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In less than a year I have destroyed two perfectly working Smiths Tachometers simply by installing them to my A65T. The first one was working fine for several miles then BAM... The tach cable broke and the instrument froze up never to function again.

More recently, I rebuilt my motor over the winter. Started up fine first time today with a NOS Smiths tach and new tach cable. Everything was going well on its maiden voyage on local streets. Then, 20 minutes into my ride in 3rd gear my back wheel locked up. I grabbed the clutch and thought, oh, oh, the motor's seized. Not so... The tachometer froze once again rendering it useless, and the cable snapped along with it.

The oil pump was rebuilt by Ed V so everything in that area should be ship-shape. The tachometer was working very well until this happened.

What on earth is going on!? This is a real disappointment.

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just a thought to consider,the last drive cable i fitted to my Commando the inner cable was about an 1/8" too long. when i assembled it i noticed i could not get the outer casing pushed flush with the body of the tach.after ensuring that the cables were properly seating in their drives it was apparent that shortening the inner cable was the only way out. thought about fibre washers inside the cable nuts but i didnt have the correct size. its not a great idea to do this,as the individual wires tend to unravel if you use a side cutters. best go is a disc grinder with a 60 0r 80 grit wheel. it seemed to me that the resulting thrust to the mechanism was not a good idea. some thing to check as i cant think of anything else that might cause your problem. im confused with your post though,tach's arent driven by the rear wheel and no cable has the the beans to lock the rear wheel

Last edited by jaycee; 06/15/14 9:35 pm.
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I have heard that too long an inner cable can mess up a gauge.
These are 3:1 gauges, made for A65? NOT 4:1, made for Triumph.


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The current (some or all? ) cables are too long and the square on the ends not properly formed, some work is needed so the cable is the right length and the square can move in and out of both ends cleanly. If you don't don't this then the cable exerts pressure up into the gauge and the bearing inside is destroyed. Sounds like this has happened twice to you.

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Jeez, an expensive lesson to be sure. So how does one know the acceptable length of the cable? What end of the cable do you cut? Is there a tutorial/instructions available somewhere so I don't botch this up again?

Both my tachs were 3:1.

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well common sense would tell you that you should be able to seat the whole affair with gentle hand pressure BEFORE you screw on the nut

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Hi,

Originally Posted by jaycee
common sense would tell you that you should be able to seat the whole affair with gentle hand pressure BEFORE you screw on the nut

Harsh ... and not always a good guide ime.

Many years ago, I was on a European tour when the T160's original speedo. cable broke in France. Next stop was Paris so I made my way to the Triumph dealer and he supplied a Venhill cable. I fitted it and within a few miles, the needle started to flick back and forth before eventually ceasing to work; tipping the speedo. during removal and a quantity of brass filings appeared inside the glass. frown

Luckily, in those days, Speedograph Richfield (who'd taken over from Smiths) had a London shop. Took the speedo. there and they advised the cable problem. Checking very carefully showed that the inner end 'square' was very slightly oversize; just as the outer seated against the thread on the speedo., it went solid. If you didn't know about the potential problem, you wouldn't notice.

Hth.

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And they are not total rash , they can be repaired it is not all that expensive or you can do it yourself.
Google Smiths speedo restorations, speedo / tacho, both are essencially the same inside.


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On the subject of cable length- "The inner should protrude no more than 7/16ths out of the top of the outer when the crimp is pushed back inside the outer as far as it can go. At this point the inner should stick out the bottom of the outer by 5/8. "

He made the point that the 5/8 sticking out all needs to be square, if some of it is still round then it will push the inner up into the speedo/tach.The consequences of the inner protruding too far into the speedo/tach is that it destroys the mechanism.



http://www.members.shaw.ca/randell/Smith/Speedo.html

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Originally Posted by jaycee
well common sense would tell you that you should be able to seat the whole affair with gentle hand pressure BEFORE you screw on the nut


What a Dick!

There was nothing indicating improper fit on this cable. Smooth installation.

Thanks for your explanation and link Kommando. I will not trust a new cable on my next attempt until it is measured.

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I once bought into the cable length thing but now having rebuilt several smiths magnetic speedometers and tachs I believe this to be a myth. The thrust arrangement on the drive is VERY robust and I have rarely seen it fail. Almost every failure I've seen can be traced to lack of lubrication inside the instrument. The plain bearing for the needle is most sensitive. This will be dried out after 40-50 years on NOS instruments and will not last long in any case. As the brass bearing wears out, the needle starts to come off-kilter and the eddy current disc starts to strike the magnet causing the needle to bounce. A fine brass powder usually starts to accumulate in the gauge. If this is ignored, the bounce will get so bad that one day the aluminum disc is grabbed hard enough to break the needle off. While rare, have also seen the main drives/magnet bearing seized due to corrosion and lack of lubrication. This sounds like the problem with your tach.

The rear wheel locking thing makes no sense at all in relation to the tach, though. confused


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Alex, is there any way to lubricate the instruments short of dismantling?

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Alex, this needle twitch and shake is what we always called "Smiths Palsy"

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Originally Posted by Steve Erickson
Alex, is there any way to lubricate the instruments short of dismantling?


Not that I know of. Which is probably why it doesn't happen. Problem is, you want just one drop of oil on the needle shaft bearing. Any more and you risk the oil getting between the eddy current disc and the magnet, which will then also cause the needle to spin.

You CAN lubricate the main drive/magnet bearing by removing the little steel cap from the drive and getting a little bit of light oil to wick between the rotating and stationary portion of the drive. Careful, though, it is possible to get too much in with similar results as stated above.


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Originally Posted by Irish Swede
Alex, this needle twitch and shake is what we always called "Smiths Palsy"


Goes well with the Lucas flicker... whistle


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Originally Posted by Alex
The rear wheel locking thing makes no sense at all in relation to the tach, though. confused


That's what I thought. An anomaly perhaps? I know it doesn't seem logical, but I attributed it to the tach locking up at that precise moment just before the cable snapped. The bike ran fine after the lock up.

So how do I proceed? Perhaps aftermarket! wink Or rebuild the instrument AND check the cable for defects. Thanks for the info.

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Originally Posted by Bola
[quote=Alex]
So how do I proceed? Perhaps aftermarket! wink Or rebuild the instrument AND check the cable for defects. Thanks for the info.


The instrument can undoubtedly be rebuilt. That's what I would do. Don't think you have much of a choice on that. AFAIK, no one makes an aftermarket tach with a 3:1 ratio.

The rear wheel locking up...again I can't see how it would be connected.


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and I always thought the Smiths readings were a move towards standardisation using the same system of rating as for motor oil .... you know ...30/40 etc

I don't know of any aftermarket 3;1 tacho being made


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Hi Alex,

Originally Posted by Alex
I once bought into the cable length thing but now having rebuilt several smiths magnetic speedometers and tachs I believe this to be a myth. The thrust arrangement on the drive is VERY robust and I have rarely seen it fail.

The drive socket on a speedo. or tacho. head isn't bottomless and is made to particular dimensions. Now I check for it, I have seen several Venhill cables where the inner is too long for the outer and/or ends haven't been tapered sufficiently. However "robust" the thrust arrangement is (and opinions appear to vary), I doubt the Smiths designers envisaged the combination of careless/incompetent aftermarket cable makers and certain owners' belief that pliers and vice-grips are actually cable location tools.

Regards,

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Hi,

Originally Posted by Bola
So how do I proceed? Perhaps aftermarket!
Or rebuild the instrument

You might want to check with these guys or the US distributor if there's a suitable sensor/trigger to drive the relevant gauge?

Hth.

Regards,

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Originally Posted by Ignoramus


I don't know of any aftermarket 3;1 tacho being made


They are out there now....I picked one up about 6 months ago. Think it was British Only.

3:1 tacho

[Linked Image]


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Bola,

Did you confirm whether it was the tach cable that caused you to lock up the back wheel? Does not seem likely that there would be enough torque produced by the cable to do that? It would snap first? Reason I ask is that I am in about the same point on a refurbed engine and going to take it out on the road this weekend. I would prefer not to have the same thing happen!
Dave

(If we get our machines running we should meet up at a Tims sometime halfway between us and swap stories!)

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Originally Posted by Dave T
Bola,

Did you confirm whether it was the tach cable that caused you to lock up the back wheel?

(If we get our machines running we should meet up at a Tims sometime halfway between us and swap stories!)


Dave, the back wheel locking up is a total mystery to me. The bike continues to function and start fine (without a working tach) mad

Getting together at Tim's is a plan, however, now I'm chasing a leak from the rear section of my cylinder head gasket because of those damn smaller diameter bolts on the 66 barrel. Back to the machine shop!


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