MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Recommendations

Parts

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 MGB Technical - Ignition rotor

Hi All,

My bgt suddenly died after 20km very smooth drive. After checking the ignition, once again the rotor had been shorted internally. The rotor is lucas one I replaced 2 years ago due to the same reason. It just covered 2500 miles!

Replacing a new rotor, my B restart at once.

Is there any good quality rotor in the market? which brand?

From now on, I will put a spare rotor, condenser, cap....in the boot!

Thanks in advance
Ennio

Ennio Wong

It happens, but it should be rare. I have had two fail, on different cars, but they were both old if not original. Both replacements were OK, as was a replacement for a third car changed as a matter of course not failure, all Lucas. Did both failed rotors come from the same source? Any signs of condensation or tracking inside the cap?
Paul Hunt 2

Paul,

Both failed rotor were lucas for 45D4. And no condensation and tracking inside the cap.

Just wonder is this ignition coil problem? The car is 70 with Bosch blue coil (12v non-ballast).

Ennio
Ennio Wong

i have heard about this elsewhere, and just went back to electronic ign myself because I have been very diapointed with points and condensors as well recently.
Stan Best

Ennio. There have been a rash of reports, over the last several years, about Lucas rotors failing early in their use life. The problem seems to be insufficient thickness of the plastic beneath the rivet holding the rotor contact strip in place. This allows the high tension spark from the coil lead to burn through the rotor and short directly to the distributor spindle rather than being transfered, through the rotor's contact strip, to the distributor cap terminals.

I have been using some rotors, marked "Made in Italy" that I purchase at the local BAP auto parts store. Have not had any problems with them over a period of five plus years and tens of thousands of miles. Might be worth considering.

Les
Les Bengtson

I don't see why points vs electronic ignition should have any effect on the rotor. If anything electronic ignition can deliver higher HT energy which would be more likely to cause a marginal rotor to fail.

Interesting you should say there is a rivet holding the contact in place, originals didn't have this, although I have seen an electronic ignition conversion which did! Photo of the two attached. Maybe it is a blind rivet rather than a domed in the original, but it is also further away from where the spindle would be i.e. outside the cylindrical section below it, and a 25D I have which *does* have a rivet is even further outside.

Paul Hunt 2

PS. Unlikely to be a coil problem as it is the plug gap that controls the peak HT, so could be over-large gaps on the plugs. Could also be a mis-match between rotor and cap in that the gap there is too big which would also cause the voltage on the rotor to be excessive. Measure the gap between opposite contacts of the cap, halve that, and compare it to the distance from the centre of the rotor to its outer edge. I've measured a 25D at 44mm/2=22mm and 21.5mm, i.e. 0.5mm clearance, as an example. A 70 would originally have had a 25D, but a 45D shouldn't make any difference (my 73 has been running one for 17 years) and the clearance should be similar, I would have thought.
Paul Hunt 2

Les & Paul,

Thanks for your information.

The Lucas rotor that failed has a blind rivet as mentioned by Les and the new one installed also has.

According to the workshop manual, the plug gap should be 35 thou for 45D4 and I only set it to 32 thou which should not be too tough to the system.

I will check the rotor to cap clearance later.

Unfortunately, classic car parts availability is almost extincted in Hong Kong, we have to order from UK & USA. "Made in Italy" rotor seems far far away!

Ennio
Ennio Wong

Hello,

I am using Echlin cap+rotor for many years without any prob. A+ the cap has bronze post instead of Al. A lot more reliable material..

Cheers,

JGC
Jean Guy Catford

Sorry if I caused some confusion Paul, yes I was delighted when I opened up my plugs to the recomnded 40 thou and all the spark still went into the cylinders. I have a 45D4 dizzy and Bosch silicon HT leads. Knowing that rotor arms are giving trouble ATM it was great not to hit every possible snag. The points/cap was just bought to mind by lots of agro in the same area, not just on my car but it seems that some current points just go high resitance if they are left for a week or two. I imagine this is "economies" in the metal they are made from.
Stan Best

Ennio,

Try contacting Distributor Doctor: www.distributordoctor.com

He sells good quality reproduction rotor arms, with 3 year guarantee, and original Lucas rotor arms. They are a bit more expensive than the rotor arms normally for sale.

I bought one of his repro rotor arms for my MGC last year after experiencing problems with rotors failing.

Regards,
David.
D M B LEGGEAT

Hi Guys,

Thanks for all your advice.

I'm thinking how about remove the blind rivet on the Lucas rotor and then fill the hole and bond the metal strip with epoxy!

Ennio
Ennio Wong

This thread was discussed between 20/01/2008 and 23/01/2008

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now