Service Bulletin: Loose Alternator Rotor Center & Rotor Nut

Service Bulletin: Loose Alternator Rotor Center & Rotor Nut

Loose Rotor & Rotor Nut

A loose alternator rotor center or a loose rotor nut can produce a sound similar to a big end "knock". Lucas rotors are notorious for becoming loose and causing problems in your primary cover. A sound coming from the left side of your engine is usually the first sign that your that rotor is bad. A loose Rotor nut can equally give you the same sound effect. This "knocking" sound at times can be hard to locate. A trained ear and an experienced technician can usually trace down the sound.


1970 Triumph Service Bulletin Loose Rotor

An original Triumph service bulletin on loose Lucas rotors and loose rotor retaining nuts. Dated 1970. Click the image to download your copy.


This official Triumph service bulletin was released on September, 2nd, 1970 directly informing dealers and mechanics of the loose Rotor and rotor nut problem. Although this is a Triumph service bulletin, the same Lucas rotors where fitted to BSA motorcycles, and even Norton motorcycles. This information is "universal" and should be applied to all models from singles, twins, and triples.

"Lucas rotors do not have a welded center..."

The first misconception about Lucas rotors is that Lucas rotors do not have a welded center. The term "welded center" has been used loosely and is a mere rumor. What causes a bad center is simply wear. When the rotor fails it causes play rocking back and forth. The sound can usually be heard at a lower RPM or at idle. If you "blip" the throttle it can also be heard.
If your engine is producing a "knocking" sound, we highly recommend removing the primary cover, and the alternator first assuming that the sound is coming from that area of the engine. It would be a real pity  to pull the engine apart to only find that the rotor was bad. (It has happened before)

"A bad rotor could cause serious damage..."

Running an engine with a bad rotor could cause serious damage to your engine and case. In fact, your life may be at risk as well. If the rotor center where to give way at 5,000 RPM it could be devastating. If you hear a sound, check your rotor and replace it as soon as possible.

Loose Lucas Rotor Example



Above is a video of a loose Lucas rotor in my personal 1969 BSA Lightning 650. The back and forth play was more then a 1/4". This was the original Lucas rotor, dated 1969.


  • Classic British Spares

    @Dave Yes quite of bit of play… Dave, that was a good thing you the cover off. That could have catastrophic. Thanks for reading today :)

  • Dave

    That’s a lot of play in that rotor! But unfortunately I can do one better…
    I bought a 1968 BSA 441 Victor Special this past summer and fortunately it was not in running condition when I picked it up. I pulled off the primary cover back at home and lo and behold… there was NO rotor nut or locking tab washer!!! And no trace of them in the primary case at all…That could have been a real mess if I’d got the bike started and roared off down the street. Gives me shivers just thinking about it!

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