MKIII stator/rotor clearance strategies

Hi all,

I'm running into a problem that seems fairly common on Commandos. My 3 phase stator/rotor upgrade has lead to a clearance issue between the two when mounted. So far fettling has not effectively resolved the issue. I can get it down to ALMOST a smooth .08mm clearance at the tightest spot, but the difference in gap from one side to the other is huge, and I have burned through one stator hoping what I got to previously was close enough. So I really want to make sure I do it right this time. The zone where it's tightest seems somewhat endemic to commandos:



I've done a lot of digging into suggestions for fixes, which have included:

the milk plastic shim method
bending the studs
hammering the stator
reaming bigger stud holes in the stator
grinding down the studs
removing material from the carrier on the right side where it closely follows the OD of the stator
milling down the OD of the rotor

...The list is long. But there doesn't seem to be a very clear consensus on best practices. My loosened assembly doesn't seem to have enough wiggle room for shims to work, and I don't understand how hammering or bending studs is going to move a part that is captured pretty tightly by the carrier plate (and the carrier plate in turn by the inner primary cover). Drilling bigger holes in the stator / grinding down the studs / modifying the carrier plate seem like they might potentially accommodate more movement, but what's the risk of stuff vibrating back into an interference condition? Milling down the rotor would be an interesting option if I had the setup to properly do that, but then the already large clearances on the left get even larger.

I'd appreciate any wisdom here because I'm a bit stumped as to the best fix for the problem.

Thanks,
Anthony

Anvill said:

Hi all,

I'm running into a problem that seems fairly common on Commandos. My 3 phase stator/rotor upgrade has lead to a clearance issue between the two when mounted. So far fettling has not effectively resolved the issue. I can get it down to ALMOST a smooth .08mm clearance at the tightest spot, but the difference in gap from one side to the other is huge, and I have burned through one stator hoping what I got to previously was close enough. So I really want to make sure I do it right this time. The zone where it's tightest seems somewhat endemic to commandos:

I've done a lot of digging into suggestions for fixes, which have included:

the milk plastic shim method
bending the studs
hammering the stator
reaming bigger stud holes in the stator
grinding down the studs
removing material from the carrier on the right side where it closely follows the OD of the stator
milling down the OD of the rotor

...The list is long. But there doesn't seem to be a very clear consensus on best practices. My loosened assembly doesn't seem to have enough wiggle room for shims to work, and I don't understand how hammering or bending studs is going to move a part that is captured pretty tightly by the carrier plate (and the carrier plate in turn by the inner primary cover). Drilling bigger holes in the stator / grinding down the studs / modifying the carrier plate seem like they might potentially accommodate more movement, but what's the risk of stuff vibrating back into an interference condition? Milling down the rotor would be an interesting option if I had the setup to properly do that, but then the already large clearances on the left get even larger.

I'd appreciate any wisdom here because I'm a bit stumped as to the best fix for the problem.

Thanks,
Anthony

Click to expand...

Try the simplest method first. The plastic milk container shim worked for me. Simple and cheap.

I had the same problem...couldn't get sufficient clearance in one spot with the plastic shims.

I chose to enlarge the holes in the stator. I was then able to adjust for proper, minimum clearance all the way around with almost no variation. I placed some loctite on the threads AND to the area on the back of the stator where it sits against the mounting boss and used a serrated lock washer.

It's been in place with no shifting since 2012.

Given the position (3 o'clock), is the chaincase sitting properly?

Anvill said:

I've done a lot of digging into suggestions for fixes, which have included:

the milk plastic shim method
bending the studs

Click to expand...

Wouldn't work for the 850 Mk3, because, unlike pre-Mk3 models, the three 850 Mk3 stator studs are only long enough to attach the stator to the outrigger, they do not continue through the primary into the crankcase and attempting to bend the four Mk3 primary studs is unlikely to move the position of inner primary case, therefore, won't adjust the position of the stator...

Anvill said:

hammering the stator
grinding down the studs

Click to expand...

Also wouldn't work for the same reason that the ring of the stator is positively located within the outrigger...

Anvill said:

removing material from the carrier on the right side where it closely follows the OD of the stator

Click to expand...

That is one option for the 850 Mk3, as then...

Anvill said:

reaming bigger stud holes in the stator

Click to expand...

...would be possible.

Also, the outrigger is positively located to the primary case by four stepped joints so a small amount of metal could be removed from the joint allowing the position of the outrigger and stator to be moved* but bearing in mind the outrigger also supports the outer end of the anti-backfire device so moving the outrigger by any significant amount could potentially misalign the anti-backfire assembly.


*(Edit: There's also a dowel adjacent to the rear stepped joint)

Suggest you take the mean measurement diameter of your sator and compare it with a couple of others.
If no difference, lightly skim it in a lathe.

Looks new. Why not consider sending back to the supplier? Alternatively I would be checking how concentric the bore is relative to the mounting stud holes.

Lovel said:

Looks new. Why not consider sending back to the supplier?

Click to expand...

The lack of clearance in that area seems to be a reasonably common Mk3 problem (my Mk3 was the same) so replacement of either the stator or rotor probably wouldn't correct it.

Assuming there is actually adequate difference between the stator bore and rotor ID, then all you gotta do is open out the stator holes and use the milk bottle shim trick. Job done.

Fast Eddie said:

Assuming there is actually adequate difference between the stator bore and rotor ID, then all you gotta do is open out the stator holes and use the milk bottle shim trick. Job done.

Click to expand...

It's not that simple for the Mk3 for the reasons stated in my first reply because the stator ring is tightly located within recesses in the outrigger so the stator position cannot move without the outrigger also moving which in turn is 'keyed' to the inner primary case.

L.A.B. said:

It's not that simple for the Mk3 for the reasons stated in my first reply because the stator ring is tightly located within recesses in the outrigger so the stator position cannot move without the outrigger also moving which in turn is 'keyed' to the inner primary case.

Click to expand...

Ah, got it.

Ignore my ‘fix’ then !

Thinking out loud, if you have access to machine tools, why not create some “waisted” studs that would allow the stator to be shifted around a bit? Then use the plastic shim method. FWIW

I had the same issue with my bike ...The first thing needed to know is if the rotor is running true on the crankshaft . Do the clearances remain the same throughout a full revolution of the rotor ?? Is the stator parallel to the rotor ??

On mine , i opened up the holes on the stator ever so slightly . I loosened up the rotor and then took a piece of .010" brass shim and wrapped it around the rotor . I found that i eventually had to put a bit of shim under the top left side of the stator . Doing this closed up the larger gap on the left and opened up the right . Snug everything up evenly and go from there .. If lucky you might even get it the first time .

Derek Wilson said:

Thinking out loud, if you have access to machine tools, why not create some “waisted” studs that would allow the rotor to be shifted around a bit? Then use the plastic shim method. FWIW

Click to expand...

Stator?

Ron Hulton said:

On mine , i opened up the holes on the stator ever so slightly . I loosened up the rotor and then took a piece of .010" brass shim and wrapped it around the rotor . I found that i eventually had to put a bit of shim under the top left side of the stator . Doing this closed up the larger gap on the left and opened up the right . Snug everything up evenly and go from there .. If lucky you might even get it the first time .

Click to expand...

Unfortunately, it seems it's not being fully understood that the metal ring of the stator is tightly located to the Mk3 outrigger (see my first post in the thread) so the stator is basically locked in a position that machining the studs or attempting to move the stator on the studs won't work.

I appreciate the responses everyone. I have tried loosening the bolt attaching the primary to the cradle along with the four studs off the motor and there’s just not enough wiggle to open up the pinched spot, the plastic shim method is what I used the first time and it didn’t work for me.

I have tried the original Lucas rotor (same OD as the replacement) and found the same pinch spot. This zone is consistent throughout the rotation of the crankshaft so I think the shaft is true. And the first 3 phase stator I fried was a wassell, exact same issue fit wise so I’m pretty confident a return of the item will not resolve anything.

I even tried buying a new Andover Norton outrigger plate in hopes it might be better centered but it’s telegraphing the pinch zone the same as the original.

LAB has articulated my concerns well in that the stator is very captured in its assembly. So it seems like my options are then a combination of modifying the outrigger plate along with drilling out the holes on the stator/shouldering the studs, or finding a way turn down the rotor. It’s honestly more modification than I am comfortable with, but it doesn’t really seem like there are other viable choices. I may try to go the rotor route so it at least affects less parts. Hopefully the reduced output the larger clearance will create won’t kill the effectiveness of the charging system. It’s pretty frustrating I can’t just get it to sit centered though.

Anvill said:

I appreciate the responses everyone. I have tried loosening the bolt attaching the primary to the cradle along with the four studs off the motor and there’s just not enough wiggle to open up the pinched spot, the plastic shim method is what I used the first time and it didn’t work for me.

I have tried the original Lucas rotor (same OD as the replacement) and found the same pinch spot. This zone is consistent throughout the rotation of the crankshaft so I think the shaft is true. And the first 3 phase stator I fried was a wassell, exact same issue fit wise so I’m pretty confident a return of the item will not resolve anything.

I even tried buying a new Andover Norton outrigger plate in hopes it might be better centered but it’s telegraphing the pinch zone the same as the original.

LAB has articulated my concerns well in that the stator is very captured in its assembly. So it seems like my options are then a combination of modifying the outrigger plate along with drilling out the holes on the stator/shouldering the studs, or finding a way turn down the rotor. It’s honestly more modification than I am comfortable with, but it doesn’t really seem like there are other viable choices. I may try to go the rotor route so it at least affects less parts. Hopefully the reduced output the larger clearance will create won’t kill the effectiveness of the charging system. It’s pretty frustrating I can’t just get it to sit centered though.

Click to expand...

L.A.B. said:

Stator?



Unfortunately, it seems it's not being fully understood that the metal ring of the stator is tightly located to the Mk3 outrigger (see my first post in the thread) so the stator is basically locked in a position that machining the studs or attempting to move the stator on the studs won't work.

Click to expand...

In my case it seemed to indicate that the stator was not parallel to the rotor and using a feeler gauge you could feel the difference as you slid it in and out . You are 100 % correct about the stator being tight on the outrigger studs . By slightly opening up the three holes in the stator i was able to put a shim between the stator and the outrigger plate in the upper left side . That helped square the stator to the rotor .

Anvill said:

Hi all,

I'm running into a problem that seems fairly common on Commandos. My 3 phase stator/rotor upgrade has lead to a clearance issue between the two when mounted. So far fettling has not effectively resolved the issue. I can get it down to ALMOST a smooth .08mm clearance at the tightest spot, but the difference in gap from one side to the other is huge, and I have burned through one stator hoping what I got to previously was close enough. So I really want to make sure I do it right this time. The zone where it's tightest seems somewhat endemic to commandos:



I've done a lot of digging into suggestions for fixes, which have included:

the milk plastic shim method
bending the studs
hammering the stator
reaming bigger stud holes in the stator
grinding down the studs
removing material from the carrier on the right side where it closely follows the OD of the stator
milling down the OD of the rotor

...The list is long. But there doesn't seem to be a very clear consensus on best practices. My loosened assembly doesn't seem to have enough wiggle room for shims to work, and I don't understand how hammering or bending studs is going to move a part that is captured pretty tightly by the carrier plate (and the carrier plate in turn by the inner primary cover). Drilling bigger holes in the stator / grinding down the studs / modifying the carrier plate seem like they might potentially accommodate more movement, but what's the risk of stuff vibrating back into an interference condition? Milling down the rotor would be an interesting option if I had the setup to properly do that, but then the already large clearances on the left get even larger.

I'd appreciate any wisdom here because I'm a bit stumped as to the best fix for the problem.

Thanks,
Anthony

Click to expand...

Anvill said:

Hi all,

I can get it down to ALMOST a smooth .08mm clearance at the tightest spot, but the difference in gap from one side to the other is huge, and I have burned through one stator hoping what I got to previously was close enough.

Click to expand...

Did you mean to say .08mm? That's about 0.003" (if my math is right) and the standard is 0.008" (0.2mm) so you're way off. I've plenty of Wassell rotors and stators without trouble but not on a MKIII.

L.A.B. said:

It's not that simple for the Mk3 for the reasons stated in my first reply because the stator ring is tightly located within recesses in the outrigger so the stator position cannot move without the outrigger also moving which in turn is 'keyed' to the inner primary case.

Click to expand...

I don't know much about MKIIIs - Is it possible that the inner primary is cocked or in the wrong location? It would seem that his gap is way off and from what you said, there's no good way to move the stator.

The solution is fairly obvious, IMHO.
Step 1. Oversize the three locating holes in the stator to allow lateral movement. Or if you have assess to a lathe use the Norman White method and waist the studs to allow the movement.

Step 2. To address LAB's point made in previous comments: Carefully remove material from the one shoulder that is restricting lateral movement. Unfortunately in this case it is the longer surface, but no big deal. Use a dremel and a small cutting tool to relieve the face that limits movement. This is not a precision surface. Or, if all you have is a file, remove material on the outside of the stator. Use a piece of paper to keep track of the contact point and focus your efforts there.

Ron Hulton said:

You are 100 % correct about the stator being tight on the outrigger studs .

Click to expand...

In mine and apparently also Anvill's case (maybe others?) it's because the circular stator laminations locate tightly in the area marked by the red arrows rather than the stator being tight on the studs, the stator being set in one position once fitted to the outrigger so opening out the stator holes alone doesn't allow the stator's position to be adjusted.



I get your point about squaring the stator to the rotor. I don't recall that being a problem with mine.

Stephen Hill said:

The solution is fairly obvious, IMHO.
Step 1. Oversize the three locating holes in the stator to allow lateral movement.

Click to expand...

You cannot just do that on the Mk3 (see above) as metal also has to be removed from the outrigger as mentioned.