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MG Midget and Sprite Technical - Beware another bad rotor arm

Another rotor arm to avoid, this was in a new distributor from Aldon Automotive a few years back - so hopefully they no longer have them fitted(?).

The information and photos are from MGB owner Richard Thompson.

I'd not seen one like this before.

Richard had an interment problem with it until he discovered it and replaced it with the original 'Red Rotor Arm' from the Distributor Doctor in 2017 and has had no problems since. - http://www.distributordoctor.com/red-rotor-arms.html

Second photo is of the rivet after Richard removed it showing very good evidence of it playing up.

Not all rotor arms that are red in colour are 'red rotors' and the Distributor Doctor's original 'Red Rotor Arms' have since 2012 carried the DD marking.








Nigel Atkins

Nigel,

I think that's not actually a rivet but is being used to hold the brass plate in place. The type of fastener I have seen before but never used and is like these https://www.rivetwise.co.uk/products/index.asp?section=dodge_spred_lok_inserts-2130 , it's threaded inside and intended to be pressed into a hole and a thread screwed in which expands the insert to hold it in place. The internal thread is evident in your 1st pic and the fingers allow for enough flex for insertion. Maybe it was too large and too close to the centre so allowing the rotor arm material to breakdown causing arcing.
David Billington

Nigel
Look familiar--!
This is a Bosch replacement button for a Lucas dizzy
I've been a little confused with your pushing of red rotor buttons in the past--Several manufacturers produce red buttons, black buttons, grey buttons, I actually had a clear one and a clear cap on my MGB once, that was interesting in the dark
The rotor button you have pictured there is a Bosch button , any rotor will fail,,BUT only when the rest of the system isn't up to scratch As soon as you have any little missfire from a plug or a lead, even over lean mixture, the rotor will be under stress and possibly die as the spark has nowhere else to go-

willy



William Revit

David,
thanks that explains the grooves. At first on a smaller picture I thought Richard might have put a thread in to extract the "rivet" but then in zooming in I could see they weren't a thread but grooves.
Nigel Atkins

Nigel
It is a thread ! - I have a spare and its a Bosch in a genuine Bosch box - part no 1 234 332 848 (ISTR ii bought it when I had the distributor fitted on the R/R)
The thread may well be 8BA as its a good fit (my metric taps only go to M3 and the smaller metric screws I have are a poor fit.

Willy, when mine failed it was a new pertronix in a new D45 distributor/ quality lead set and new plugs which had only done a few hundred miles but I had just fitted my HIF6.
How does the mixture being weak cause a problem?
Shortly after the problem I had it back on the rolling road and they tried some needle changes.

R.

richard b

Richard,
I've just had to delete my post as I thought perhaps Willy had either a newer Bosch arm than the ones I bought or another make sold as a Bosch.

The fact that you and Richard both had this rotor fail in a 45d Aldon dissy might be a coincidence or just a bad batch, either way they were bad so perhaps for others best avoided or be made aware of at least, especially if they have an Aldon dissy fitted. If you lift the dissy cap off and find one of these and you've had intermittent problems them it might be the cause. Even if not I wouldn't chance it but others can make their own decisions.

I'll still put my post to Willy to explain the red bit of it all.
Nigel Atkins

Richard,

When the mixture is weak it requires a higher voltage to initiate the spark which puts more stress on the ignition system HT components. I had the problem on my first VW Passat where it started to misfire as it was sometimes arcing from the plug cap metal sheaths to the head, it ran OK on the move except occasionally when going over a bump the engine would falter or stop, I eventually tracked it down to a perished manifold to brake servo hose which was leaking enough to cause lean running and I guess bumps caused it to leak enough to stop the engine but correct itself when stopped as it could be restarted to continue. It should have been picked up at MOT but wasn't, the perished section was on the underside of the hose so not visible from above.
David Billington

Found it, as just one example.

With a yellow box with Bosch over it and at a lower price than I can normally get my Bosch rotor arms for, and still not like the other Bosch arms I see for sale.

BOSCH 1234332848 rotor arm
Product code: 1234332848
£6.66 - £7.99 (inc. VAT)

(cross reference replaces Lucas DRB104C)(? I've not checked, just copied info) -

Auto Electrical Spares - https://www.autoelectricalspares.co.uk/bosch-1234332848-rotor-arm-4418-p.asp



Nigel Atkins

Willy,
now that's interesting if that's a genuine Bosch rotor arm as the Bosch and Beru rotor arms I've bought don't look like that at all. See the photo, and key, below of an example of a Bosch rotor arm as I'd recognise it (the Beru look similar but more of a brown tint to the red colour).

The thing about red is that is not just the colour but a description of a particular make of rotor arm that has then been copied and the name/description also used on the copies.

I don't know if its the same your way but back in the day someone might say they've bought a "hoover" when they bought a vacuum cleaner, the name became synonymous with the product, so even though a competitor's vacuum cleaner, such as Electrolux, had been purchased it's still referred to as a "hoover" as if manufactured by the company Hoover.

See here for the original 'red rotor arms' that are now marked DD to distinguish them from the copies - http://www.distributordoctor.com/red-rotor-arms.html

Rotor arms have been one of the piss-poor quality parts available going back a good while now, they got so bad that suppliers started copying the Dissy Doc idea of producing a better quality for those that know how bad what they were offering were. Moss over here still offer lower quality rotor arms for the tight-fisted classic vehicle owners because obviously there's still demand. Moss over here offer the Dissy Doc(?) ones (well they're marked DD) as a high quality alternative and tell you why, but they still also offer the cheaper ones. - https://www.moss-europe.co.uk/rotor-arm-25d4-high-quality-gra2101hq.html?assoc=142196

Dissy Doc's explanation of why he had his own made - http://www.distributordoctor.com/rotor_arms.html

Key to photo below -
. 1 = Bosch symbol (and part number elsewhere)
. 2 = black stuff (insulation?)
. 3 = blade end against arm material, no gap
. 4 = double thickness wall



Nigel Atkins

Richard
You asked-
"How does the mixture being weak cause a problem? "
If the mixture is lean enough to cause a missfire under load the increased power that is being put out by the coil to try and fire the lean cylinder has to go somewhere and if there's a weak spot in the system like the poor old rotor or equally thin insulation inside the ignition coil itself then that is where it will go----
A lot of modern fuel injected cars that are running ever leaner and leaner suffer from stray sparks, even coil on plug cars get tracking down the outside of the spark plug insulator quite often and that's a jump of probably 30mm or so that the spark goes cause it's easier to do that than jump the plug gap in a lean cylinder
willy
William Revit

So if I weaken my mixture I can damage my rotor arm? I don't understand.

I don't see how on an unintelligent engine like a Spridget engine, unless it's been fitted with an ignition management system with a feedback loop to the coil, a weak mixture will increase coil voltage, and thereby damage the rotor arm.

The points open, the magnetic field in the coil collapses, inducing a voltage of circa 40000 volts. The coil has no idea if the mixture is weak or rich, and no means to alter it's output voltage even if it did.

My understanding is, a weak mixture will create a hotter combustion chamber and damage the plugs, but I see no way that feeds back to the rotor arm or coil.

A weak mixture needs a bigger spark to ignite properly. But surely only if you open the plug gap to try and attain that fatter spark, then that might create a higher induced voltage, and thus greater current flow through the rotor arm, which could damage it. But our old banger have no means to alter ignition strengh on the basis of a weak mixture. Do they?

The original rotor arms we used to get, could cope with this. I've often run weak and never fried an old black rotor arm. If the rotors are being fried, it's because they are not up to the job.



anamnesis

If you don't understand, you don't understand
I'm not going to get into a engine school here--but just to answer your question-
Lean mixture means higher combustion temperature, temperature and pressure affect secondary circuit resistance, The higher the resistance the higher the voltage output from the coil will be-and if it's lean enough to missfire the coil output will be sky high and looking for the easiest path it can find----you can look at electricity flowing like water, it's like sticking your thumb over the end of your garden hose while it's happily flowing, the pressure gradually builds up until something lets go
William Revit

I'm with Anam. No way can a basic coil respond to fluctuations in mix - or anything else for that matter. Granted mix fluctuations may well affect the plug voltage required, but then to accomodate this you'd have to change the coil, or/and fiddle about with points gap, plug gap, change plug etc.
Oggers

Glad it's not just me then Oggers.

I do understand. But not how a weak mixture automatically leads to a greater voltage and increased current flow.

It's not voltage that heats metal and melts a rotor arm, it's current.

Increased resistance causes less current to flow, not more.

To get the same current flow for increased resistance, yes you need to increase voltage. But unless you put in a coil with greater output, or one that has the means to increase it's output, you won't get greater voltage, and thus greater current flow. Isn't that right?
anamnesis

Look at it this way - electric is lazy in the same way computers are thick, electric will always go the shortest path of least resistance sometimes going strongly where it shouldn't, like an idiot following TwatNav instructions into a river.
Nigel Atkins

Anam

Yes - agree with all of that. V = IR if I recall my basic physics!

Coil is basically just windings. It kicks out a constant plug voltage regardless - unless it breaks down internally.
Oggers

An--
"Increased resistance causes less current to flow, not more. "
Exactly, you do understand
With a high resistance out at the spark plug end, the coil produces more voltage to try and get to earth and if it can't get to earth at the plug it'll take an easier route somewhere, If there's no easy path like for example an inferior rotor then it will try to get to earth at the next easiest spot, most likely in the coil and eventually break down the insulation in the coil and kill that instead--
eg.
The first thing to do if I'm presented with a car with a misfire at idle is to hook it up to my favorite tool, the scope, and straight away you can see if the problem is electrical or mixture related by the spark pattern presented on the screen
William Revit

"With a high resistance out at the spark plug end, the coil produces more voltage to try and get to earth"


No it won't. Coil voltage is constant. Current may vary though depending on resistance.
Oggers

Nah--
A typical coil pokes out about 40000v
Normal running the output from the coil 15-20Kv max to get the spark across the gap initially then 3-4Kv during firing
The voltage will only increase with a higher resistance to try to fire the plug--wider plug gaps lean mixture faulty plugs or leads etc--
With an open circuit, for example with a plug lead off, this is when the coil will produce it's full power eg. 40Kv to try and get to earth

The problem with lean mixture is that the coil has to produce more power to fire across the plug to start with, then because of the light fuel load the spark duration of the plug is also reduced leaving excess power remaining to oscillate back into the lead/coil or out through any weak point
This is also the reason why resistor plug leads are recommended for electronic ignitions ,to prevent this residual stray current from returning back into the system and killing it

William Revit

Seems to me we are in some kind of agreement.

Without looking it up, I recall the basic coils fitted to sprudgets, were circa 25k output maximum voltage from the secondary windings. It doesn't matter what external factors you change, that's all you get. The original rotor arms were more than capable of handling that. Weak mixture or not.

Many people put 'sports' coils on, which are capable of 40kv max. But you are unlikely to see 40kv, even if you weaken the mixture; and if you do, the original rotors were able to handle it.

The problem is not caused by a weak mixture. It's caused by a rotor arm that isn't made properly, or is made of the wrong materials, or both.
anamnesis


"The problem is not caused by a weak mixture. It's caused by a rotor arm that isn't made properly, or is made of the wrong materials, or both."

I hate doing the ""thing
As you say the problem is not caused by a weak mixture alone but if the rotor arm is of inferior design the weak mixture can be one of many reasons why it has been deemed to be the place to go for the spark to jump
This is where this started with Richard asking-
How does the mixture being weak cause a problem?
I think I've answered that question-

I'm going for a beer-
Cheers
willy
William Revit

Except that, that kind of implies, that someone with a crap rotor arm can compensate by enrichening the mixture; when the correct response it get a decent rotor arm.

I think 🤔 I'm right in saying, compression ratio has more effect on gap voltage requirement than mixture. The higher the cr the higher rhe voltage requirement. So you could say reduce the cr. Or, just get a decent rotor arm. I'm going for breakfast. 😉
anamnesis

Anam

I think that every component from coil primary to coil secondary to main HT lead to rotor arm brush to rotor arm to distributor cap contact to plug HT lead to plug electrode and finally to plug gap are all under stress the instant the points open. So the quicker the discharge, the less stress for those components I.e. that electricity wants to flow to "earth".
The plug gap discharge is complex and is affected by various fuel, temperature and pressure factors, as you and Willy have alluded to. I'm far from an expert on this but can appreciate the need for decent components, the rotor arm being one of them.

Coffee time nearly!
Bill Bretherton

"The plug gap discharge is complex ---".

Amen to that. Have a read of Paschen's law. Lol.
anamnesis

Just to finish off,
The first half of your last post----
????? It doesn't kind of imply anything, no one has said richening the mixture would compensate for a dicky rotor, Don't know where you pulled that from

Second half, yes, compression ratio can be a factor but as i said before i'm answering Richard's question on can lean mixture effect the load on the system including the rotor---which i think has been well and truly answered

over and out

willy
William Revit

"--- even over lean mixture, the rotor will be under stress and possibly die as the spark has nowhere else to go-"

Clearly the implication is, making the mixture richer will save the rotor arm. Which I agree is the wrong solution.

I said basically, if the rotor can't take a lean mixture without failing, the rotor is crap and needs replacing.

The original black rotor arms were easily able to take the maximum coil voltage without failure. I know because until recently I still had an original black rotor arm on my Sprite, and have had a 40kv coil on it for yonks (not that it made the engine make more bhp lol, but it FELT faster 🤣), and have run a bit weak for as long as I can remember.

Hey ho.
anamnesis

Hey ho's right-------I'm over this
There's a BIG difference between running lean and overlean

"Clearly the implication is, making the mixture richer will save the rotor arm."
No that's not what i said or mean at all, I'm talking a fault like leaking manifold gasket or something like fuel starvation or a blocked injector etc---causing extra heat/pressure in a combustion chamber from being OVER lean
You can twist what i say round to suit what you think but if the mixture is correct, then richening it further won't fix a faulty rotor ever--I didn't say that at all-I'm talking OVER lean
I get guys in my classes that want to twist and argue things and at the end of the day they haven't learnt much compared to them that want to learn and get something out of the lesson

William Revit

I think it's a cunning fail safe device. If you run excessively lean you are likely to blow holes in your pistons but the Cunning Plan is for the rotor arm to fail so the engine cuts out and saves you a very expensive repair bill !
Those of you fitting quality red rotor arms beware of unintended consequences
GuyW

Willie. Thank you for that! If anything from this thread, I learned from it. I got what you were saying. Again, thanks mate.

This is a good place the pull the plug on this thread.

Cheers

Gary
79 MGB

I'll join you with a beer!!
Gary Hansen

Guy, that's fantastic, I know a good few places where that mental agility would be very appropriate, very apt for many I've meet in the motor trade.

The quality of the excuse is all that matters.

😉
Nigel Atkins

I've sent an email to Bosch UK but it only got through their email receipt acceptance on Friday so I might hear back next week or the week after, all being well.

ETA: I was only asking them about the rotors not lean or other lean. 😅

Nigel Atkins

You can't get a quart into a pint pot; or so the saying goes.

Lean on it all you like, the original quality of rotors don't go bad because of an air leak, because there is only so much voltage the coil can produce, and a decent rotor can take that.

What's contentious about that? Nothing as far as I can see.
anamnesis

Hmmm, yeah I like that idea. Install a crap rotor arm to act like a fuse, as protection against an air leak. 😁
anamnesis

No, not protecting against an air leak, but against a holy piston.
GuyW

Been watching Songs of Praise?

Nigel Atkins

Just checking Anam's comprehension.
GuyW

Oh, no, I don't think he's compressive at all.
Nigel Atkins

Holy spirit of petroleum batman. 😊
anamnesis

I'm a bit disappointed to be honest,on two levels

Firstly, I'm disappointed in myself that I haven't been able to get the point accross

and secondly and probably more disappointing ---I'm disappointed that this has turned into a bit of a mockfest

bit sad really

willy
William Revit

I don't agree that it has become a mockfest. There's often a degree of 'humour' as an aside to the discussion. It's a form of drift, and is good natured. I'm sorry you are offended; --- definitely not intended by anyone to anyone I'm sure.

As for the point. You made your point. A weak(er) mixture requires a bigger spark. A weak mixture alters the conductance of the spark gap. As the conductance goes down, the voltage across all resistive components goes up, as the magnetic field in the secondary winding continues to collapse, and eventually reaches the maximum voltage attainable by the coil.

My point is, the voltage from the coil is limited. The "sky" is NOT the limit. The limit is defined by the ratio of the number of turns in the primary windings, to the number of turns in the secondary windings, and the input voltage.

Before Lucas or Bosch or other manufacturer's rotors arms, reduced in quality or changed in construction, or materials used, the rotor arms didn't fail prematurely because of an air leak, because they used to be made in such a way, as to be able to handle the maximum coil voltages. But of course they did wear out. Like the points and condensor they have always been a consumable. But they didn't fail when an air leak made the engine run rough. I've never seen any text that says something like, --- remember to replace your rotor arm if you develop an over weak mixture.

But now I guess, if you can no longer buy decent rotor arms, it may well be a requirement. But not if you can still buy decent rotor arms,-- or have, as I do, a few old black ones.

Back in tbe day ( queue violins), I used to replace points and rotor every year even if they didn't need it ( they came bundled and were dirt cheap). I bunged the barely worn stuff in my spares box with other part worn stuff, because I'm a horder, lol. They are now very useful to have. I have lots of useful old, but barely worn stuff.

Wish I could say the same about me, -- old but barely worn 🤣.



anamnesis

Willy, I wasn't mocking you or anyone with my comment about the failsafe device. My apologies if you thought I was. I was just attempting to bring some levity to a thread that was in danger of becoming an increasingly bitter confrontation.
GuyW

I'm not offended at all, no need to apologize to me
Reading A's last post he obviously has a handle on how the system works so basically he's been taking the p@ss all along, not impressed at all with that sort of thing
In the General section,maybe some extra banter etc has a lighthearted place, but in the technical section it's a bit mischievous in my eyes and makes it hard for people reading this to sort out the truth from the rest of it
William Revit

No I haven't been taking the piss all along at all. Just pointing out what I know to be the case. Glad you aren't offended. 🙂
anamnesis

This thread was discussed between 28/04/2021 and 03/05/2021

MG Midget and Sprite Technical index

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