'74 Commando Alternator Rotor

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thunderbolt

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I have been reading the International Norton Owners Association Tech Digest in relation to my '74 Mk2 Roadster.

On page 5-2 they say that the alternator rotors on older Nortons ('68 - '73) have a tendency to self destruct and a welded rotor should be fitted in place - P/N 54202275 or P/N 60600684 (Lucas P/N I assume).

I have checked the P/N on my rotor and it is P/N 5421006L. Last two digits are a bit hard to read.

Would my rotor be okay or should I replace it?

In the article it says if the rotor self destructs it can take the end of the crankshaft with it - scary.

Thanks in anticipation.

Cheers Don
 
Well yours is a 74 so I wouldn't worry about it, I am still running my original rotor in my 74 motor but it had a new centre replaced about 35 years ago when the original centre had movement in the rotor, haven't done anything to the rotor since.

Ashley
 
thunderbolt said:
On page 5-2 they say that the alternator rotors on older Nortons ('68 - '73) have a tendency to self destruct and a welded rotor should be fitted in place - P/N 54202275 or P/N 60600684 (Lucas P/N I assume).
Click to expand...

None of the rotors are or were "welded".
If you want to fit the latest specification rotor then it's Lucas 54202299
www.rexs-speedshop.com

Genuine Lucas Alternator Rotor 54202299 | RM20

Genuine Lucas rotor, p/n 54202299. RM20 6 pole 74mm, diameter with welded centre. Fits all Lucas alternators numbered RM18 and above. When comparing prices – the price shown includes VAT (UK tax is removed for overseas customers at check out)
www.rexs-speedshop.com www.rexs-speedshop.com

www.baxtercycle.com

54202299 – New Lucas Reproduction

www.baxtercycle.com www.baxtercycle.com
 
Last edited:
found this:
"Another issue with rotors is that they may become loose on their centers. You can check for this buy clamping the rotor between two spacers in a vice and trying to turn the magnet portion. If there is any detectable play the rotor is no longer usable.
These can not be fixed, don’t even try.
If the rotor comes apart while the engine is running you will destroy your primary drive and risk locking the rear wheel or bending the crankshaft..."
www.jrcengineering.com

LUCAS ALTERNATOR TIPS | JRC Engineering, Inc.

Lucas Alternator Manual 87 pages Lucas Alternator Tips and Hints The alternator system fitted to British bikes from the 1960...
www.jrcengineering.com www.jrcengineering.com
 
My 74 has the rotor ending in 006 and no issues with it. The check is easy to do.
 
When mine came loose (on a '76 T140, so should have been a 'good' one) it sounded like a big end was on its way out. Remove the stator and the failure mode is easy to detect.
I read an old magazine article whare a French garage used Citroen car suspension star spring friction damping washers to clamp the outer & inner parts together.
 
B+Bogus said:
I read an old magazine article whare a French garage used Citroen car suspension star spring friction damping washers to clamp the outer & inner parts together.
Click to expand...

Yes, the 'Belleville washer" mod.
An example here:

cracked rotor, Belleville washers, and upgrade questions

I'm the original owner, 1974, so it came to me with the Belleville washers mod from new. I'm going back with an RM23 and it's rotor will need the star washer,(on order) and had a couple questions: --Stator has two green/yellow wires....problems? --replacing stock unit with a180w rectifier...is...
www.accessnorton.com www.accessnorton.com
 
The one item I would check is the gearbox shell layshaft bearing, if its the original ball bearing then its failure could be more sudden and catastrophic than a rotor failure.

You need either some documentation or info that a previous owner has done the upgrade to a roller bearing or ball bearing recommended by Mick Hemmings. If not then its an open up the gearbox to check.

Mine failed at very low speed and with pre warning ie the kickstart moved backwards on acceleration but they can just go at high speed and lock up the rear wheel.
 
YukoNorton said:
found this:
"Another issue with rotors is that they may become loose on their centers. You can check for this buy clamping the rotor between two spacers in a vice and trying to turn the magnet portion. If there is any detectable play the rotor is no longer usable.
These can not be fixed, don’t even try.
If the rotor comes apart while the engine is running you will destroy your primary drive and risk locking the rear wheel or bending the crankshaft..."
www.jrcengineering.com

LUCAS ALTERNATOR TIPS | JRC Engineering, Inc.

Lucas Alternator Manual 87 pages Lucas Alternator Tips and Hints The alternator system fitted to British bikes from the 1960...
www.jrcengineering.com www.jrcengineering.com
Click to expand...
Well that's not true if your rotor is lose you can get a new centre installed, mine was done 35 years ago when mine became lose in the rotor and a local classic bike shop sent it away for a new centre to be put in, cost at the time was $20 but when I got it back the new centre keyway wasn't cut deep enough and had to file the key down and later I used a broach set to cut it to the right size, well that was 35 years ago and I am still using the original rotor, hasn't come lose since.

Ashley
 
ashman said:
Well that's not true if your rotor is lose you can get a new centre installed,
Click to expand...

Not possible with later (most) rotors as they have a solid (hexagonal) core.

www.accessnorton.com

Lucas Alternator rotors

I am working on a 1970 750 Commando. First cleaning the parts and putting it back together, it came in boxes, then I will take it back apart to 'restore' it. That being said, it came with several alternator rotors and I remember reading that Lucas went to welded rotors sometime in the 70's. I...
www.accessnorton.com www.accessnorton.com
 
L.A.B. said:
Not possible with later (most) rotors as they have a solid (hexagonal) core.

www.accessnorton.com

Lucas Alternator rotors

I am working on a 1970 750 Commando. First cleaning the parts and putting it back together, it came in boxes, then I will take it back apart to 'restore' it. That being said, it came with several alternator rotors and I remember reading that Lucas went to welded rotors sometime in the 70's. I...
www.accessnorton.com www.accessnorton.com
Click to expand...
All good as mine was done back in the early 80s, was done at a machine shop where the classic bike shop got its work done, I wasn't working at the time so a $20 repair was better than buying a new rotor and its held up all this time.
 
ashman said:
All good as mine was done back in the early 80s, was done at a machine shop where the classic bike shop got its work done, I wasn't working at the time so a $20 repair was better than buying a new rotor and its held up all this time.
Click to expand...


Maybe so, I'd say you've been lucky as the problem with the rotors is the body becomes loose on the hexagonal core (hence the Belleville washer mod.) causing the magnets to shuffle and destroy the rotor resulting in it "exploding" and a new core sleeve isn't likely to prevent that if the process has already started.
www.accessnorton.com

Diagnosing a failed alternator rotor

Here's a video - Maybe hard to tell, but just LOOKING at it, it looked fine. Not a single punching or pinning that is typically done to extend the life of the rotor (but which actually contributes to a more catastrophic failure mode).
www.accessnorton.com www.accessnorton.com
If in doubt or there's any indication of looseness (knocking?) then better to fit a new late rotor in my opinion rather than attempt a repair and the repair is likely to cost as much or more than a new rotor plus the hassle involved as helpful local machine shops are not as plentiful as they once were.
 
Last edited:
In case anyone is wondering how the so-called "welded" rotors are constructed:

What is the best Alternator Rotor available today (2019)

Are those rotors balanced, even if so a recheck could never hurt. I remember reading an article although can't recall the exact numbers where an ounce of out of balance out there on the end of the crank was (iirc) equivalent to 126 lbs at reline (4 cylinder bike) Once that early style center...
www.accessnorton.com www.accessnorton.com
 
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