Classic British SparesKlempf British PartsBaxter CycleBritBike Sponsor SteadfastCyclesSRM EngineeringLucas Classic MotorcycleIndustrial tec supplyHepolite PistonsThe Bonneville Shop

Upgrade your membership to: Premium Membership | Gold Membership | Life Membership | Vendor Membership | Site Sponsor Membership
Welcome to BritBike Forum!
Britbike forum logo
Member Spotlight
Lannis
Lannis
Central Virginia
Posts: 13,908
Joined: July 2001
ShoutChat
Comment Guidelines: Do post respectful and insightful comments. Don't flame, hate, spam.
Search eBay for motorcycle parts in following countries
Australia, Canada, France, Holland, Italy, United Kingdom, USA
Previous Thread
Next Thread
Print Thread
Page 1 of 2 1 2
Joined: Jan 2006
Posts: 7,810
Likes: 135
Britbike forum member
OP Offline
Britbike forum member
Joined: Jan 2006
Posts: 7,810
Likes: 135
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-



GrandPaul (does not use emoticons)
Author of the book "Old Bikes"
Too many bikes to list, mostly Triumph & Norton, a BSA, & some Japanese
"The Iron in your blood should be Vintage"
1 member likes this: Allan G
Support Your #1 BritBike Forum!

Check out British motorcycles for sale: British Motorcycles on e-Bay UK, British motorcycles on e-Bay North America
Joined: Feb 2019
Posts: 29
Likes: 1
F
Britbike forum member
Offline
Britbike forum member
F
Joined: Feb 2019
Posts: 29
Likes: 1
Excellent tech tip! It's easy to check and certainly easier than an engine teardown. smile Thanks for posting.

Noel

1 member likes this: GrandPaul
Joined: Oct 2003
Posts: 11,281
Likes: 26
R
Britbike forum member
Offline
Britbike forum member
R
Joined: Oct 2003
Posts: 11,281
Likes: 26
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


Don't hide 'em, Ride 'em !!

RF Whatley
NE Georgia, USA
2 members like this: GrandPaul, Beach
Joined: Jan 2006
Posts: 7,810
Likes: 135
Britbike forum member
OP Offline
Britbike forum member
Joined: Jan 2006
Posts: 7,810
Likes: 135
Originally Posted by 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...
I was actually going to mention using Chan-L-Locks, but it slipped my mind as I was recording. Probably because I found it loose immediately by hand...

Good call!

Last edited by GrandPaul; 03/13/21 7:02 pm.

GrandPaul (does not use emoticons)
Author of the book "Old Bikes"
Too many bikes to list, mostly Triumph & Norton, a BSA, & some Japanese
"The Iron in your blood should be Vintage"
Joined: Mar 2006
Posts: 4,118
Likes: 23
R
Britbike forum member
Offline
Britbike forum member
R
Joined: Mar 2006
Posts: 4,118
Likes: 23
i thought the knocking at lite throttle loads on my A65 was rod replaced rotor and all quiet i corn-fess i was fooled for a while


1972 Triumph T120
1968 BSA A65
1968 MGB Roadster
1979 Chevrolet Camaro Berlinetta
1969 Honda Mini Trail
1939 farmall f30 tractor
2004 Honda Shadow Aero

1975 yamaha xs650b
1972 Norton commando
2 olive drab WWII military bicycle replicas
Joined: Jan 2014
Posts: 1,509
Likes: 142
N
Britbike forum member
Offline
Britbike forum member
N
Joined: Jan 2014
Posts: 1,509
Likes: 142
Originally Posted by 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.


thumbsup


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.
My T120 has been like that for around 3 or 4 years.
Not perfect but a workable fix. It also works if the keyway on the crank is knackered as is the one on mine.

Joined: Aug 2001
Posts: 7,165
Likes: 82
Britbike forum member
Offline
Britbike forum member
Joined: Aug 2001
Posts: 7,165
Likes: 82
Excellent video Paul, thank you!!


Jon W.


1957 6T Thunderbird 650
1968 T100R Daytona 500
1971 TR6R Tiger 650
1970 BSA A65F 650
1955 Tiger 100 - Project
1971 BSA A65 650 - Project
1972 Norton Commando 750 "Combat"


"Every time I listen to AC/DC, so do my neighbors"

1 member likes this: GrandPaul
Joined: Feb 2019
Posts: 479
Likes: 190
S
Britbike forum member
Offline
Britbike forum member
S
Joined: Feb 2019
Posts: 479
Likes: 190
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: Hillbilly bike
Joined: Jan 2014
Posts: 1,509
Likes: 142
N
Britbike forum member
Offline
Britbike forum member
N
Joined: Jan 2014
Posts: 1,509
Likes: 142
I've pegged thick ss washers to the rotor to achieve the same thing a few times.
As you say, it's not for everyone but i don't see anything wrong with it.

Joined: Oct 2008
Posts: 7,176
Likes: 135
Britbike forum member
Offline
Britbike forum member
Joined: Oct 2008
Posts: 7,176
Likes: 135
I've never had a rotor separate, but a loose primary chain can make the same sort of noise. Check that before removing the primary cover.


Knowledge speaks. Wisdom listens.

72 T120V cafe project "Mr. Jim"
72 T150V "Wotan"
92 BMW K100rs "Gustav"
Joined: Dec 2002
Posts: 3,368
Likes: 32
L
Britbike forum member
Offline
Britbike forum member
L
Joined: Dec 2002
Posts: 3,368
Likes: 32
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: Beach, GrandPaul
Joined: Jan 2006
Posts: 7,810
Likes: 135
Britbike forum member
OP Offline
Britbike forum member
Joined: Jan 2006
Posts: 7,810
Likes: 135
I don't mind criticism, as long as it's CONSTRUCTIVE.

I got two earfuls of constructive criticism when I first learned that Norton Commando swingarm spindles get loose in their root tubes, resulting in a dangerous wobble at the rear. I had taken extensive care to replace the bronze bushings, which almost never need replacing, used the high end stainless vernier adjustable isolastics, a rod-linked top steady, and didn't bother to simply try wiggling the rear wheel side to side!

Hey, lesson learned (for the better).

Last edited by GrandPaul; 03/14/21 3:34 pm.

GrandPaul (does not use emoticons)
Author of the book "Old Bikes"
Too many bikes to list, mostly Triumph & Norton, a BSA, & some Japanese
"The Iron in your blood should be Vintage"
Joined: Oct 2003
Posts: 11,281
Likes: 26
R
Britbike forum member
Offline
Britbike forum member
R
Joined: Oct 2003
Posts: 11,281
Likes: 26
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. My T120 has been like that for around 3 or 4 years. Not perfect but a workable fix. It also works if the keyway on the crank is knackered as is the one on mine.
When these rotors fail, they actually Explode ! inside the primary case. Some pieces jam into the stator, others fly between the primary chain and sprocket. The end result is that they destroy the primary case and lock the rear wheel up. Dis-engaging the clutch may or may not be an option at that point.

I'm a degreed Mechanical Engineer. Take it from me the dynamic forces involved at 5500 RPM are astronomical. A mere 2 bolts is not going to stop a rotor from exploding. The forces are acting outward, radially. A bolt into the sprocket is acting at 90 degrees to the forces, acting axially. And, your bolts are placed in "shear".

The root problem is the rotor is not a homogeneous part. It's composed of a steel center hub, 6 magnets, and all held in place by molded aluminum. Exactly which one of the 8 pieces were you hoping to secure with your bolts ?

If that's the way you want to "fix" your bike that's your business, but please don't come on here touting it as a solution for others. I have seen first hand the devastation these rotors can cause, you obviously have not.

All the best.


Don't hide 'em, Ride 'em !!

RF Whatley
NE Georgia, USA
Joined: Oct 2003
Posts: 11,281
Likes: 26
R
Britbike forum member
Offline
Britbike forum member
R
Joined: Oct 2003
Posts: 11,281
Likes: 26
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.

Hope this helps.


Don't hide 'em, Ride 'em !!

RF Whatley
NE Georgia, USA
Joined: Jan 2014
Posts: 1,509
Likes: 142
N
Britbike forum member
Offline
Britbike forum member
N
Joined: Jan 2014
Posts: 1,509
Likes: 142
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.

Last edited by NickL; 03/15/21 2:49 am.
2 members like this: Beach, Hillbilly bike
Joined: Feb 2019
Posts: 479
Likes: 190
S
Britbike forum member
Offline
Britbike forum member
S
Joined: Feb 2019
Posts: 479
Likes: 190
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: GrandPaul
Joined: Aug 2011
Posts: 143
T
Britbike forum member
Offline
Britbike forum member
T
Joined: Aug 2011
Posts: 143
Excellent video! Really interesting.

Joined: Oct 2003
Posts: 11,281
Likes: 26
R
Britbike forum member
Offline
Britbike forum member
R
Joined: Oct 2003
Posts: 11,281
Likes: 26
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


Don't hide 'em, Ride 'em !!

RF Whatley
NE Georgia, USA
1 member likes this: GrandPaul
Joined: Mar 2005
Posts: 10,659
Likes: 76
J
Britbike forum member
Offline
Britbike forum member
J
Joined: Mar 2005
Posts: 10,659
Likes: 76
Plus 1 for RFW.

The forces that cause this are not appaent to the naked eye. The rotor is not in a state of constant rotation. To quote Kevin Cameron. "The flywheel does not turn smoothly, but in a series of fairly violent jerks." It is this hammering effect that leads to failure. With each "violent jerks" (power stroke) of the twin cylinder engine, any weakness is eventually taken advantage of.

While in theory clamping would seem appropriate, it can be less effective than the original design used to retain the magnets.

Last edited by John Healy; 03/16/21 5:39 pm.
Joined: Feb 2005
Posts: 877
Likes: 10
M
Britbike forum member
Offline
Britbike forum member
M
Joined: Feb 2005
Posts: 877
Likes: 10
Aren't the latest rotors referred to as 'welded' rotors? Or is memory failing me again, anyway, if indeed they are welded just exactly what is welded? Can't be the magnets to the hub or to the alloy so what's the difference with the later design? Mark R.

Joined: Jan 2014
Posts: 1,509
Likes: 142
N
Britbike forum member
Offline
Britbike forum member
N
Joined: Jan 2014
Posts: 1,509
Likes: 142
In suppose i'd better replace the one on bike then, it's only been
modified like that for about 3 or or 4 years.

None of them are welded that i've seen.
The inner Hex is steel, the magnets are arranged on the hex and the ally is
cast over the lot.
Even a dumbass like me can see it's a crap way of making something but
as Lucas had to make them for about sixpence, that's what you get.

The ones i've bought from India for the old enfields are better made.
They fit ok.

Last edited by NickL; 03/16/21 10:53 pm.
Joined: Mar 2011
Posts: 400
Likes: 20
L
Britbike forum member
Offline
Britbike forum member
L
Joined: Mar 2011
Posts: 400
Likes: 20
How funny is this? I was out in the garage a couple nights ago taking apart the primary on my 70 Bonneville to fix the clutch and lo and behold, the rotor was floppy on the center. Purely coincidentally this post shows up. All of these posts, especially including the warnings, are a good reminder to check this every time you (meaning, I) have the primary cover off.

Joined: Feb 2008
Posts: 5,633
Likes: 248
Britbike forum member
Offline
Britbike forum member
Joined: Feb 2008
Posts: 5,633
Likes: 248
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: Beach
Joined: Jan 2014
Posts: 1,509
Likes: 142
N
Britbike forum member
Offline
Britbike forum member
N
Joined: Jan 2014
Posts: 1,509
Likes: 142
Would people call the original design a bodge? Yes (certainly i would)
Done for a price. Relying on crap cast ally to hold on a small hex centre
is little better than the method i used. It is also running in shear over small
hex flats that provide the perfect ramp for jacking the magnets out.
I bet the chap who designed that had a mechanical degree as well.

The pegs i used were not a friction point they are drilled into the rotor
and screwed into the sprocket. The whole thing is solid BUT i accept the
pegs are in shear. They are 3/8 Grade 8 pegs.
The other method i have used is to peg 2 large ss washers to the rotor this
is then clamped with the centre by the fixing.

Joined: Jan 2006
Posts: 7,810
Likes: 135
Britbike forum member
OP Offline
Britbike forum member
Joined: Jan 2006
Posts: 7,810
Likes: 135
All very interesting.

I KNOW (believe me) that some people don't have $75 to spare for a new rotor, so these bodges are attractive.

But I encourage you to maybe start a savings jar and toss a few coins in it every day. As soon as it's over $75, BUY A NEW ROTOR!

Last edited by GrandPaul; 03/17/21 2:49 pm.

GrandPaul (does not use emoticons)
Author of the book "Old Bikes"
Too many bikes to list, mostly Triumph & Norton, a BSA, & some Japanese
"The Iron in your blood should be Vintage"
Page 1 of 2 1 2

Moderated by  Jon W. Whitley 

Link Copied to Clipboard
Job CycleBritish Cycle SupplyMorries PlaceKlempf British PartsPodtronicVintage MagazineBSA Unit SinglesBritBike SponsorBritish Tools & FastenersBritBike SponsorBritBike Sponsor






© 1996-2021 britbike.com
Powered by UBB.threads™ PHP Forum Software 7.7.5