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MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MG Y Type - DKYH4A rotor size.

Using the top photo as a guide, I've been running Y3348 for 12 months, apparently set up fine on both the ignition side (using the red DD rotor A), and the fuel side. Today, a sudden cutout towards the end of a 10 mile run. I suspected ignition, so looked under the distributor cap - to find clear scorch marks and surface damage along the latter two-thirds of the rotor arm. The second photo is an attempt to show that.
I fished out two older spares from the glovebox, picked the cleaner one, and she fired up instantly and took me home. BUT - the two spares are much longer in their reach, both at 40.4 mm as opposed to 31.68 on the red DD rotor; and one has a significantly longer "tail" on the brass arm. Could it be that the shorter reach red rotor is not suitable for the DKYH4A, and that the spark, in attempting to bridge the large gap to the distributor cap terminals, has gradually damaged the rotor arm?
Put another way, should I stick to the cleaned-up originals with the longer reach - and if so, which is correct for this distributor - the one with the longer "tail" (19.55mm), or the shorter at 14.55mm.
Hoping folk with more experience can help.
Thanks as ever.
John.





J P Hall

John- looks like you need to have another measure up to me. To measure the reach/length of the rotor you need to measure from the centre of where the carbon post runs out to the end of the blade---They all look the same to me. The longer part on the other end of the two black rotors is an unneeded counterbalance, bit of an overkill.not needed.
Out of the two black rotors I'd go "B" for two reasons
#1- less worn where the carbon post runs
#2- If you have a choice go for the one without the rivet- the rivet is a shortcut for the spark to track out to earthing on the dist. shaft. The usual cause of death to rotors is either spinning the engine over on the starter with the plug leads off the plugs--High resistance in the plug leads or maybe monster plug gaps.
I've got a fella passing your way over the weekend, he's got room for that TC on the truck--just saying.

willy
William Revit

On the length of that tail on "C" ,it doesn't need to be that long on your car, you can see the deposits on the red rotor where it's been arcing, nowhere near around as far as that long one.
William Revit

Thanks as always, Willy.
You're right - the measurement from centre pole to end of brass arm is 21.05 mm give or take a bee's wick. The counterbalance design on the black ones sure fooled me. The red (failed) rotor is marked DD 4, which I assume means it's a good quality Distributor Doctor one; so what actually causes that damage to the arm and eventual ignition loss - is it something I can prevent?
Your mate is very welcome to call in for refreshment, especially if (a) he has your skillset, and (b) he brings his wallet.
John.
J P Hall

The biggest killer of rotors is cranking the engine over with the ignition on with the plug leads off--that'd include maybe a dead plug lead or if one happened to drop off while you're motoring along.Basically anything that prevents the spark getting out to the spark plug, then it has to go somewhere so usually burns a hole in the rotor or out through the coil tower. Everyone seems to recomend the red rotors- BUT which red rotor, I don't think the dist.doc. one is the one but not sure---I'll check-
Only stirring you up about the TC, I can't see you parting with that. I've spent a fair bit of time on my special lately and got him ready to paint--more or less
It hasn't seen daylight since 1959 so that'll be a shock for it.
Cheers
willy
William Revit

OK---the red DD rotor isn't the one to get it's the genuine lucas red one, also watch out china has invadaed with red CherryBerg ones as well so make sure you're getting genuine lucas in a box
William Revit

Roger that re. the "Genuine Lucas" rotor Willy, although I've read of all sorts of offerings being marketed in Lucas boxes. B above has the original Lucas emblem on it - I bet you don't get that anymore. Do you know of a source of genuine Lucas, or at keast genuinely made to Lucas specs?.
Yeah, I realise what you're about with the TC. I dream of the day I can get at it.
Will check plugs, leads, points, everything I can think of today in an effort to eliminate the CAUSE of the arcing/sparking/degradation of the rotor arm.
Thank you. John.
J P Hall

https://www.angclassiccarparts.co.uk/products/morris-minor/morris-minor-series-ii-morris-1000/ignition/red-performance-rotor-arm-lucas-morris-minor-1000
William Revit

Bottler, Sir William - ordering 4 as we speak. Thanks as ever. John.
J P Hall

Check the coil polarity.
Bob Wood

This'll be interesting-
William Revit

Righto: Positive earth; White from coil +ive to control box, white/brown from coil -ive to dizzy.
Correct I believe, but happy to stand by, Bob and Willy.
John.
J P Hall

And in further news: after just a 10 minute run today, I was curious, and had a look at the rotor: sure enough, already a clear and definite scorch mark on the trailing half of the rotor. 10 minutes!
Willy, all leads are in as new condition and firing is even on all 4 cylinders.
I'm still at a loss as to the cause, especially with it reoccurring immediately after cleaning everything as recommended in the wsm, and making sure points gap is correct. The only thing I'd say is maybe order a new set of points and a new condenser?
John.





J P Hall

In my opinion- rrrrs about'
If your car is pos earth then the ignition power is neg. so your white ign. wire should be hooked to neg. side of the coil and dist wire White/'black' should be on the pos. side
It'll run ok as is but you're cheating yourself out of about 20% of your spark and the spark will be going through your plugs backwards from the earth tang to the centre post instead of the correct centre post to earth direction.
There are a couple of ways of testing spark direction but that's just a waste of your time--just swap them over.
willy
William Revit

Yup, that makes total sense Willy. I expect Bob will concur. I've just proved that the coil will "work" whichever way round, but that eventually the car will complain.
My bad. I'll reverse the connections tomorrow and confirm findings after a bit of driving. Thank you so much for your willingness to help, Willy and Bob.
J P Hall

Hi John,
Have you had your distributor rebuilt? - how worn is the shaft?

Coil polarity is important, but I'm not sure it's causing your issue.

Keep at it - you will work it out.

Tony
The Classic Workshop
Black Mountain
A L SLATTERY

I haven't, Tony. That would be worth doing, as the Y is a keeper. No obvious slop in the shaft, but who knows what might lurk down inside. Also I know you're a fan of remapping to suit modern 91 ron fuel.
I'll fish out the name of that place you recommended.
John.
J P Hall

Our YT was purchased in 2007, and was already setup for negative earth, but still retains an original CB/SW ignition coil.

After reading this thread, I thought I should check coil polarity, and lo and behold the CB terminal was still wiring to the dizzy. Now swapped over.

The coil is going to fail one day, and maybe it should have a long time ago, but it's nice to have an original Lucas critical part working so well after all these decades.

Cheers

Tony
The Classic Workshop
A L SLATTERY

Well fancy that, Tony! - the wire swap might make a difference to your YT's power too - I certainly feel that Y3348 is now back to running as well as it did when first set up properly, 12 months ago, before the crud built up on the rotor blade.
I checked today after 2 runs (just 10kms each), and sure enough the rotor blade (brass) remains clean as a whistle. Just a slight dulling where I applied 2000grit emery paper.
This has been yet another learning curve.
Cheers all. John.
J P Hall

Learning is good John -- keeps you/us interested. And as Tony has just shown, never take anything for granted---check stuff yourself --trust nobody--unless they've got beer of course--lol
Full points to Bob for thinking of checking the coil polarity, I'm feeling guilty there for taking it for granted that it was hooked up correctly- my apologies for that.
You must be getting your Y pretty right by now---Time to get onto that TC.

willy
William Revit

Willy - mate, I've drawn so much benefit from your wisdom and your willingness to share it. You cannot feel bad just because I keep finding new and more obscure ways of making mistakes. I love this hobby, it excites me to tackle different jobs every day - 4 years retired already - plus of course running around with whipper-snipper and mower. Wouldn't be dead for quids.
Yes, the Y is overall pretty tidy, everything done bar a couple of little niggles - I put one Jackall pipe a bit too near the d/side rear axle; and one trafficator solenoid needs a cleanout. Neither stops Joanne from using the car, but I promise to check the distributor rotor on a regular basis!
John o'the north.
J P Hall

A chilly 10 degrees here this morning.

J P Hall

10degrees--who'd live in Queensland.
Interested in the colour of your car,I'm just doing mine now in Connaught green, looks similar to yours but i'd say from the pic. i'm a bit darker. Which green did you go for.
There's a little trick I can run you through explaining the carbon deposits on your rotor if you want, I used to use it in apprentice training schools for the stubborn ones that wouldn't believe spark travels in a given direction.

willy
William Revit

(a) I would very much like to know more, Willy - whenever and however you wish. Let me know how best to communicate.
(b) I went for a Jaguar dark green, the tin says HER, and for some reason I thought there was a 003 involved - I can follow up if you need more.
In the sun, the green jumps out more, but in the shade it can be mistaken for black. A couple of piccies to follow - plus a teaser of the ex-hillclimb TC hiding in the corner. Coincidentally, it's a very similar colour.
John.




J P Hall

... and the sleeper: you can see the slab firewall with various conveniences.
John.




J P Hall

What a magic pair of cars, yep I'd say my green is possibly darker? but that TC, -fantastic patina, there's no way i'd restore that, 'just' a real good clean up, get it going and use it as is.
I'll put a little short lesson/experiment up on here for you to play with that'll explain the sparks. Bit flat out for the next couple of days, so over the weekend it will have to be.

willy

By the way,
HER= Jaguar British Racing Green / Brooklands Green
William Revit

Absolutely 100% what I plan to do with the TC Willy - when I thin the flock I'll part with the BGT and the Skewes/Needham built TD, but never the Y or the TC.
Sorry I seem to have meandered off my original topic, the DKYH4 rotor issue - but your info about the direction of flow of electrons will bring us back, and will be hugely appreciated. I wouldn't mind betting plenty of folks here are not clear on the issue.
John.
J P Hall

JP Hall
If you Look in John Lawsons book he writes
rotor arm should be Lucas400051
John YB0362
JC Jebb

Thanks John - your info prompted me to look at Barney Gaylord's excellent notes (MGAguru, item IG 111) on rotor types, different designs, failure points etc- all of which backs up Willy's comments. One Australian Supplier, Classic Fasteners, describes the rotor as being for CLOCKWISE direction of shaft, on the basis that the direction should be read from underneath - whaaa? I always thought it was ok to stand on my feet, look down at the distributor and call it anticlockwise.
John.
J P Hall

I believe Lucas measured the direction of rotation from the bottom for some reason John.
Ian
ian thomson

John
If you can bear with me for a bit I'll try and explain the polarity connections for your ignition coil. and why your rotor was getting carbon/burn marks.

Earlier you mentioned electrons--I don't really want to get all serious here, the aim is to explain as simply as we have space for here why it's important to have your coil connected correctly. If you want a quick electrons lookup, look here--

https://web.engr.oregonstate.edu/~traylor/ece112/beamer_lectures/elect_flow_vs_conv_I.pdf

The ignition system is designed to give out a high voltage negative output which looks for a positive earth- It doesn't matter if your car is neg.earth or pos.earth the coil needs to and will when wired correctly, see it as pos.earth - why you ask. The electrons(spark) need to travel from the coil,through the distributor to the spark plug where the spark jumps from the centre electrode to the earth tang.
Electrons work better if they're jumping from a hot place to a cool place and the greater the temp. difference the better/easier the spark will jump.
So here we have a spark plug which by design has a centre electrode that runs quite hot-the range of heat is decided by the length of the centre electrode/insulator assembly-the longer the electrode the hotter the plug--shorter,cooler.
From this you can see the spark will be happier jumping from the warm centre electrode to the cooler earth tang on the base of the plug.
This is where the problem is with the coil running in reverse--the spark will be jumping in reverse from the base tang to the centre electrode---it'll start and run ok but as the electrode gets hotter it will be more difficult for the spark to jump accross and can take up to 15-20% more effort from the coil when under load--This is where a coil that would normally be sufficient for the car will reach it's limit that 15-20% earlier under stress and can break down under load/high revs.
So it's important your coil is poking out a neg spark looking for a pos. earth.(electron flow direction)
Although the coil needs power from the battery to charge up, when it collapses and sends out it's spark it doesn't care if the car's battery is neg. or pos. earth. it will just send out it's neg. spark to find somewhere to land, and it's the coil/plug leads, and distributor's job to guide this spark to the plug where it can do it's job--
I can hear you thinking , what if the car is wired neg. earth.--doesn't matter, with 30,000volts going down the line it just doesn't take any notice of the piddly 12v system. it just blasts it's way out there.

Now connecting the coil--IF the coil's posts are marked pos/neg it's easy-
Pos earth car--coil pos to dist. coil neg to ignition
Neg earth car--coil neg to dist, coil pos to ignition
But
If the coil is marked CB/SW or Dist/Ign you have to be carefull. the coil 'could' be either pos or neg earth.
Unless you know the history of the coil and that it is for a pos or neg earthed car. Most English cars were pos earth but a lot of US cars were neg earth with the same coil markings.
Testing-
For me and most w/shops it's simple, hook the coil lead up to the scope, if the sparkline goes upwards she's all good - if the line goes down the coil's hooked up back to front.
You can test at home if you have an old analogue volt meter with a swing needle. Hook the positive lead of the meter to the engine block, neg lead to either of the coil primary posts. turn the dial up to the highest voltage so it can't see 12v then crank the engine a little--don't start it up. If the needle flicks up then the coil connection is correct-if the needle tries to go backwards the coil wiring needs reversing.
If you don't have an old meter or don't want to risk it getting killed then there's another way which is fun--
Best done in the shade-
Rig one of your plug leads up on something so it's about 10-15mm away from the spark plug and start your car up making sure it's running ok and you can see the spark jumping to the plug, you might have to adjust the gap down a bit depending on how good the coil is.
Now with a graphite/blacklead pencil. holding the pencil by the wood. poke the point of the pencil into the middle of the spark---If the coil is correctly wired in there should be a good solid spark from the plug lead to the pencil then a fluffy/flared spark out to the plug. If the solid spark is between the sparkplug and the pencil and then flares out the other side to the lead then the coil primary wiring needs swapping over.
See pic.
So this is what was happening with your rotor, instead of the spark jumping from the rotor to the cap posts, the fluffy spark was jumping from the posts TO the rotor.
I drilled holes in a cap once to watch what happened and it was lit up like a Christmas tree inside with the coil reversed
Now, PLEASE, if you're playing with this, don't get you finger on the end of the pencil and don't go holding the plug lead by hand, rig something up to hold it in place.
These sparks can ruin your life.
If you've ever had a decent boot from a coil you'll know the direction of flow is definately FROM the coil TO Earth not round the other way.
BE CAREFULL





William Revit

Willy, thank you for taking the time to put together that explanation. As an enthusiastic layman, this knowledge is priceless as enables me to understand the "whys and wherefores" of this wonderful hobby. I honestly believe that, with your approval of course, it should be put in the Y Type Register website (Paul Barrow?) - although of course the principle applies far and wide.
If I knew how, I would suggest a change to the original thread title - to add something like "Coil connections explained"?
Anyway, off I go to borrow an analogue meter from a mate in the car club - I don't plan to attempt the pencil trick until other avenues have been exhausted!
Several more local runs, and I can report a still-shiny rotor tip and smooth running. Awesome, as they say! Thanks also to Bob Wood for the coil suggestion.
John.
J P Hall

John- the explanation was just for you and your rotor , it doesn't need posting around everywhere or renaming., that's not what we're about.
IF you're going to borrow someone's meter, make sure the car doesn't start or the meter will probably be toast--maybe do it cold with no choke so it doesn't fire up, you only need to crank enough to see which direction the needle flicks. and make sure you're on the highest possible voltage scale on the meter.

willy
William Revit

Totally understand, Willy - nuff said. I'll follow your instructions to a T, and post my findings.
John.
J P Hall

Honestly John ,if it's going ok now I'd be leaving it be and finding something else to do------------------TC
William Revit

I hear you, brother - just a quick confirmation of the Y's electrical health, then it's a daily driver while I get back to other chores. This has been a tremendously helpful conversation, and I hope of interest to others. John.
J P Hall

Thanks for the explanation Willy. I have often wondered though, if there is a difference in coils manufactured for positive and negative earth cars. As you have explained they will work when connected the "wrong" way round but do we also need to know how they are connected internally? I feel that the direction of winding of the primary and secondary coil windings must have some effect and, perhaps, also where the common connection between them is made. If my suspicion is right we should all be measuring the actual direction of the spark rather than simply relying on the markings on the coil.
Ian thomson
ian thomson

I am pleased the problem has been found.. It is information that I remembered from a Lucas 3 day training course in Birmingham 50 years ago.
Bob Wood

Ian - interesting point; I'll be interested in Willy's view.
Bob - see what you've started! Where abouts in Qld are you?
John.
J P Hall

Ian--not going to start a school here, so briefly.
Ign. coils have two windings, primary and secondary or we could name them (roughly)input and output.
I take it from your question that you know how coils work-
Both windings are connected internally to the primary Pos, terminal by design so that the secondary coil, having one end of it's winding connected to Pos. has a Neg. charged output from the other end of it's coil which is the output to the sparkplug. And that's where it's important to have the coil primary wiring polarity correct to make sure that output is a Neg. charge.
So basically, all ign. coils should have the same internal connections
It's just the CB/SW (+/-)markings that confuse some.
I would guess that they marked them that way back then thinking that no-one would have a need to turn their battery around- and any neg earth car from this era would have their coils marked in reverse CB/SW (-/+)---same coil just marked up in reverse.

willy
William Revit

Also, there are some German Bosch coils about with the DIN Standard markings 1-15 where 1= NEG 15=POS
William Revit

Well, after 3 weeks of almost daily 20 minute runs (just 10 mins each way, no decent long runs yet), I can confirm instant starting, willing running, and just the standard smudge of brown on the rotor. No buildup of crud, no pitting or heat-scarring. Great outcome - and as a bonus, the driver's side trafficator leaps to attention like the left one - it was a bit sluggish before.
Thank you Willy!
John.
J P Hall

This thread was discussed between 14/06/2024 and 14/07/2024

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