speedometer gearbox/cable -- what's wrong and how do i fix it?

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Peavey Jeltz said:
Looks rebuildable with a donor guide ring and some wire nails for rivets. How's the ring gear? Don't bin it.
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given enough time and money, i'm sure it is rebuildable, but not sure it's worth the effort. ring gear looks OK - didn't notice anything broken. as far as "don't bin it", the parts will be cleaned, bagged and tagged, along with everything else, and saved for the next guy.

i'm thinking that guide ring may aid in positioning the gearbox laterally on the axle. with the ring pretty much gone, (WAG) the last install forced it to too close to the rear hub cover causing it to bind and ultimately fail. not sure I have a clear understanding on how everything is supposed to fit together. probably get a better feel for things when I get all the new stuff. BTW, received an email from Ella @ old britts - package sent yesterday, so hope to have it Saturday and back on the road sunday. :)
 
joe czech said:
given enough time and money, i'm sure it is rebuildable, but not sure it's worth the effort. ring gear looks OK - didn't notice anything broken. as far as "don't bin it", the parts will be cleaned, bagged and tagged, along with everything else, and saved for the next guy.
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If, as you say, the gear teeth are still in reasonably good condition then it is rebuildable for very little money as all it would need is a guide ring (if you can find a good used one from an old unit and either rivets or screws) although it's not unheard of for owners to have removed the drive gearbox and found the guide ring missing!


Some of us prefer to lubricate the drive gearbox with oil in preference to grease.

Comnoz recommends Jewel Amber oil....: https://www.accessnorton.com/Norton...se-fitting-for-speedo-drive.8613/#post-113397

...I use whatever heavy gear oil happens to be handy.
 
gortnipper said:
Interesting that they list four lengths of Norton cables for 1964 - 75, 5'2"-5'9".

I bought a Venhill cable for my rebuild, N01-7-100. At 5'9.5" it is decidedly too long.

Does anyone know the proper length for a 06-7904?
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hey, good morning Dave (from the other side of the planet anyway)
old britts has the length listed at 5'9.5" (an AN assembly - #NM25087). I would think there should only be one OE length - given only one "factory correct" routing - independent of the handlebars since the speedometer is in a fixed position. speculation on my part....
 
Mine loops.too much between the speedo gearbox and the frame behind z plate, so it ends up resting on the folded up center stand. There is also plenty of slack under the lower fork yoke.
 
L.A.B. said:
If, as you say, the gear teeth are still in reasonably good condition then it is rebuildable for very little money as all it would need is a guide ring (if you can find a good used one from an old unit and either rivets or screws) although it's not unheard of for owners to have removed the drive gearbox and found the guide ring missing!


Some of us prefer to lubricate the drive gearbox with oil in preference to grease.

Comnoz recommends Jewel Amber oil....: https://www.accessnorton.com/Norton...se-fitting-for-speedo-drive.8613/#post-113397

...I use whatever heavy gear oil happens to be handy.
Click to expand...
a bit puzzled --
not sure i'm understanding or visualizing how this whole thing goes together. i'm thinking there some sort of ring, either cast or machined into the gearbox housing that's completely worn away on my assembly. I do have some very thin alloy shavings mixed in with the grease - looks like something is definitely worn and completely gone. probably should soak that thing in solvent for a thorough cleaning and get a better understanding of what's going on here. i do know I want to fix this problem only once, and move on. thanks (LAB) for the oil link.
 
gortnipper said:
Interesting that they list four lengths of Norton cables for 1964 - 75, 5'2"-5'9".

I bought a Venhill cable for my rebuild, N01-7-100. At 5'9.5" it is decidedly too long.

Does anyone know the proper length for a 06-7904?
Click to expand...

5' 9" According to the Smiths Motorcycle Applications list Edit: For the Commando. Shorter cables are for other models.
 
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Actually, thinking about it, if cleaned and reassembled then it would probably continue to work 'as it is'.
 
gortnipper said:
Mine loops.too much between the speedo gearbox and the frame behind z plate, so it ends up resting on the folded up center stand. There is also plenty of slack under the lower fork yoke.
Click to expand...
I thought that was normal - ???
mine was routed straight down from the speedometer, down past the lower fork yoke, routed down the RH frame member - zip-tied a couple places, near the front ios mount, routed under the iso mount and routed horizontally along the frame to the gearbox drive assembly. there seemed to be plenty of slack - I assumed to allow for full swingarm movement. I did have a very loose fit zip tie on the lower horizonal frame member to restrict any excessive movement. the cable did have a tendency to rest on the center stand (in the folded-up position). twice I tore out the end of the cable. i'm assuming is was from issues with the gear box speedo drive.
 
L.A.B. said:
Actually, thinking about it, if cleaned and reassembled then it would probably continue to work 'as it is'.
Click to expand...
beginning to think I really don't have a clear understanding of what's going on here. first step - solvent clean everything. one thing I need to figure out it how to get the input drive separated for the housing.
 
joe czech said:
one thing I need to figure out it how to get the input drive separated for the housing.
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The ring gear is what I would consider to be the 'input' and according to your photo it has already been removed.

The 'output' gear that drives the cable need not be removed if there's nothing wrong with it.
 
L.A.B. said:
The ring gear is what I would consider to be the 'input' and according to your photo it has already been removed.

The 'output' gear that drives the cable need not be removed if there's nothing wrong with it.
Click to expand...
I was thinking "ring and pinion" terminology. anyway, the gear that drives the cable has some binding - need to disassemble everything for cleaning and inspection. i'm thinking the failure was due to lack of maintenance. my understanding it needs lubricant every 5000 miles. this thing looks like it's never been serviced. lots of caked and baked crud internally. I think once everything is cleaned it may not as bad as I thought.
 
joe czech said:
the gear that drives the cable has some binding - need to disassemble everything for cleaning and inspection.
Click to expand...

Unless it is bad then I suggest you flush it as best you can and relubricate. The spindle bush and end cap are press-fitted and crimped so best not disturbed if possible.

joe czech said:
i'm thinking the failure was due to lack of maintenance.
Click to expand...

But, it didn't fail, did it? It was the cable that failed as the speedo and therefore the drive gearbox was apparently working normally until the cable dropped out.
The grease fling problem almost certainly the result of the hub plate rubbing on the drive gearbox causing it to overheat which is a fairly common problem.


joe czech said:
my understanding it needs lubricant every 5000 miles. this thing looks like it's never been serviced. lots of caked and baked crud internally.
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Not just lubricated but periodically cleaned otherwise the crud builds up to the point where it impedes the rotation of the ring gear.
 
L.A.B. said:
But, it didn't fail, did it? It was the cable that failed as the speedo and therefore the drive gearbox was apparently working normally until the cable dropped out.
The grease fling problem almost certainly the result of the hub plate rubbing on the drive gearbox causing it to overheat which is a fairly common problem.
Click to expand...

leaning toward the theory that the speedo gearbox locked up and rotated with the rear wheel assembly. as a result, it pulled the end of the cable out. guessing and speculating here....

anyway, thanks for everything. you've been most helpful!
 
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joe czech said:
leaning toward the theory that the speedo gearbox locked up and rotated with the rear wheel assembly. as a result, it pulled the end of the cable out. guessing and speculating here....
Click to expand...

I think you could be right. The speedo gearbox ended up in its normal position!

Your 'Stopped for lunch' picture appears to show the grease fling mark in your other photo is already there on the hub (at roughly the 6 o'clock position) and the speedo gearbox slightly rotated.
 
L.A.B. said:
I think you could be right. The speedo gearbox ended up in its normal position!

Your 'Stopped for lunch' picture appears to show the grease fling mark in your other photo is already there on the hub (at roughly the 6 o'clock position) and the speedo gearbox slightly rotated.
Click to expand...
...good eye!
 
Sorry first look here today/now.
Want a wrap up?
1. Top hat definitely goes in from the outside entirely to centralize the drive, as the Smiths original hole is to big for the commando axle. If it was inside it would NOT centralize anything. The top hat is steel and does not get crushed...
2. Inside potmetal flat, ring is where the internal wheel spacer sits. This potmetal surface becomes crushed and squishes/extrudes outward. This causes the surface to curve and bow. The speedo body therefore is pushed toward the wheel trim cover.
3. Two things happen: speedo felt holder rubs on wheel cover makes heat and also speedo drive gear is bound up between the riveted ring AND the now not flat speedo body and they fight each other to retain the speedo gear and friction eat at the retainer ring and potmetal body...more heat more melted grease.
4. The speedo body is clearly no longer flat. I have a few press tools to flatten the body. This withdraws the seal holder from rubbing the wheel trim cover, and stopps the gear from grinding away at both...
5. Unfortunately the inside surface of the body where the wheel spacer abutts is permanently crushed and thinner than original. It can loose grip and the speedo rotates if the body is not flattened to reduce the grinding effect

6. I also prefer a thinner than grease like gear oil inside these units.
Yes I fixed a few and looked very closely at the failure mode.
7. if you don't flatten the body the ring and pinion no longer line up!!! ugly gear mesh
 
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dynodave said:
Sorry first look here today/now.
Want a wrap up?
1. Top hat definitely goes in from the outside entirely to centralize the drive, as the Smiths original hole is to big for the commando axle. If it was inside it would NOT centralize anything.
Click to expand...

Interesting. I remember when I was assembling my rear wheel that the gearbox hole was an EXACT fit for the axle. So much so that it would not slide on unless held perfectly straight. Next time I have the wheel off I'll take some photos.
 
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