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MG MGA - Rotor arm problems

I've attached a photo. This shows three rotor arms of slightly different styles. One of the round-ended ones secures the brass blade to the bakelite body with a rivet, the other does not. Both the round-ended ones have a spring blade inside the base to ensure a no-rattle fit onto the distributor cam, and the square-ended one doesn't. What they all have in common is that they are all duff. There was a fourth one in the series, but I can't find it at the moment (must have thrown it too far).

I have had to have my car recovered so many times now with this same problem that I am getting rather depressed about it. One of the round-ended ones failed on Sunday, and I substituted the spare (the one without the rivet only to find that that didn't cure the problem, so I had assumed that I had a coil problem (after changing the points and the condenser as well, by the roadside) and had to pay £135 for a recovery (a fine time to recall that I had forgotten to renew the recovery insurance). A couple of days work doing some careful substitution now shows that it was the rotor arm after all. I have now fitted the car with a filthy old original item I picked up off the floor of a friend's shed, and carry a new spare, which I have just checked.

Why am I getting this constant problem? None of these are a tight fit in the cam, so they're not getting cracked by being forced on. I think I can detect some slight burning by the tensioning blade inside the round-ended ones, but none of them show any clear evidence of cracks that would cause tracking. The distributor cap itself is a good fit, and the coil is a Lucas sports coil, which seems OK.

Any ideas?

G


Gus Gander

The current crop of rotor arms are junk, they are high carbon and poorly riveted.

Fortunately in the USA we have Jeff Schlemmer, he rebuilds distributors and also has just had a batch of 5000 rotors arms made for the 25D distributor, they are a non-carbon plastic and bonded rather than riveted.

He sent me a prototype several months ago to try out on my high-mileage daily driver BGT, it has performed flawlessly.

Email Jeff at jeff@advanceddistributors.com tell him I sent you.

Mike
The Wiz

I found a photo showing the rotor arm installed on my BGT.


The Wiz

For me there is a common demoninator here: The distributor cap. That's where my suspicions lie. Have you tried another cap?

Steve
Steve Gyles

Wiz,
Does that screwdriver have a nut on the end of it? (not the handle end!) L&LOL.
Peter.
P. Tilbury

In over 40 years of MGA driving I've only had one rotor failure. That was last year on a recently purchased new one. I put a (very) old one back on. I've driven over 5000 miles this year.
Can you say "crap"?

GTF
G T Foster

That "screwdriver" is a highly sophisticated engine/gearbox alignment tool. :-)

GT, there was a time rotor arms were good quality, your old one will probably last many years. Unfortunately the same cannot be said for new ones, hence Jeff having them made to his specifications.

The Wiz

I think I finished my rebuild of my MGA about 7 years ago. I used the original rotor arm and dissy cap with new leads. Never had it fail to date. I am the sort of person who you would call tight (skinflint) so I do not buy a part until the old one is finished and as yet neither the cap arm no spark plugs have given any concerns.
I think based on that the it is possibly the new rotor arms that are poor quality.
Bob (Robert) Midget Turbo

Real problem in the UK with race cars using Lucas rotor arms. Basically as someone said, they are crap!! Even genuine, if they are genuine, Lucas branded are rubbish.

So many racers are misfiring and non finishing just because of these damn rotor arms.

The insulation is breaking down which is not helped by high output sports coils.

I tried to get a manufacturer of ignition systems to make 'competition' rotor arms. He said what would I pay. I said £25 if it meant finishing a race!!
Has he made them? Has he buggerery!!!

I do not understand this, when there is obviously a demand for them.
A Ford that I run on a Lucas distributor, I am going to convert to Bosch. Genuine Bosch parts are (should!) be OK.

I suppose it is possible to modify a Bosch unit to fit a B series.
Must look at that.

CP
Colin Parkinson

Hi, An American company makes ignition components for MGA/MGB vehcles. The component brand is "Blue Streak" These non lucas parts include points, condensor, rotor, distributor cap, and ignition coils. I have been using blue strak components for years, and they are vastly superior to Lucas and other "junk" components that are available. I once had a bosch brand distributor rotor that was of excellent quality and that lasted a very long time, but have been unable to find a source to supply more.. I can heartily recommend the blue streak line of ignition parts, if they are available at your local parts store. You may have to inquire if they are available. Cheers, Glenn
Glenn Hedrich

On my MGB I fitted a 123 integrated electronic distributor- that uses Bosch rotor arms they make a dissy for the MGA- I may go down that route if I have any problems with my 25d setup which appears to work perfectly at the moment and I am tempted to leave well alone
Paul
MGA mk 2 Coupe
P D Camp

Hi i have had the same pain 3 rotors in a week , spoilt a holiday in yorkshire only saved by pals in a tin top , fed up with the a & its electrics so decided to SELL it on our return & buy an MX 5 !! got the MX (nice car)then i bought PETRONIX complete distributer , could not believe the change in the car so did not sell , went to Le Mans Classic ,also did a MG France rally =1576 miles return ,did not miss a beat , went like a train ,35 mpg ,still have the MX but it just sits waiting to go for a run, shame , we only use the MGA now , better than using recovery!!! good luck COLIN
colin daly

Steve: concerning the distributor caps, I have circulated three on the car and they all seem to work OK. The one on it now is a repro one that looks fairly well-made. The two others are all lettered up as Lucas, but one of them (the one that was on it when it failed) seems to have the contact posts quite well burned. I have a few of these about because I have two Sprites as well. The three cars share a surprising number of parts.
I am sure of the rotor failure because I spent a while yesterday just changing the arms over while leaving everything else as-is, and seeing if it worked or not. I had originally made up my mind that the repro arms tracked because the blade had been held on by a rivet, so that setting this rivet was likely to have cracked the plastic. Now I have at least one faulty one that isn't riveted, so I am, once again, at a loss. Previous experience of tracking failures always lead me to expect a gradual failure, with intermittent missing first. This time it just went out like a light.

By the way: I noticed, while originally looking for the fault, that when I changed the caps over to check on the possible failures, that water had got in along the HT leads, and the little pins that screw in to fix the leads to the cap were quite rusted. Not a problem, I should imagine, concerning having the sparks or not, but it's not going to help, and the threads could tear when dismantling and ruin an otherwise good cap, so I'll be sure to grease this up well in future.

Thanks for all the suggestions. This is in some danger of turning into a blog.

G
Gus Gander

Strange world. I have been subject to the sames spares providers over the past 12 years as any of you and I have never had a problem with a rotor arm. Carbon HT leads yes, mechanical advances yes, but never a rotor, never a condenser and never a cap. Weird how some people suffer some types of faults and others something completely different.

Steve

Steve Gyles

I have not had any problems with rotors failing, but I did note that after installing a pertronix in my distriburor the rotor did not fit all the way down onto the shaft like it did without the pertronix. I thought the rotor might hit the contacts in the cap, so I filed the bottom of the rotor off until it fit down onto the top of the shaft as before. Could this elevating of the rotor when used with a pertronix be causing some of the rotor failures?
Ed Bell

I also have been using Lucas rotors from Moss and have had no problems at all, not a misfire. Has anyone investigated what has happened to the ones that have failed? You would think it would be obvious.

Ralph
Ralph

I've had Pertronix for about 7 years now, and the original rotor arm fitted well. Have not had a problem with it. I think it is about 12 years old.
Peter.
P. Tilbury

I'm going to break convention here and post a link to another forum:

http://www.mgexperience.net/phorum/read.php?1,919046

I have no business relationship with Jeff Schlemmer, I just use his products, they are at least as good as Blue Streak and half the price. With the weakness of the dollar shipping to the UK sould be very reasonable.

Mike

The Wiz

I guess I'll pipe in since I have a surplus of good spares lying about.

From my investigation, the problems I find with new rotors is an internal short from the tip of the rivet holding thebrass contact in place to the distributor shaft. The high carbon content in the plastic leads to internal burning. Gus, you may ahve a different issue putting a MUCH greater load on teh rotor - your distributor is likely out of phase. Where do you see burnng on the contact? If its on the CW edge, then you should screw your vacuum advance all the way to the "R" side of the adjustemnt and reset your timing at the clamp. You may be out of phase, firing the cylinders between the distributor cap terminals (stressing all the ignition components.) The weakest link will fail first, but other problems will follow.

Blue Streak (Standard) still makes their own points and condensers, but the caps and rotors have been farmed out to an overseas supplier of dubious quality. That's what lead me into this expensive design and aquisition of GOOD rotors.

Rotor height is of little condern with the Pertronix unless you own an MGTD. That's the only distirbutor with a real clearance issue.

Here's a pic of the rotors I have. Packaged 2 per box since everyone could use a spare!


Jeff Schlemmer

Jeff,
That's an interesting observation. I must admit that I tend to do timing rather by ear. Until the recent failure I had estimated the ignition to be a little retarded, just from the sound and performance. Now that it is running again with another rotor arm, I think it is rather over advanced (and tries to stall the starter motor) so I propose to reset this anyway.

The rotors that failed don't always show where. The only ones I have that DO show a sign show a slight smoky burning on the tensioning blade inside the cup. They don't all have this blade (I see that your new ones do, and they look as if they are modelled on the last Lucas type available). I had assumed that the type that has a rivet was likely to track, but I also have at least one failure where there wasn't a rivet, so I take your point about the carbon content of the bakelite (or whatever it is).

Another variable you mention that I hadn't considered: I have been running a set of those carbon HT leads, and they're on there now. Do I understand you to say that these may be making some contribution to the problem? I have an adequate supply of the black and yellow striped stuff, with some nice old 'Champion' plug caps to make up a new set.

A purchase of a pair of your rotors seems shrewd. Do you take PayPal?

Gus Gander

Gus,
If you're running the original type side-entry cap, that's a BIG part of your problem. You can't jam a screw into carbon plug wires and have them last long - the wire will deteriorate quickly where its broken by contact with the retaining screw. You need to run a stranded (solid) core wire with that type of distributor cap. When the wire breaks down and the spark is forced to jump a larger gap, the coil will be forced to produce more voltage to jump that gap, and that helps to deteriorate the cap and rotor. By getting the voltage down to a "normal" level, you extend the life of your ignition components.

Yes, my rotors are modeled after the original type with a thermally bonded contact to eliminate the rivet. I sell them via Paypal for $15 a pair in the US and Canada, $18 a pair shipped to the UK.
jeff@advanceddistributors.com
Jeff Schlemmer

This thread was discussed between 26/09/2008 and 01/10/2008

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