[TR] Speedo and Tach Cable Problems, Part 2

[Rick]

Yesterday, Mike wrote

 

"There is apparentally only one mfgr of all the speedo cables we get  here
in 

the US. Margaret has seen failures of  her rebuilt speedos because  the
inner 

portion was too long. The new cables were being made too long.  They should
be 

 3/8 (Check this-I think its 3/8 -but its in my shop  manual). 

 

She campaigned with all the suppliers to get the cables made correctly
until 

she finally cornered the manufacturer. She told him about all the  speedos 

and tachs which were getting broken because the inner portion was too  long.
He 

laughed and told her she shouldn't complain, it was good for her  business.

I used a grinding wheel to shorten mine, then a fine file to dress the
edge.

 

Mike Moore"

 

Mike,

 

With all respect to Margaret and the folks at MOMA, the free length of the
cables (differential length between inner and outer components) is not the
problem. When I first tried to solve this problem last year, after smoking
my tach using a new Moss cable, I realized that the inner cable was
compressed and simply ground off the length to make it work. The result was
an unraveled cable at the ground-off end. I made a 0.125" square
cross-section pig and inserted it into the dizzy (I was addressing the tach
problem at the time) and found that it inserted an inch or so, more than
enough to accommodate the entire pressed end of the cable. At this point my
mental light-bulb finally illuminated (it's a bit dimmer these days and it
takes me longer to arrive at the obvious) and I mic'ed the cable ends to
find the root problem in the oversized cross-section dimensions.

 

I'm belaboring this point a bit because the manufacturer of the cable knows
perfectly well that the cable, from a free-length point of view, is in spec.
Attempting to fix the cable thickness problem by lopping off several mm of
free length is a kluge that will ultimately fail, and does nor force the
manufacturer to fix the true dimension problem.

 

I applied the same technique to a new speedo cable and was gratified to find
that the persistent +/- 5 mph wobble in the speedo disappeared. It's now as
steady as the speedo in my Dodge pickup (Dodge pickups are our gold
standard, right?)

 

Rick in Seattle

 

1960 TR3A

1970 BSA 441 Victor

1975  Norton 850 Commando Mk3