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MG MGB Technical - Lucas Type 25 Rotor Arm Problems

I own a 1970 MGB GT 1800 (Mark II). Following a breakdown diagnosed with high tension problems (spark leaving coil to distributor but no power to the plugs)I had replaced plugs, HT leads, distributor cap, rotor arm, and coil. Problem solved only to return months later. This time I replaced the luminition electronic ignition with CB points and condensor with no benefit. Then I remembered reading an article in Safety Fast (november 2003, page 22) about a problem encountered by KB Higgins with the rotor arm on his MGB Olympic engine. The pattern rotor arm had failed. Reasons given were :- poor plastic which fails when hot or the metal rivet used to fix the brass contact plate to the rotor is too close to the cam and shorts through.

I have now gone through 4 rotor arms since November 2003 on very few miles. I purchased one rotor from the MG Owners Club spares and 3 from a local motor factor manufactured by Intermotor and Power Train.

Am I alone in this problem. What is the solution. KB Higgins suggestsed in his article obtaining an old type rotor arm which are made from bakelite and have the brass contact plate monded onto the plastic without the metal rivet. I don't know where to find one. Any ideas most welcome.

David Thaw

David. The problems with the current rotor arms have been on-going for some time. I have been using the Italian made rotor arms rather than the Lucas ones with no problems. I believe the brand is Intermotor, but that may be the brand name used by the parts store (BAP) which carries them. You may want to see if anyone can get an other than Lucas rotor and test it yourself. Les
Les Bengtson

On another thread, someone says that his rotor problem was caused by a poor motor ground. Might want to look in that direction. Can't hurt, won't cost much, might cure the problem.
R. L Carleen

I have had this problem many times over the years on MGs, Triumphs and Jaguars, I have never yet found out why a fairly new and perfect looking rotor arm should suddenly cease to function, I just carry a spare at all times.
The Wiz

After I bought my '73 B (9/2001) I did a tune up with replacement parts (ie not Lucas) and had the brass piece on the rotor arm actually separate from the plastic while I was driving. Luckily I had put the old one in the glovebox with the points (also installed pertronix at the same time). I recently bought a Lucas rotor arm and have driven a couple hundered miles without incident.
Zach Dorsch

This thread was discussed between 21/08/2004 and 22/08/2004

MG MGB Technical index

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