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lucas exploding rotor ?

Discussion in 'Norton Commando Motorcycles (Classic)' started by stu, Jun 8, 2018.

  1. stu

    stu

    Joined:
    Jul 22, 2012
    a buzz at speed which I thought might be the isos out of whack turns out to be the alternator rotor vibrating off centre. I had the top head steady come loose recently so maybe the motor bouncing around knocked off the fine clearance required. I used the plastic milk carton feeler gauge (non magnetic) to gap all round, back to spec but turns out my rotor is of the exploding variety (Lucas number ending . . 2006) So are there any signs of early failure to look out for or best just replacing straightaway given the buzz ? and if so whats best alternator to go for ? I'm a 1973/late 750 - all stock / daily rider
    cheers
    Stewart
     
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  2. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    If you’ve confirmed it as one of the exploding variety, surely it’s just not worth the risk of putting it back in?

    I’m sure folk will chime in that they’ve done 10s of thousands of miles on one without issue.

    But personally, I would not put such a known extra failure mode into my engine. There are enough of those already !
     
  3. kommando

    kommando

    Joined:
    May 7, 2005
    If the magnets are showing movement shown by running a fingernail across the OD and feeling for ridges then replace now before it takes the stator with it later, problem being what is the best replacement between Wassells/Sparks , if LAP do a rotor to go with their stators I would go with that one and not the others, which have had issues in the past,
     
  4. baz

    baz VIP MEMBER

    Joined:
    May 26, 2010
    + 1
     
  5. eskasteve

    eskasteve VIP MEMBER

    Joined:
    Sep 24, 2012
    My opinion is that if you have a "go to" parts supplier then I'd contact them for advice concerning choices. I deal almost exclusively with Old Britts for Andover Norton parts and CNW for aftermarket stuff. Neither have ever steered me wrong when I needed advice. If you are located outside of the USA then Old Britts is off the table as they no longer ship outside the US.
    I had an OEM rotor explode back in 1977 in my Combat. The damage FAR exceeded what a complete new charging setup costs. Rotor, stator, primary chain, and both the outer and inner primary case all toast. In addition I had to get a friend to come pick me up with his truck and he was a total Honda devotee. 150 miles of lectures on the evils of English motorcycles.
     
    Last edited: Jun 9, 2018
    Fast Eddie likes this.
  6. Bernhard

    Bernhard

    Joined:
    Apr 20, 2011
    You need a rotor with welded magnets !
     
  7. kommando

    kommando

    Joined:
    May 7, 2005
  8. stu

    stu

    Joined:
    Jul 22, 2012
    Thanks for the replies - I wound up replacing the stock rotor for a modern lucas from RGM which looks almost identical but shouldn't disintegrate. The rotor nut took a fair bit of weight on the breaker bar to loosen up and seemed like a lot of torque tightening it back up to spec again.
    Unfortunately the buzz is still there - a resonating "frraatting" noise that kicks in at about 3200 rpm and above with light vibration through the footpegs . .
     
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