British car industry except Ford (US firm ) and Vauxhall used Lucas components. when i started in the garage in 1964 everything was Lucas distributed through a network of dealers. The parts were common across ranges. You can swap lobes. I have replaced a lot of distributor bearings and they were standard. rotor arms sometimes differed but there was a standard arm. Caps could also be different with straight or side outlets for ht It was only later that other distributor types came in. For 1946 - 1980 most of the distributors can be rebuilt. The most common fault was worn bearings, then springs gone then vacuum leaking. At the same time there was standard starter which fitted most cars and a standard 22amp dynamo. Solenoids and 7inch headlamps were standard as were 51/2 inch four beam.
Charles the 25 series should all be the same. there are still parts available over here. Now we have a Lucas owned shop and it is just a mile away so if anyone has a part number I can check if it is still available.
You can rebuild the vacuum advance, you can undo the crimping and replace the flexible sheet. It is a pretty crude device. The accuracy of these units is rubbish absolute rubbish.
As far as the advance curve goes you can use virtually any without much difference. During my lecturing years, I spent a lot of my time researching the computerization of ignition. We actually built computerized units to run British Leyland A series and E series engines.
If you are really stuck as long as the distributor turns you can use the HT side, using a 16F628 pic from Microchip with a hall effect sensor on crankshaft and a potentiometer sensor on the throttle a ULN2003 Darlington Driver for the coil switch and a bit of code. You can even simulate the vacuum by measuring the throttle position and engine speed.
Modern engines need accurate timing to get through all you guys fancy emissions. These British proper engines can fire the spark anywhere from 45d before to TDC and still run. since the burn time between individual cylinders can vary considerably the actual accuracy of the timing is pretty remote. remember the whole point of the advance is to ensure that fuel burn time which is relatively constant irrespective of engine speed is completed with maximum cylinder pressure being developed just after TDC on full load with the burn being completed more effectively under lighter loads by advancing start time. One of the student experiments was to run a A series engine on advance from -90 to 90 starting at zero. We had converted distributors with no advance weights and handles to move them and markers on crank flywheel. amazing just how far a A series would run. under load it would pre-ignite quite quickly.
Tom had a Aerial NH Hunter 1954 single cylinder manual advance retard and exhaust valve lifter. if you tried to start it with advance high and valve shut the kick start came back up as quick as it went down. If a car got too close, full advance ignition, lift exhaust valve and open throttle, sheet of flame and loud explosion out of exhaust, they soon moved back.
Charles Irvin writes:
The Lucas CAV catalogues list every vehicle produced for the year of the catalogue, then they have a breakdown of every electrical component on any given vehicle. They then go into a listing of each part by type (lamp units, lenses, starters, solenoids, generators, alternators, etc.), complete with part numbers & applications THEN, they go another step: they list ALL rebuild kits, as well as separately available rebuild parts for each component. THEN, they have a picture, or drawing, of each part! THEN, they have, at the back of the catalogue, a numerical part number listing, complete with superceded part numbers! (hence why the people that have these catalogues won't part with them: some parts may have superceded numbers 5 or 6 times over a 20-year period!)
The catalogues were available for a single make & model type (soft cover - may have been limited to make only - MG, Triumph, Austin/etc. -, I can't remember), or as a master catalogue for all vehicles of all makes (hardbound). No - I don't have any, but that friend of mine that used to have the shop had tons of 'em - and of course, he wouldn't let me out the door with one. (he still won't part with any of them) AFAIK nobody's put these catalogues on a CD yet, but if you find one, they're red (hardbound), or red/black (softbound). Tom jarred my memory a bit: I think the ONLY mechanical difference between a 25D4 and a 25D6 is the number of cam lobes on it! I think the castings are the same, just a different part number stamped on the outside. That was pretty much how they did it.
Tom Gross writes:
As Charles says, they're pretty much all the same thing. Once you get a distributor in good condition with the right number of cam lobes to fit into the hole, engage whatever slot-pin-gear is used to transfer the drive, and figure out how to hold it in place, I'd think you would be good to go. I never crossed the numbers between the NADA parts book and the regular parts book to see if the numbers for the distributors were different. Seems like they could be the same. I can't think of any differences except for maybe the advance curve.
The real difficult-to-get part is the vacuum advance. There are people who have rebuilt ones available, though. Last year I used a magneto with mechanical advance on my 500 cc single cylinder motorcycle. Having one cylinder makes it very easy to tell the effect of messing around with the timing. My bike seemed to run just as happily at around 28 degrees advance as it did at 38. How fast I moved the lever to advance it as I sped up it didn't seem to matter much, so I just pretty much left it around 34 degrees advanced, except when sitting at a traffic light. One time I did notice a definite effect on me when I forgot to retard timing all the way, and then tried to kick start it.