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Tachometer Needle Wraps Around
#794802 01/04/20 11:32 pm
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1967 BSA Lightning Grey Smiths Gauges. Tach is rebuilt (by me) due to this problem. Tach works smooth when driven by drill, etc. Works smooth and accurate when the cable is installed to the rear of the case but the keeper is NOT screwed on. If I start to tighten the cable keeper the gauge picks up RPMs and a needle waggle even though actual RPM is constant. If I tighten the keeper fully, the needle wraps around.

Best guess is that the squared portion of the cable is too long and that tightening the cable forces it into a bind. Before I reach for the cutting wheel to shorten it, are there any other suggestions or thoughts on what my issue might be?


Last edited by Redshirt; 01/04/20 11:32 pm.

Redshirt

'67 BSA Lightning--A collection of Helicoils held together by matching numbers
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Re: Tachometer Needle Wraps Around
Redshirt #794803 01/04/20 11:35 pm
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Cable is too long


Bill
1974 Norton Commando
1966 Lightning
1965 Lightning Rocket
1966 Norton Atlas
1967 Norton Atlas
1948 Panhead
Re: Tachometer Needle Wraps Around
Beach #794832 01/05/20 10:04 am
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Originally Posted by Beach
Cable is too long


Plus 1, there is a specific length is can not be beyond. I guess much shorter and it would jump the square also. Sadly I don’t have this information to hand.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Re: Tachometer Needle Wraps Around
Redshirt #794885 01/05/20 7:47 pm
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yep + 1 on cable too long ...........instead of cutting the cable you might be able to try various thickness spacers on the end of the inner effectively shorting the squared bit sticking out ...i cant tell u what the square length should be but i know its critical

the tricky thing about just lopping the end off the square bit is that it tends to kind of unwrap and you get a frayed end but Im sure there is a way around that ........solder maybe?


"There's the way it ought to be and there's the way it is" (Sgt Barnes)
Re: Tachometer Needle Wraps Around
Ignoramus #794930 01/06/20 2:30 am
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Originally Posted by Allan Gill
Originally Posted by Beach
Cable is too long


Plus 1, there is a specific length is can not be beyond. I guess much shorter and it would jump the square also. Sadly I don’t have this information to hand.

Originally Posted by Ignoramus
yep + 1 on cable too long ...........instead of cutting the cable you might be able to try various thickness spacers on the end of the inner effectively shorting the squared bit sticking out ...i cant tell u what the square length should be but i know its critical

the tricky thing about just lopping the end off the square bit is that it tends to kind of unwrap and you get a frayed end but Im sure there is a way around that ........solder maybe?


Thanks Allen and Ignoramus,

I'd hoped to go at it again today but the other vehicles took all the time. My calipers have a depth gauge so I can check out how long is too long for the square section. Ignoramus, good tip on the unravel risk. I have this awesome low-temp solder powder in liquid flux that should be magic for this issue. Just dip and torch. Puts solder into the most the most resistant of places and metals. I will post an update this week.


Redshirt

'67 BSA Lightning--A collection of Helicoils held together by matching numbers
Re: Tachometer Needle Wraps Around
Redshirt #794942 01/06/20 5:14 am
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Chronometric and magnetic inner cables have a slightly different “protrusion” beyond the outer. It’s not much but it’s significant. Google that.

I have found that on some cables there is a nylon or brass ferrule on the inner that can shift up and down the cable.

Otherwise...spacers


'51 C11 in a '54 C10L frame. Back on the road...
'70 Triumph Trophy 500. Next on the bench for a refresh!
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Re: Tachometer Needle Wraps Around
Redshirt #794989 01/06/20 3:31 pm
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You could put an oring of a similar size to the ID of the knerled nut so it keeps the cable outer further away from the instrument. you could then fit the end of the cable with the ferule fitted to the other end stopping it riding further up the cable... Just a thought


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Re: Tachometer Needle Wraps Around
Redshirt #803330 03/29/20 9:52 pm
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Sorry to take so long to get back, but I just got around to what I consider a satisfactory repair yesterday and tested today. Yes, the cable was too long. I cut off as much of the lower end as possible, but the press fit brass collet at the top was still too tall and bound on the tach fitting. I needed just 1/8 of an inch more so I clamped the cable in the vise, placed my smallest socket over the cable to rest on the brass fitting. I them hammered on the socket the minimum necessary to slide the collet down the cable. It worked. No bind and smooth (ish) tach function. The joy of repro parts . . . . Thanks everyone for chiming in.

Redshirt


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'67 BSA Lightning--A collection of Helicoils held together by matching numbers
Re: Tachometer Needle Wraps Around
Redshirt #803385 03/30/20 5:57 am
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i had a the same problem with new uk made tacho cable some years ago (VENHILL) the nature of the cable doesnt like bench grinders. i used a 60 grit garnet sanding disc in a 4'' high speed angle driver to relieve the excess a little at a time till the housing flange would rest easily against the tacho housing


Moderated by  Allan G, Jon W. Whitley 

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