Classic/Antique Car Repair: Speedometer Cable, speedometer cable, moss motors

Question
I know you mention your expertise in MG's dates from 1956, and mine is a 1952 TD, but maybe the question is generic enough.  

My speedometer stopped working recently, due to a failed cable.  The cable was broken close to the gauge end.  I purchased a new original style cable from Moss Motors, and the installation seemed easy enough.  The new cable broke close to the transmission fitting as soon as the car moved.  The speedometer is not frozen.  Using the broken end of the cable, I inserted it into the speedometer spigot and twirled it with my fingers. It turns freely, and the needle moves appropriately.  There were no kinks in the cable when installed.  Do you have any idea what could be wrong?  Is there a trick to this seemingly simple installation?  I got another cable, but I'm hesitant to install it until I know what went wrong.  Any help will be greatly appreciated!
Carlos Diaz

Answer
Hi Carlos,
That is strange so I would do it this way, Jack one rear wheel off of the ground. Look closely at the drive on the trans. while someone rotates the raised rear wheel in the forward direction. Note that the drive does rotate. Now check with a piece of the old cable to see how deep the cable goes into the drive. compare that with how much cable sticks out of the new cable housing. This is to confirm that there is not an end force on the cable. If it is OK meaning there is no end load on the cable when it is installed. If it measures ok install it on the trans. drive. Now rotate the wheel again and watch the speedo end of the cable rotate and note how much of the cable sticks out of the cable housing. Now with your old piece of cable check to see how much cable will slide into the speedo. Check that measurement against how much cable is sticking out of the housing to see that there is not going to be an end force on the cable when it is installed. Remove the cable from the trans. mount and install the cable onto the speedo. By hand at the trans end see if you can operate the speedo. I don't remember if the TD cable had a flange on the cable itself or not but if it does you need to confirm that tightening either end does not clamp down on that flange (if it has one) thus locking the cable solid.
This is the procedure used to add a speedo cable to a mismatched speedo/trans. when a conversion is done.
The only thing that can do what is happening to your cable is either an end load on the inner cable or a clamping of a cable flange when installed.
I hope this helps.
Howard