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Jon W. Whitley
Jon W. Whitley
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Thread Like Summary
Allan G, Beach, GrandPaul, Hillbilly bike, NickL, Stuart Kirk
Total Likes: 20
Original Post (Thread Starter)
by GrandPaul
GrandPaul
If you've ever had what sounded like a rod/bearing knock and tore your engine apart only to find the bearings were all good, you must have overlooked the more likely (and MUCH easier) solution - replacing your failed alternator rotor. The body of the rotor on Lucas alternators have a tendency to loosen off their hubs.

I made my second youtube video ever, verifying what I know was the knocking problem on my '67 Bonneville-

Liked Replies
by RF Whatley
RF Whatley
Great video.

That's the way I test them too... except that I grab them with a huge pair of slip joint pliers. The reason being is that some of them are just as broken, but they can't be turned by hand. If the magnet outer moves independently of the steel hub, even a tiny, tiny bit, then they must be replaced. There is absolutely no repair.

I test them anytime the rotor is removed for a primary chain replacement or whatever. The incidence of failure is way to high to not have this as a regular check.

thumbsup
2 members like this
by leon bee
leon bee
Stuart and Nick: I love these real-world fix ideas. Used to be, you kinda had to keep these things to yourself around here, unless you liked a lot of criticism.
2 members like this
by NickL
NickL
First up, sorry RF i didn't mean to offend you.
The fact that the rotors separate and explode is something i am aware of
and have witnessed several times over the years with varying results.
Yes they are normally quite severe.

The hex in the centre pushes the magnets out as the loose outer rotates on the hex,
they have nowhere else to go.........
If you stop the outer rotating on the hex they stay in place as designed. This
is done by locking the two parts together. In the original setup, molten ally is
poured over the hex and the magnets to form the rotor. The ally is reliant on
the hex centre to stop it rotating. As long as the two parts are locked together
there is no problem.
The fix i use can only be carried out if the rotor is not really bad such as magnets
sticking out of the ally, but is quiet suitable for a slightly loose outer.
As i said it may not be for everyone to carry out the repair i have used but like
Stuart i don't see a problem with it.

The original rotor is a pretty crap design anyway but cheap.
Leon, if i was worried about criticism, i wouldn't have bought or ridden these
old crates for as long as i have.
2 members like this
by quinten
quinten
, mind the gap

[Linked Image from walridgemike.files.wordpress.com]
as the hex hub center loosens the hub trys to accelerate faster than the magnets and potmetal .
the magnets are hammered and cammed outward ... creating more and more space be be cammed outward
... kind of cool if its not your engine .
2 members like this
by Falcon_52
Falcon_52
Excellent tech tip! It's easy to check and certainly easier than an engine teardown. smile Thanks for posting.

Noel
1 member likes this
by Jon W. Whitley
Jon W. Whitley
Excellent video Paul, thank you!!
1 member likes this
by Stuart Kirk
Stuart Kirk
Originally Posted by RF Whatley
If the magnet outer moves independently of the steel hub, even a tiny, tiny bit, then they must be replaced. There is absolutely no repair..
Originally Posted by NickL
You can actually use the two stud holes in the drive sprocket and put 2 holes in the rotor then lock the two together that way with a couple of threaded pegs.
That's a good idea. ET rotors on BSA twins are secured that way.

Here's another idea. If you have a lathe to work with, you can machine the rotor to make a flat surface for 2 concave lawnmower blade serrated spring washers and sandwich the rotor between the washers. The washers have to be bored to fit over the end of the crankshaft and the rotor nut. The washer grips the aluminum body and not the steel center. This effectively silences the knock but the timing marks, if any, may not be accurate when you're done. But then they weren't accurate anyway if the rotor was loose. I've used this repair several times in the last 50 years but I will freely admit this solution won't be for everyone..
Here's a link for a typical blade washer.
http://www.texaslawnmowerparts.com/...e-Cupped-serrated-washer-38-_p_7706.html
1 member likes this
by Stuart Kirk
Stuart Kirk
Originally Posted by RF Whatley
Originally Posted by Stuart Kirk
That's a good idea. ET rotors on BSA twins are secured that way.
Sorry, sir. It's a VERY bad idea.

Also, ET rotors are TIMED using a single pin into the engine sprocket. That pin does nothing to keep the ET rotor from exploding.
Hi RF. I guess I didn't expect this to be such a contentious subject. I didn't mean it to be and I apologize. I do have additional comments.

Thank you for pointing out the pin's timing function but we must recognize that it also moves the drive input from the hexagonal core to to the aluminium body of the rotor. This actually eliminates the cam lobe effects of that 6 sided core that tries to push the magnets outward and possibly "explode" the rotor..

The spring washer strategy accomplishes exactly the same thing by by gripping the aluminum, not the core. NickL makes the same point and I agree with his thinking.
Originally Posted by NickL
The hex in the centre pushes the magnets out as the loose outer rotates on the hex,
they have nowhere else to go.........
If you stop the outer rotating on the hex they stay in place as designed. This
is done by locking the two parts together.

It seems that the overriding issue is to not let a rotor problem go too long. If you hear a worrisome noise from the lower end, take the time to find out what it is. The ones I have repaired have been loose enough to be audible and sounding like a rod or main bearing noise at idle or just above.
1 member likes this
by Tridentman
Tridentman
Richard--speaking as one Richard to another---never underestimate the power of friction.
In Nicks configuration the normal failure mode of the rotor is overcome by the friction clamping all the parts together so they do not move relative to one another.
If the coefficient of friction is high enough, if the clamped area is appropriately positioned and the clamping force is high enough then it will work.
In Nicks case it obviously has worked for several years.
Is it the "correct" solution to the problem of a loose rotor?--No.
Would a lot of people call it a "bodge"?-- Yes.
However engineering is not about "correct" solutions-- it is all about making something that works.
If we never think out of the box it will be a sad day IMHO.
1 member likes this
by RF Whatley
RF Whatley
Originally Posted by NickL
First up, sorry RF i didn't mean to offend you.
No one has offended me. I'm not mad or upset, I'm simply being emphatic.

Each of us has blind spots. There are areas of knowledge that are not our specialty. (Mine seems to be women. laughing ) When I try to talk about a subject like astrophysics, an astrophysicist will look at me like I'm crazy. Running concurrently with this is the human tendency of smart and/or successful people to believe they know more than they do. A shining example of this is when movie stars believe they know how political policy ought to be run better than anyone else. You can listen to these people and immediately your face winces as you listen to them. It's NOT that they are stupid. They are rational, intelligent people, but they are simply trying to operate well beyond their area of expertise.

All this leads to my brother-in-law's favorite saying, "You don't know what you don't know." In other words, it's easy for anyone to get "bitten" because normally rational and intelligent people try to work in an area they don't really know about. They don't know the basic "pitfalls" in that specific area, and so the end result is that they end up falling right into a big "pit" that is glaringly obvious to those with some knowledge in that area.

If I told you that we could bolt the cylinder head on by having a bolt come from the side of the engine, you would all laugh. No! The forces of compression and the method of construction tell us all that the head was fitted to the engine in the vertical, therefore to oppose the forces of compression, the head bolts must also be in the vertical. Everyone clearly sees that.

But what some are saying here is that we can bolt or clamp the alternator rotor from the side (axially), and effectively oppose radial forces. That is, the forces would be acting 90 degree to the holding force. For the same reason you cannot attach a cyl head from the side, you also cannot hold/ retain/ constrain a Lucas rotor for exploding with clamping forces from the side. It's a very similar situation. One is plainly obvious, the other is harder to see unless you work in the field.

Then on top of all that we have cast aluminum that cures with a very large grain structure making the breakage probable and inevitable.

Sorry to beat a dead horse... I think we simply leave it at.... If your rotor is even a tiny bit loose, it will only get looser. So start and end with replacing it immediately.

All the best. thumbsup
1 member likes this
by NickL
NickL
I think that's enough of all this.
As i said i'll just get another rotor and smile when i remember this thread.
Life's too short to be taken seriously.
1 member likes this
by Stuart Kirk
Stuart Kirk
OK, I've finally got to it. I pulled the rotor covers on both my unit singles. The B50 has my loose rotor fix, the B44 doesn't appear to. I tested the B50 rotor by marking it with a sharpie and then tapping on the edge of the recessed Lucas logo in both directions to find any looseness. It is tight.
[Linked Image from i.postimg.cc]
You'll notice the face of the rotor has been machined as well as the recess (the hub is also machined down flush with the aluminum) for the washer to fit. I've been riding this bike for years and letting my early 20's sons thrash on it, so that has meant lots of wheelies and trying to keep up with modern bikes. This bike was a $500.00 swap meet basket case that had thrown a rod so now it has a new liner from LA Sleeve, and a Carrillo rod and JE piston from Ed V.
[Linked Image from i.postimg.cc]
Now, the B44 does not appear to be modified. There is no special washer or machining visible and I don't remember if I fitted only one washer to the back or not. The fact that the crankshaft threads don't come up flush or proud of the nut makes me think I might have. But it checks tight also.
[Linked Image from i.postimg.cc]
Interestingly, the B44 with the zinc pump was not wet sumped, the B50 with the iron pump was. Go figure.
[Linked Image from i.postimg.cc]
So anyway, I feel reassured that a rotor can in fact be kept together using a cupped washer strategy of some sort of pinning approach. Here are shots of an original washer and a modified washer. Notice the teeth are trimmed down to avoid digging into and damaging the aluminum [Linked Image from i.postimg.cc]
and the domed side is machined to fit the back of the nut better.
[Linked Image from i.postimg.cc]
So that's my take on the whole loose rotor issue. We clearly need to check for looseness because that can lead to bad damage if neglected for too long. But, if the rotor isn't very loose, one of these methods can extend the useful life quite a lot.
1 member likes this
by quinten
quinten
its all good till it isn't

[Linked Image from triumphrat.net]
1 member likes this
by Stuart Kirk
Stuart Kirk
Originally Posted by quinten
as the hex hub center loosens ......the magnets are hammered and cammed outward ...
Another impressive failure. And your assessment and description of the cause is a good one.

That hammering and camming is exactly what pins or cupped washers prevent.
1 member likes this
by tridentt150v
tridentt150v
I made up a spacer to replace the original spacer so that it has a recessed inner ledge/lip. Careful measurements of the recess means that the spacer will lock both the alloy and the steel centre to the drive sprocket once the rotor nut is done up. I did this years ago. But it wasn't to fix a problem, it was to stop a problem in the first place.
IMO the the alloy and steel centre shear because of the whip generated by the rotation>>> inertia and recoil. If you can remove or abate this whip by locking the components so they are effectively a single unit then - in theory - there will be no shear.
I've never seen the magnets come loose or fall out where the steel centre and alloy are intact.
I've never seen or heard of a triple with this issue....because the rotational whip is different/smoother ...120' vs 180' = eliminating or lessening the whip is a good thing.

I've done this on a few twins [after convincing the owners of the benefits or at their request] and none have ever had an issue.
1 member likes this
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