Intro and Alternator Question

[ccambern]

Hey all,

I wanted to introduce myself and ask a quick question...

I'm restoring a late NC 250. Started life a semi-complete Scrambler and will end up as a cafe racer. I've done a lot of work on race cars and karts and recently a light resto on a '73 Vespa, but this is my first vintage bike. I live in Lansing, MI. It's cool to see an active Duc Singles board and it sounds like there are quite a few Michiganders here.

Anyway, I'm working on getting the motor back together and I'm beginning to think about electrics. The motor came with the 2-pole alternator and brass flywheel. I'm planning to covert to 12V and (maybe) electronic ignition. A while back I picked up a 4-pole alt on ebay. The seller claimed it was NC Ducati, and it certainly looks right, but I noticed yesterday that it's too big to fit inside the brass flywheel. I know the 40W alts came with an alum flywheel, but I thought the brass would work... No part numbers that I can see, and I haven't checked to see if it fits the case yet. Any ideas? Maybe the alt is actually from a WC?

Thanks in advance!

-Clark

[wcorey]

I'm not much further along than you as far as knowledge of these particular alternator systems go but can clear up a few things.
I'm doing a widecase build that came with (probably) the same brass rotor type alternator. First, you likely have a four (magnet) pole rotor with a 2 coil stator, the aluminum ones are generally a six pole rotor with a 4 coil stator, not sure but either may also come in the other material. Both the 6 pole and the 4 pole rotors will fit on WC and/or NC as far as I know. As you have noticed the stators/rotors are not cross compatible, due to the different diameters of each.

I believe that they put out a high enough ac voltage that to achieve 12v is simply a matter of using a 12v regulator. Power output is where it gets dicey, I guess there are various ways to tap into some additional current but there are different options and ways the stators come wired which complicates things a bit. There is already quite a bit of reading/info available here but it's spread over a few threads and a lot is not specific to what I'm doing.

I have both types of alternators and will be using the aluminum rotor for the lighter weight and I'm hoping for some advantage in amperage output. It seems to me that the magnets on each type are close to the same strength but obviously one has two more of them. I also noticed that it would be physically possible to fit 3 of the '2 coil stator' coils into the space available in the 6 pole rotor, with some heavy mods to the mounting plate of course. I'm going to look into this further and also the possibility of using a brit bike type aftermarket regulator/rectifier, because I have some... This is where I am presently, haven't got much further on it and need to do a lot more research. Will need to get the motor together before I can try any of it, unless I get really ambitious an make a test rig.

I'm quite certain Bob will jump in here at some point as he seems to be somewhat of an authority on Duke singles electrics and is more than willing to share his knowledge.

I'm also trying to dig up some info on ignition alternatives, particularly these very prolific cheap aftermarket cdi boxes/pickup coils for generic honda 50 style 'pit bikes', need to figure out if the advance curves are in the ballpark and other such things.

Bill

[DewCatTea-Bob]

By: 'ccambern'...
" The motor came with the 2-pole alternator and brass flywheel. "

____ By "2-pole", did you actually mean '2 coil' ? _ Cuz if there were such a thing as a 2-pole alternator, then it's rotor-flywheel may only have just 2 magnets.


" A while back I picked up a 4-pole alt on ebay. The seller claimed it was NC Ducati, and it certainly looks right, but I noticed yesterday that it's too big to fit inside the brass flywheel. "

____ Alright, correct me if I don't understand you correctly...
You've found that your purchased 2-coil stator is too large to fit within your original narrow-case small-sized brass-rotor !?
If so, here's what I'm currently thinking...
__ Most of us -(people who know a bit about DUCATI-singles), don't realize that the first wide-case models of 1967, (which were imported into the U.S.), did not come stock with the common large-sized 6-pole alternator which all later wide-case models came with.
The wide-case 250/350-Scramblers of 1967 came with a larger 4-pole 30-watt alternator, which looks very much the same as the smaller 4-pole 40/28-watt alternators of the narrow-case models.
Without having one of each size-type in hand, the easiest way to tell the two size-types apart, is by the differing mounting-screw hole-patterns of the n-c & w-c type stator-plates.
Due to those two very different bolt-patterns, those two stator-plate types are not interchangeable !
__ So please check the mounting-hole patterns of your two different sized stators, and then let us know what you've found.


" I know the 40W alts came with an alum flywheel, but I thought the brass would work... "

____ Actually, late n-c Mk3-models came with the lighter small-sized flywheel-rotor, while the off-road n-c models came with the small-sized brass version. _ And those two rotor types are indeed interchangeable !


Tillater,
-Bob

[Thevin]

Welcome, I'm over in Spring Lake, not far away..

[DewCatTea-Bob]

By wcorey...
" First, you likely have a four (magnet) pole rotor with a 2 coil stator, the aluminum ones are generally a six pole rotor with a 4 coil stator, not sure but either may also come in the other material. "

____ Well, there were actually a number of alt.stator/rotor combo types...
The first type was the 4-pole stator with the small-sized -(4 & 5/8") brass-rotor, then later, also with the small-sized alum.alloy rotor.
Then next was the 6-pole (with 4 coils) alt.stator with it's medium-sized -(1mm under 5") alum.alloy rotor.
Then came the still larger wide-case versions, with 6-pole (with 6 coils) alt.stator and large-sized -(1mm over 5") alum.alloy-rotor -(for 250), and brass-rotor -(for 350).
(I know that there was also a 6-pole stator with 5 coils, but don't recall what model it came from.)


" There is already quite a bit of reading/info available here but it's spread over a few threads and a lot is not specific to what I'm doing. "

____ So just what are YOU doing? _ I'm sure I could help you out with it !


" I also noticed that it would be physically possible to fit 3 of the '2 coil stator' coils into the space available in the 6 pole rotor, with some heavy mods to the mounting plate of course. "

____ That SOUNDS like a good idea...
Back in the '80s, I had seen some Jap-bike stator which had 3 such separate power-coils spaced 120-degrees apart, and the radius-cut of their core-protrusions was better matched (than that of such Ducati power-coils), with the inner-radius of the 6-pole Ducati-rotor !
I had planned to make a 1mm thick steel adapter-plate to fit those same 3 Jap-bike power-coils to a wide-case DUKE but, just never got around to doing that. _ So I'd really like to see something of the sort tried-out !
__ It seems too bad that Ducati never simply thought to try including a second-pair of power-coils mounted between their two original power-coils, (that would've only required a square iron-core, in place of the two separate coil cores), as that -(4 coil 4-pole stator) would've increased -(doubled) that type of alternator's output, in a more simple manor (than retooling for their 6-pole model).


" I'm going to look into this further and also the possibility of using a brit bike type aftermarket regulator/rectifier, because I have some... "

____ Be sure to keep in mind that you'll then likely have to switch-over to a positive-ground system.


" I'm also trying to dig up some info on ignition alternatives, particularly these very prolific cheap aftermarket cdi boxes/pickup coils for generic honda 50 style 'pit bikes', need to figure out if the advance curves are in the ballpark and other such things. "

____ Something along those lines made for Honda-XL250/350s, ought to work about perfectly.


DUKE-Cheers,
-Bob

[ccambern]

OK... seems like I got my terms crossed up.

1. The engine came with a 2 coil stator and a 4 pole brass rotor.
2. The stator I bought on ebay has 6 poles and 4 coils. It fits the mounting holes in my narrow case, so it must be a NC stator.
3. The 4 coil stator is too large to fit inside the original brass rotor, so presumably I need to look for a "medium sized alum rotor."

Anyone have a medium sized rotor laying around that they're willing to part with?

Thanks for the help.

-Clark

[DewCatTea-Bob]

" The 4 coil stator is too large to fit inside the original brass rotor, so presumably I need to look for a "medium sized alum rotor." "

____ That's quite correct, the 6-pole stators are too large to fit inside the 4-pole rotors! _ And that's a good-thing too, as otherwise it seems you might have gone-ahead & mismatched your alternator, and thus would've ended-up with relatively little power-output.
__ I HAD assumed that you had bought an extra 4-pole stator so that you could rewire it up ahead of time, for the day when you'd replace your stock-stator & wiring. _ But now I wonder what your actual plan was for another stator... Did you have any certain intentions for using the 6-pole stator in place of your original stator?


" Anyone have a medium sized rotor laying around that they're willing to part with? "

____ Right, you're going to need a 6-pole ('126mm') rotor, to match-up with your '60-watt' type stator. _ Those were used on all narrow-case 5-speed Monza, Mach-I, & Sebring models.
__ In case you don't find one, why have you not considered using your original 4-pole stock-stator for whatever your purpose?


DUKE-Cheers,
DCT-Bob

[bettyann]

There's a 60 w monza/sebring rotor and stator on e-bay,they usually go cheap,one sold on e-bay 4 days ago for 50.00$

[ccambern]

Thanks! I've been watching that one.

[ccambern]

So I got a hold of the correct aluminum rotor to match my stator. However, it spun on the crank during its previous life. The center isn't too terrible- not blued at all, but somewhat galled. Seems like there are a couple of options here:

1. Clean it up with emery cloth and run it (perhaps with some green loctite).
2. Clean it up on a lathe- not sure if backcutting will move it too close to the stator.
3. Swap in the center from my Bronze stator. I haven't measured yet, but I think the centers are the same. I'd have to pick up a solid rivet tool or perhaps use button heads, but I think it's doable. I'm just wondering- how good are the tolerances? If I knock the old rivets out, will I need a jig to get everything re-centered?

Any thoughts or experience with this?

Thanks,
Clark