Stator Installation Problem

I’m installing a new three phase rotor and stator in my 72 after re-shimming the inner chaincase. The right hand stator mounting stud is off about 1 mm to the left. This is pushing the stator off the rotor to the left, thereby widening the rotor-stator gap on the left and decreasing the gap on the right to below .008” spec’d in the Manual.



Pictures show my old stator and rotor, which had fit fine, and now won’t fit in the same way the new stator won’t fit. There are no cracks in the chaincase stud mounting bosses, and the studs don’t appear bent. The manual states that if the gap is wrong then the studs must be realigned. how do I do this?

On a positive note, I installed DynoDave’s clutch pushrod seal and it looks great.


Amy help will be most appreciated. Thanks, Ben

Are you sure you’ve not gone from one extreme to the other and spaced the chaincase out too far? It seems kinda logical to me that if you pull the case in that direction, that stud will indeed move as it has?

If you’re sure that’s OK, personally I would drill out the offending hole, drill them all out if needed, but start with that one first and see how you go. The other alternative is to be d the studs. But I don’t like that idea personally. I see nothing wrong with opening out the holes if that’s what’s needed for you to ensure a proper gap, when the nuts are tightened, the stator ain’t goin’ nowhere. Use the beer can / milk carton wrapped around the stator trick to ensure you have clearance all round.

If it’s a new stator, you should measure the ID carefully and compare to the OD of the rotor. Some new ones are wrong and have inadequate clearance from the beginning, giving the assembler zero chance of getting the gap right.

I agree with what Eddie says. The rear case alignment can be tricky. Also, I have the centre chaincase nut on and tightened when doing the stator alignment as that rules out any distortion when outer case is fitted. Use a socket as a spacer over the stud to emulate the outer case.

Even with new studs installed mine was off to one side (irrespective how it was mounted) so I opened up the holes slightly and as Eddie says I used a cut up a plastic 4pt plastic milk container cut up as a spacer

this is a well known problem with the stator/rotor gap .....as Fast Eddie said: open the 3 holes to have more movement to adjust the gap all around the rotor. and most important as Nortoniggy said: when you have got the adjustement right then tighten the center nut and controll the gap all around again before you close the primary.

Maybe putting a washer or two under the stator on the two front studs would bring it back into alignment?

Like FE:
Did you measure the ID of the stator?
only the metal matters
It has been a (long) while but, one I got was small and I will have to be bored on a lathe before I can use it. Obviously aftermarket, wish I kept track who the supplier was to try and find out who the manufacturer was...

I’ll measure the ID of the stator and report. Since the problem also occurs with my old stator, which previously fit fine, I think it’s the chaincase.

I had it apart to fix a significant rumbling in the chaincase that was caused by the final drive chain contacting the chaincase inner sealing plate. I disassembled and added a washer to the center stud, and this cleared up the contact. The chaincase was vibrating pretty heavily previously. Maybe this distorted the studs relative to the rotor shaft. I think I’ll take it apart and look everything over.

Thanks everyone.

I have found that on all of the new genuine Lucas rotors and stators I’ve played with, the tolerances are tighter than the originals.
This means that the air gap between the two is noticeably smaller.
Factoring in crank flex, they run closer than I am comfortable with.

I have mentioned before about what I do to address this, and what works for me:

I also regularly see a pinch point at between 2 o’clock and 3 o’clock which necessitates enlarging the holes in the stator and waisting the studs a little.
But I don’t think that concentricity of the stator around the rotor is your particular issue, on the grounds you say it was absolutely fine beforehand.

dont forget the plastic encapsulation on three phase is bigger than single phase and often fouls the stud spacers

check the backside and see if there are any witness marks on the plastic moulding , i have often had to trim the LAP stator`s with a knife to get side clearance between spacer and encapsulation

I have both opened the holes up a bit and relieved some of the epoxy around the mounting bosses with a sanding drum on a Dremmel.

Isn't this process called "fettling"? I was under the impression Nortons required this to run...

Interesting string. I happened to have a “Lucas” branded 3-Phase stator on my shelf for my next build. Part No.47252. I also had the alternator that came out of my current Commando – looks original, but who knows.

Took some quick dimensions and was surprised on how close the bores were (both 2.965”). Was not able (lazy) to measure the bolt circle for the mounting holes. But the mounting holes themselves were pretty darn close (.312” – old v .315 new). It does appear the width of the 3-phase was greater and the distance to the central armature greater so a small shim to center relative to the rotor will be necessary.

Given the various stators and rotors from different manufacturers there could be significant deviation from what I measured.

When I assemble a Norton primary my reference surface is the Engine crankcase pilot diameter and gasketed mounting surface. That is home when fully torqued. I shim the mounting stud to inner case with the inner primary fully bolted to the engine. The transmission then floats relative to that.

Given all that “precision”, now for the crude part. When I have a rotor that is not centered to the stator I take my small brass hammer and tap on the mounting studs until a business card moves within the stator/rotor gap without restriction. Hopefully I don’t have to remove for a while.

Crude but effective.

I dunno, but I suspect that these modern alternators might not necessarily be shoddily made, the tighter tolerance might be intentional. Modern alternators on modern engines run with tighter tolerances, because they don’t need big ones. Maybe the manufacturers think they’re ‘improving’ these old designs?

If so they’re wrong. I set up mine once with a very careful .010” clearance, and it touched quite badly. I believe this can only be down to crank flex. So I set the next one up with twice that, a .020” gap (.040” undersized rotor) and noticed no difference in operation. So far, there is no evidence of touching.

FWIW, I had the new stator and rotor measured up, the stator mounting holes were concentric to the bore and the bore was round. The rotor was concentric to the central hole and was round. Everything was good and correct, there was just way TOO LITTLE clearance. I simply had the rotor turned down to give me the desired clearance.

I would advise anyone fitting a new stator and / or rotor to at least measure the gap twixt the two BEFORE assembly.

problem with the commando chain case is it is separate to the crank case so any miss alignment in the bolt holes or discrepancy in the mounting flanges
will accentuate its self at the stator mounting studs a 2 thou miss alignment at crank case face will become 10 thou at the crank end , also we need to factor expansion contraction of the alloy cases
and nominal play in the main bearing or bad crank to bearing fit will also cause issues on overrun a thou play at the main bearing will become 4-5 thou at the crank end on over run

i do not think it is down to any design change in the stator rotor diameters , each set of cases / crank alignment will have its own idiosyncrasy and will require its own careful assembly
we also have to accept that most engines of this age will have had many owners and may have had cases changed which may be a less than perfect fit
1970`s production tolerances were not the best and would not be acceptable in 2020

Scout63 said:

... I disassembled and added a washer to the center stud, and this cleared up the contact...
This Caused

Click to expand...

Fast Eddie said:

Are you sure you’ve not gone from one extreme to the other and spaced the chaincase out too far? It seems kinda logical to me that if you pull the case in that direction, that stud will indeed move as it has?,,/QUOTE]
May have caused this...

Click to expand...

I think I know what is wrong but need to investigate further. ( Guzziology reminds us to fix the problem, not the symptoms). In replacing the primary plates, I did not seat the felt washer in the plates such that the rear drive spindle slides through it. I think this is pushing the rear of the inner chaincase out, flexing it such that the rear stator spindle is pushed forward. This created the extra clearance on the center mounting stud, and is causing the rear drive chain to spin the inner primary plates. I’ll disassemble tomorrow and check.

had a binding problem, from about the 12-3 o'clk position, on my 74 w/ an aftermarket Wassell stator. the only way I could get a proper fit was to do a "waist" cut on all three (3) mounting studs. as I recall, I removed .016" from the studs where the rotor mounts. I was able to get a near perfect .010" air gap around the final assembly. personally, I would modify the studs before drilling out the stator mounting holes. I know, as Scout63 said, about fixing the problem and not the symptom, but this was by far the easiest solution.

Thanks Joe. I’ll take the inner case off again and report. It just doesn’t make sense for the original stator and rotor to fit right during the previous assembly and not now. Both the original and replacement stator fit on the studs fine when the inner chaincase is removed but not when assembled. I’m thinking now it must be flexing of the case since the case is not fitting over the final drive sleeve gear. As it is now when I spin the rear wheel the inner primary cover washer plates spin with it. That must be the sleeve gear pressing on the inner plate rather than extending through it and through the felt washer.

Scout63 said:

Thanks Joe. I’ll take the inner case off again and report. It just doesn’t make sense for the original stator and rotor to fit right during the previous assembly and not now. Both the original and replacement stator fit on the studs fine when the inner chaincase is removed but not when assembled. I’m thinking now it must be flexing of the case since the case is not fitting over the final drive sleeve gear. As it is now when I spin the rear wheel the inner primary cover washer plates spin with it. That must be the sleeve gear pressing on the inner plate rather than extending through it and through the felt washer.

Click to expand...

good luck. I usually don't hammer engineers on things that were designed 50 years ago (hindsight's ALWAYS 20/20), but IMO, there are too many things, with tolerance stack up's that all must come together for a perfect fit - with the inner primary case mount being first. if the case mounting is off by just a little, minor issues will cascade into larger problems. this is one area where the design should have allowed for minor adjustment on that stator during installation, especially when dealing with aftermarket parts. my 2-cents, FWIW.