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MG MGB 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
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

I'm not sure why Nigel started this thread as it just repeats what Richard has already said with the same photos, and Willy's picture seems to show exactly the same type as Richards.

It's Distributor Doctor red rotors that are being pushed, not red rotors per se. Dist Doc started producing red versions of Lucas black rotor arms when copies of those from some sources were only lasting days. Then cheapo manufacturers jumped on that bandwagon and Dist Doc started moulding 'DD' into his so buyers could beware.

It's difficult to tell from Richard's second photo exactly what went wrong with his. The problem with the Lucas copies is that a rivet is used to secure the contact into the moulding and that is too close to the spindle. Others also have a rivet but positioned further out that may be OK. DDs like Lucas originals don't have a rivet.





paulh4

It's got me intrigued a bit, found this Renault rotor and it has that exact same rivet---threaded
I'm wondering if they use the thread to mount the spring loaded contact arm on the rotors requiring them--a universal sort of thing allowing them to dual purpose a single rotor for two different aplications
I still believe a rotor won't normally die on it's own, it's caused by excess load caused by high resistance further along the system
The combo of resistor leads with resistor plugs would put enormous strain on a rotor or even something as simple as a lead falling off

willy


William Revit

Paul,
I started this thread as the information was formerly in a long thread about ignition problems, and thread drift, both of which I contributed.

I wanted to highlight this rotor and red/'red'/"red" rotors generally.
Nigel Atkins

Willy, Paul,
it's not just the raised rivet that was (is still?) the problem -

" ... rotor arms have been failing because the typical "mix" used in the injection moulding [nowadays][still?] contains more carbon blacking and is therefore more conductive. Still more importantly, the rivet which holds the brass inlay into the moulding is slightly longer than the original, bringing it too close to the spring clip on the underside."

http://www.distributordoctor.com/rotor_arms.html

Also there's the discussion on whether it's threaded or grooves, two so far say threads which wasn't clear on the photo I put up.

Nigel Atkins

Whilst faults in the HT circuit can caused a raised voltage which may well break components down there have been enough cases where new 'no name' black rotors broke down in hours but when replaced by a correct Lucas or Dist Doc item they don't for many to have come to the empirical decision that one is better than the other.

You would need to remove the Bosch contact from the moulding to see if it was pinned or riveted under that threaded boss, I suspect not. What's at the bottom of the hole - metal or moulding?
paulh4

This thread was discussed between 28/04/2021 and 30/04/2021

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