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 don't want to put a rotor into my bike that might "explode" under higher rev's. I had that happen on a G15 I had back in the 70's, it was real ugly!
Any information in regards to the date would be appreciated.

Here's what the 'exploding' type look like, but I'm still using it. I'm not sure when they changed.

This is how my '72s look, with two time mark bosses cast in but both looking the same with same groove in them not one plain and one grooved like this one. I put nail polish and some scratches on the correct side time mark. The weakness to worry about is the inner metal bush coming loose to wiggle which I guess could get to point it bind and tears up stuff.

Just did a quick internet search and came up with 54202299 is a welded center Lucas rotor number. Frankly I can't see any difference visually between the unwelded ones. I know my original '72 rotor came loose, but didn't come apart before I replaced it.

They apparently don't let go all at once but just get looser and looser in center till too unstable to reuse.

The old type (numbered .......2006) with pressed-in centre tends to explode due to the loosening centre. The later type (numbered …….2298) has a welded-in centre and will not explode. Check this out before you refit the existing Lucas rotor to prevent severe damage!

Seen one explode. Following buddy he pulls over to pull off brakelever/footrest and primary cover with tools at hand in pouch . Wiggles off cover and giant atom bomb of pure white smoke headed skyward. Cleaned out small mound of bits n' pieces , he bungys cover to back seat and turns off all lightings and we limped him home. Luckily a nice summer day park outing. :|

I wanted to see what was inside these rotors so I sacrificed an old rotor (part number 54212006). I melted all of the zinc away and this is what I found. It appears as though the hub is made from a hexagon bar stock and the magnets are simply attached to the hub. Then there are laminated 'caps' that are placed on top of each magnet. Each of the laminated caps are held together with one rivet. You can see that the laminated caps are made in such a way that they are held in position once the zinc casting is poured. Even though the magnets were weak this rotor appeared to be in good shape. However, on closer inspection of the hub I could see that the magnets had been rocking and actually wearing into the hexagon hub. It was showing early signs of going bad. Here are a couple pictures of the rotor with the zinc removed:

Alternator rotor with zinc removed


Exploded view of the hub, magnet and laminated cap


According to Norton Owner's Club Service Notes booklet, they claim that part number's 54202275 and 54201143 are a welded type rotor. They state that the magnets themselves are welded to the hub. If that is true than I imagine that they simply tack welded the magnets to the hub. Then once the rotor is completely assembled and final machined, the rotor would be re-magnetized. The new Lucas rotor that is available is part number 54202299 and it claims to have a 'welded' center. There are other aftermarket rotors that claim to have welded centers as well.

Here is another variation of a rotor that is installed on my 1974 Norton Commando. My bike was built in June 1974. It's part number is 54201144 and this was ink stamped on the back of the rotor. Under close examination I believe that this rotor is simply a 54212006 rotor that had the front and back surfaces machined so that the rotor would be sandwiched between two large bellville spring washers. I believe the thinking is that the torque loads from the crankshaft would be dispersed along the faces of the rotor in hopes of preventing the magnets from rocking on the hub. Thus preventing the magnets from pushing outward against the laminated caps. I am not sure if this is a fix however the rotor on my bike has 20,000 miles on it and it seems to be in good shape. Here are a couple of pictures:

Rotor on a 1974 Mark IIA


Backside of the rotor with the ink stamped number



It would really be nice to see how the currently manufactured rotors are actually put together. Is there anyone willing to sacrifice their new rotor so we can see?

Thanks for those pictures Peter, interesting. Rotors can be re-magnatized, I forget the name of the machine, but someone used to do it for about $50, I remember reading about it. Mine is weak, so it won't work any more with just the MC2, but it seems to charge and work fine with the Pazon and a battery.

Thank you all for your input, now I a off to my stash of rotors and see what I have. It's always something isn't it.

Or one thing or another. :|