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MG MGB Technical - 25D distributer refurbishment

Dear All,
i bought very cheaply (about 7 UK pounds) a 25D distributer with vernier adjustment and vac advance module - second hand of course, but its essentially because i want to learn a bit about the internals and how they all work without risking doing something bad to my own distributer, which will remain firmly on my car (73BGT). i am pretty ignorant about how all of this works to be honest and have never had the cance to raelly sit down and have a good look. I am thinking it would be a nice kitchen table project for me to learn a bit if i were to try to recondition this. Can anyone give me some basic pointers as to what i would be looking for as regards worn bits needing replaced, if bits like springs etc can stiil be purchased anywhere, or even some recommended reading - eventually i would like to end up with a nearly good as new spare distributor, then see if it is better than the one i have on the car (ineviatable, unless/until i screw it up!). i think i recall someone on the this BBS is an expert at this (is it Jeff, or have i got it wrong) and so perhaps you could also tell me if this is a waste of time as it would never be up to much anyway (hope not as i have convinced myself i got a bargain!).
thanks in advance (pun intended) i will try not to be too retarded (and will stop now).
m rae

Mick, Go to this site.
Look in the Ignition section. Look for "Tuning the Lucas distributor PDF" This article has very good information on how to tune and rebuild Lucas distributors.

Clifton Gordon

Hi Mick.

Great idea !.. I suggest that you consider doing the same with an alternator and a starter motor.

Try to track down a copy of "The AA Book of the Car" which was published in the 1970's. Copies occasionally turn up in charity shops, second hand bookshops, etc.
It contains lots of clearly presented info (including lots of colour drawings and diagrams) about how cars of this era work.

The biggest problem area is wear in the bearing between the main shaft and the body of the distributor, there should be very little sideways play. The centrifugal weights bearings do wear.
New centrifugal springs are availible, and yes, one should be 'tighter' than the other.

Don't forget that the vacuum advance and centrifugal advance characteristics of your 'new' distributor may not be the ones specified for your car, the numbers stamped on the appropriate parts will tell you what you have.


Mick, if you want the book Don references, use

There are usually several available in the UK at used book dealers, listed at bookfinder. I found one here in the USA, and its very helpful when I want to know HOW something works.

John English

You are dealing with a euro spec distributor and engine. In the US, the later distributors were tuned for emissions, and not for performance. Check the specs for your distributor and compare it to those for an earlier distributor. Odds are the specs for the earlier distributor will produce more power and driveability than the later specs.

With you test distributor you can set it up different ways and see what works best.

Barry Parkinson

nice one chaps - i will begin the big read and start playing (better move the breakfast dishes out of the way first!).
will let you know how i get on in time.
thanks again, really appreciate your time.
m rae

Mick, here are some basics to look for that will physically show you how much wear there is in your distributor:
Check the endplay of the cam assembly on the shaft. As you hold the distributor, any vertical movement in the cam assembly over a few thousandths of an inch is wear that can be corrected by machining the shaft shorter.
Remove the points plate and rotate the cam assembly while holding the drive gear on the bottom of the shaft. This will operate the advance mechanism and show you if it moves freely, binds, or if the springs have deteriorated. A relatively snug operation typically means its in good order. Binding or a very loose operation for the first few degrees (or weights that flop free without any advance) are signs of weakened springs. This, of course, is a "feel" learned from experience, but it gives you a good picture of what your advance mechanism is doing.

Check the fit of the weights on their pivot pins once you have it disassembled. They most likely will not be a machined fit anymore. If the holes are elongated in the weights or you can see or feel a flat spot developing on the inside of the pivot pins, you should look for another core. Measuring the vertical play in the weights is also a good determining factor for wear.

Tools required: large flat bladed screwdriver, very small flat bladed screwdriver, posi-drive screwdriver, 11/16" wrench (to hold the cam while removing the shaft screw), 3/16" drift and hammer. For further investigation, a micrometer, dial indicator with base, and of course the all important distributor tester!
Let us know what you find!
Jeff Schlemmer

Don - where are centrifugal springs available from pray? I have never been able to find any. Correct springs that is, there are alternatives from the likes of Cambridge Motorsports and Anglo but they are for tuned applications and very different to any of the originals I have. Even buying a rebuilt unit from someone who claims to supply all the big suppliers I found an incorrect maximum advance stop, incorrect vacuum capsule, and only one spring! And, of course, the original advance specs are almost totally irrelevant these days with the vastly different (and still changing) fuels that are available.
Paul Hunt 2

Hi Paul.

I bought my new springs from LMG Kent Ltd (02083034811) (following a thread on this forum, if memory serves).
They seem to be an acceptably good match for the originals, but then I didn't have any 'as new' originals to compare them with.
The performance of the car wasn't noticeably changed after I fitted them during my distributor rebuild.

D P Malin

Thanks for that, it's worth getting a get to see what they are like.
Paul Hunt 2

Oh well, no further forward. LMG *do* have springs, but they really only have them for selection in the workshop to achieve a particular curve for an engine they are working on. They would sell a pack of 4 or 5 as they receive them, and you can experiment yourself (like with Cambridge Motorsport or Anglo), but the problem is they have dozens of different packs and you would need to buy one of each! They certainly can't say which pair of springs, or even which pack or packs, are needed to achieve an original Lucas curve.
Paul Hunt 2

Hi Paul.

Sorry to falsely raise your hopes, they happily sold me a pair of springs as described above, but that was back in 2003.


an update on what was found beneath the muck.......took a lot of cleaning to even see the bits you all mentioned to me. it seems that at some time the internals have been heavily lubricated, this attracted muck and so on. anyway, its good news i think. 'up and down' play of the shaft is very little. the cam is relatively unworn, polished perhaps, but not worn down at the contact corners. springs etc seem to be good as all is nice and tight, but moving when it should (the weight srpings seem to be quite new, certainly as compared to the rest of it as they were still strong and very shiny). vertical play in the weights is also pretty minimal. finally, the vacuum advance mechanism works if i put the end in my mouth and suck very hard (as a bagpipe player, i have a strong blow and suck!). new points, condensor, rotor thing and low tension leads put in . what i've done is taken it to bits, cleaned everything, very lightly lubricated and reasembled. prior to reassembly i dremmel brushed the casing, and sprayed the vac unit silver. i now understand what a distributer does and how it does it, and have the bonus of a spare distributer/spare parts set. unfortunately, in a moment of madness i laid the uncleaned lump down on the living room carpet to answer the phone, so whilst i am happy with my handywork, i am in deep sh*t when my wife moves the dining table!
thanks again to all of you out there, when the table gets moved i am giving my wife your names!
all the best
m rae

Don't know is they have Resolve carpet cleaner over there, but I suggest you use something similar.

Your cleanup should improve things immensely the way it sounds. It's nice to be able to see the difference when you're done, too.

Try brake cleaner, worked for me in similar "situations". But try on a non visible spot first!
Willem van der Veer

Resolve? Thought that was to deal with a hangover the morning after the night before. Might also help with the post-discovery ear-ache.
Paul Hunt 2

I have just bought a boxed and unused Lucas 25D distributor on Ebay for little money and want to fit it to my UK spec unmodified 1275cc MG midget MkIII. It looks identical in all respects to my existing 25D but the numbers on the casings are different. What is the significance of the numbers and can how can I tell if this recon unit is acceptable for use on my car? Thanks, Gordon, London, UK.

Those numbers (5 digits starting with a 4) are the advance curve of that specific distributor. Go to this site to look up and compare your original to the one you purchased:

Not all of the info there is correct, but it will give you a good idea. Otherwise post the numbers here. We can probably help, and possibly tell you how it will affect your engine!
Jeff Schlemmer

If it is 'new old stock' (NOS) then the curves probably are what the reference number indicates. But if rebuilt the curve could be anything, there are some charlatans out there and very few people spend the time and effort required to tweak the springs to get the indicated curves, as original springs are no longer available. Even if you *did* get the correct curve for your engine it would only really be relevant when using the petrol available at the time i.e. 4-star leaded. With today's fuels, let alone engines with worn and alternative components, none of the original curves are probably ideal.
Paul Hunt 2

Thanks Jeff and Paul these replies are really helpful. The distributor is a fully recon unit from Lucas themselves -a "B90" recon unit in original box. Could I reasonably assume that the number (which I have not checked yet) will correspond to the correct set up? eg If it was for an MGB it will have the correct MGB set up when reconditioned by Lucas? How material would it be to the performance of my vehicle if this distrubutor was made for say a Hillman? Are the differences really that noticeable on a standard untuned vehicle? Cheers, Gordon
Gordon Lewis

Gordon, the amount of advance and the rate of advance can vary wildly on Lucas distributors of different model numbers. It could offer anywhere from 7-23 degrees of distributor advance (double that at the crank) and anywhere from a 1200-6000 rpm "all-in". It may be close to what you need, but the likelyhood of it being perfect is about .1%. If you post the numbers on the casting we can probably guide you.
Jeff Schlemmer

It's quite possible that some Lucas reference numbers were originally for different BL models, and possibly different manufacturers. If it was reconditioned by Lucas then I *would* expect the correct characteristics for the reference number on the body. But like Jeff unless the reference numbers tally then the likelyhood of it being ideal for you engine is pretty minimal. Having said that *any* 25D or 45D distributor can probably be used on any engine for which they were original equipment, even a completely different reference number. You would simply have to set the advance so that you didn't get pinking at any combination of throttle, load and rpms. In practice that is what many of us have to do today anyway, given the fuels available. Even then if you normally drive in Norfolk, then take a holiday in Scotland, you are likely to start pinking like mad. The higher the octane fuel you can get the the more you will be able to advance your timing. Retarding the timing for 'cooking' unleaded will sap performance and increase running temps markedly.
Paul Hunt 2

As Paul inferred, Lucas themselves probably put the correct springs into the rebuild. The big question is whether or not you have the correct distributor for your application!
Jeff Schlemmer

Thansk both, you sure know your stuff and its very helpful. Our local supermarket Tesco now has 99 octane unleaded which I try to use most of the time so I will stick to that when i can. The new dizzy is ref no 40771E. I bought it because my my old one is very worn in the spindle and bearings and even the MG club have no recons available at present. Even when available the quality of some of their so called recon parts is very poor. Any further advice that I can pass to my local specialist garage who will fit and tune the new dizzy is most welcome.

Cheers, Gordon
Gordon Lewis gives the specs for a 25D4 40771A (it is not clear as to what the difference suffix letters actually mean) but no application. If you look up your old one on the list you can compare specs. Even if they are different, by swapping over springs and possibly weights (although often these are common across many reference numbers) you may well be able to get close to your original with good mechanicals. If the maximiam centrifugal advance numbers are differnet you may also have to swap over the top half of the spindle. If the vacuum characteristics are different, and you capsule is good, swap that over as well. I've no idea who your local specialist garage is, but unless they are experienced with distributor repairs and tuning you would be better off doing it yourself.
Paul Hunt 2

I'm servicing my distributor too, but I'm a little confused. You said "any vertical movement in the cam assembly over a few thousandths of an inch is wear that can be corrected by machining the shaft shorter." Just how many is a "few" thousandths of an inch? Is there a factory recommended clearance spec? You also said "Measuring the vertical play in the weights is also a good determining factor for wear." Wear of what? The posts? How much vertical play is considered to be excessive, and where do you measure for it?
John Hartman

John, I hold my rebuild tolerances tighter than most original distributors. I reduce the cam assembly end play to .002-.005". The originals were in the .004-.010 range. I've seen used distributors with upwards of .030" play.

The best way to check wear at the weights and pivot pins is to use a micrometer. A variance of more than .003" in the pin diameter and the weight pivot hole is excessive, but may or may not affect the timing consistency. I have new and used repacements on hand to repair any questionable assemblies.

Running the distributor on a tester and getting consistent, repeatable results is more important than following actual specifications to the nearest .0005". If Lucas had followed their specs, we would all have less distributor problems! Their assembllies consistently vary by more than .005" in MANY critical areas. Stock curves had a 10% error factor!
Jeff Schlemmer

Would soaking the distributor body in oil as if it had a new bushing installed be beter than simply cleaning it and putting a few drops of oil on the bushing? How much clearance is acceptable between the bushing and the shaft? Is it better to use a high-temperature grease on the shaft and weight posts, or just put in a few drops of oil?
John Hartman

In theory, soaking the shaft bushing in oil is a great idea, but the whole body in oil? That would promote dust and debris to stick to everything, later causing issues.
I use grease on the main shaft and a grease-oil mix on the cam shaft. I only use oil on the weight pivots, with maybe a tiny bit of grease. You don't want the weights to stick after sitting all winter, or in cold temperatures.

How much clearance is acceptible between the bushing and shaft? Well, at .002" you can start to feel play on a clean shaft assembly. By .004" you have some real slop that can affect timing. If you grease the shaft rather than oil it, you should have about .0015" to leave "space" for lubrication and heat expansion. The closer the tolerances are on the advance assembly, the more play you can get away with in your shaft bushing. It all comes down to testing each specific distributor to see if that particular combination of parts performs well on a test machine! In example, a shaft with a fair amount of runout and a tight bushing may function the same as a shaft with no runout and a moderately loose bushing.
Jeff Schlemmer

Of course, I'd clean the body after soaking it in oil.
What kind of grease do you use?
John Hatman

Today, all Lucas "original" distributor looks to be made in india.

Have a look on :

Pierre NoŽl

Sorry John! I've heard stranger things in my time than putting an oil soaked distributor in a car! The best way to do it would be to remove the shaft bushing (bearing) and soak it in oil for several days. My replacement bushings are stored in oil.
I use Mobil1 synthetic grease on the shafts.
Jeff Schlemmer

"all Lucas "original" distributor looks to be made in india"

One manufacturer, perhaps?

Took my distributor out last week to fit a different vacuum unit, and found the points covered in oil. Cleaned them by putting a strip of paper between the closed points and pulling it gently (don't pull a torn edge through the points or it leaves fibres behind). I was surprised to find no trace of a spike. These points have been on the car for over 10k miles now. I've never adjusted them in that time - I use a dwell meter to check the gap and as long as they are with the (45D) +-5% tolerance I leave them alone, just lift the lid once per year to oil the shaft and grease the rubbing pad.

I've heard of oil-quenched contacts in heavy current switching which reduces arcing and burning, maybe it has stopped pitting of my points. Now I've cleaned it off I wonder what will happen,
Paul Hunt 2

Some quite excellent discussion here. I use the "Super Lube" brand teflon grease on the distributor shaft myself. It seems to work well over the years. Soaking the bushing in a pill bottle of oil, for a period of 24 hours, has been my practice. David DuBois recommends "pressure packing" the bushing by forcing the oil from the inside to the outside--when the oil is visible on the exterior, you have sufficient oil in the bushing.

I do not trust "pressure packing", nor "soaking in oil" to be sufficient. Hence, the use of the synthetic grease on the distributor shaft. Please note that the distributor shaft has both a "major diameter" and a "minor diameter". (This means that the ends are of full diameter and the central portion is of something less diameter. Get as much grease into the "groove" as you can.)

As to the "end shake" of the distributor shaft, the only specification that the factory workshop manual lists, this is worth considering. The end shake is the up and down movement of the distributor shaft, within the distributor body, that can be measured when the distributor shaft is inserted and the brass washer (which benefits from a good coating of grease) and drive dog are installed. The amount of end shake is based on the thickness of the brass washer and how far the distributor bushing is sittin up into the distributor body.

The brass washer, which the drive dog bears against, will wear over time. It bears against the bottom of the aluminum (aluminium) distributor body. Thus, moving the distributor bushing upwards, by a few thou, can take this play out of the system. It would also be possible to supply stainless steel spacers to take up this slop. But, simply forcing the bushing very slightly higher does the same thing and works quite well.

The system has the ability to tolerate a certain amount of "slog" in its set up and still function an an acceptable manner. It is possible to drill out the holes in the weights and install a thin steel bushing into them. It is also possible to weld up the holes and re-drill them to original size.

As Jeff notes, there were some rather loose tolerances in the original production products. (Roger Parker once wrote to me about, when he and I were both young, he went to Lucas and, "an old man, in a brown smock, assembled for me the best distributor I ever owned".) Roger, Paul Hunt, and I, are now all "old men". We have owned a number of MGs over the years and have enjoyed driving them fast. I would doubt that any of us, other than Roger's one "special, old man built" distributor was, exactly on per the factory specification.

A perfect distributor, tested on a distributor test machine, is an indeal. Jeff is working towards that goal. John Twist works toward that goal. I play with it, owning the machines necessary. But, I am not convinced it is an absolute necessity except for the very few who can drive their cars to the absolute limits of their capability (both those of the car and the driver).

Today, I had lunch with Sherwood Parker, who, as a representative of the Bob Bondurant School of Higher Performance Driving, got me an excellent deal on the Bondurant course after the 9/11 shake up. (Sherwood got me the four day course for the price of the two day course because so many people had decided not to fly out to attend.) Both of us are "retired from racing". Neither of us thought that a degree or two of mechanical or vacuum advance was significant in either our daily drivers or in our race cars.

As to Lucas 25D distributor bushings, my tech article on this subject, available at tells how to replace the old, worn bushing with a new one from World Wide Auto Parts. The owner of World Wide, Peter Caldwell, sent me two bushing, free of charge, with the proviso that I would give them a fair testing and could write whatever I thought about them. I found them to be quite excellent, replace every 30K miles, and far better than any other bushing I had tested.

John Twist, Jeff, or I (I do not offer rebuild services) could provide you with a better rebuild than the simple bushing replacement that Peter offers. But, for most useages, simply cleaning the distributor throughly, installing one of Peter's bushings, and throughly lubricating the system would be sufficient. Making sure you take the end-shake out of the system when you do so.

Les Bengtson

This thread was discussed between 19/07/2006 and 09/08/2006

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