MG Car Repair: MGB Speedo not working, nylon gears, internal gears

Question
Hi Barry,
The speedo on my 69 MGB GT is not working and hasn't been since I bought it a couple of months ago (not using it on the road yet). The previous owner said that it needed a new cable and one came with the car - not sure if it is a cable fault or even if the one provided is correct. However, is there a step-by-step method of identifying such a fault, i.e. to make sure it is the cable and not the speedo itself or the gearing? Thanks, Gary.

Answer
It is often the inner cable.  They either come out at one end, or the squared end gets rounded, or it snaps, sometimes part way into one end or the other.    
holding

1) Firstly, check that the knurled nut holding the cable to the speedo is fully done up.  If not, do it up and you may have fixed the problem.  

2) Undo the knurled nut and extract the inner cable.  It is long, covered in dirty oil, and guaranteed to mess up whatever you are wearing, so be prepared with a boiler suit, disposable gloves and a dustbin bag.

3) Clean and inspect the inner cable.  Does it have a kink in it?  Do the ends look square and not snapped off?  Have you merely removed half of it?  You may need to extract the remainder from underneath the car.  

4) If it all looks OK, then put the inner back into the outer and go for a short drive with a passenger.  Does the inner cable rotate as you drive along?  If not, you have real trouble, because it is driven by a pair of nylon gears inside the gearbox.  To replace the internal gears is an engine-out job.  Do not be tempted to try replacing just the easy one, it never works.  If you need to replace these gears, fit a new cable as well, or else the old cable will quickly destroy the new gears.