|The MGA With An Attitude
Speedo & Tach CABLE DRIVE END DIMENSIONS - ST-107
On 09 June 2007, Kris Sorensen in Ontario, Canada, wrote:
"I remember some time ago reading about the protruding length of the cable going into the speedometer being a possible problem if too long. How long should it be? My speedo either reads too high gets stuck or won't go back to zero if the cable is screwed in tight."
I'm sitting here measuring some parts with a disassembled speedometer and a new cable from Moss US. See pictures below.
Speedo cable drive parts
Depth of the cable hole in the input spindle is 17/32-inch from the back face of the threaded housing, so the cable core end should not protrude more than 1/2-inch maximum from the end of the cable housing (1/32-inch shy of the bottom of the hole).
The end of the spindle is recessed 3/32-inch inside the threaded barrel of the instrument. The square drive area is recessed 1/8-inch inside the spindle. The square drive area is just 1/8-inch long (full contact area). That gets down to 11/32-inch of depth from end of the barrel.
Additionally, the end of the cable core is slightly tapered for lead-in for about 3/32-inch, so you need that much more cable protrusion for full engagement in the square drive within the spindle. So for full engagement the cable core should protrude a minimum of 7/16-inch from end of cable housing.
That leaves a fairly narrow range of required cable extension, 7/16 to 1/2-inch from end of cable housing.
Speedo cable engagement length
To add to the confusion, the plastic ferrule on the cable core only protrudes 1/32-inch from the end of the cable housing. That means it can float another 1/16-inch into the barrel before it touches the end of the spindle. That would bring the cable end another 1/16-inch closer to the bottom of the hole. To avoid interference at the bottom, you need that 1/16-inch additional bottom clearance, so the maximum protrusion of the cable has to be reduced by 1/16-inch.
Speedo cable ready to insert
So the answer is, the cable core should protrude 7/16-inch from the end of the cable housing, +/- 1/32-inch. Problem here is, the new cable I have in hand has barely 3/8-inch protrusion from the cable housing, which is not enough by my calculation. I have been playing with various depth of engagement to be sure my measurements and calculations match the real world condition, and it does.
Speedo cable no drive distance
Speedo cable yes drive
With the cable core retracted as far as possible into the cable housing, and the housing held 3/32-inch from the end of the threaded barrel, it fails to drive. When it is screwed up tight it may have only 3/32-inch positive engagement in the square drive. It works as is, but hard to say how long it would continue work before it might wear enough to strip the corners of the square drive.
My idea of a better solution is to hold the cable core in a vice with the plastic ferrule a little above the vice jaws, then use a small tube socket as a driver to tap the plastic piece a little farther down the cable to get the proper 7/16-inch of cable protrusion (beyond end of the cable housing). If you have too much cable end protrusion, you can grind a little off the tip of the cable core.
Speedo cable connected
Addendum, December 11, 2017:
And ten years later I have a note from Nisonger Instruments verifying this dimension. Click for the 155-KB pdf document.
Thanks to R. Michael Baiada for forwarding this document.