|The MGA With An Attitude
STRIPPED ODO Drive Gear and REPAIR -- ST-103
A common cause of failure in these odometers is the shearing off of a few teeth from the fiber drive gear. Blake J. Urban (aka Bullwinkle)
This is a partial assembly of a few of the internal working parts of your speedometer, looking in at an angle from the back. Top center of the image there is a small phenolic gear that is driven by a brass single lead worm gear on the speedo input shaft. Each revolution of the input shaft advances the phenolic gear one tooth. This gear drives a ratchet pawl which in turn advances a gear on the odometer drive one tooth for each rotation of this phenolic gear. Things get a little sticky (especially if stored for long time), and the phenolic part gets old and brittle, and at some point a few teeth are sheered off from the phenolic gear (as you can see in this picture). The worm gear then stops driving the phenolic gear, and your odometer stops working.
There are two identical phenolic gears, one to drive the main odometer, and one to drive the trip odometer. It is somewhat more common for the main odometer drive to fail in this manner, as it has two more number wheels that only turn over at 1000 mile and 10,000 mile intervals. Often the main ODO drive will fail when trying to turn over an even 10,000 mile reading, as it is trying to move all of the number wheels at once (or actually two or three of the wheels at a time in sequence), and some of them have not moved for a long time.
If you are careful you can disassemble the instrument enough to replace these parts. Unfortunately the phenolic gears are not currently available as separate replacement parts, so we have to settle for using old parts gleaned from other used instruments. Even a professional instrument rebuilder would have to be using used replacement parts for repair work. So you should be looking for another used speedometer (with the same model number on the face). If you find one in working order you install it in your car. If you find one that has a known failure of one of the the odometer drives, but the other one still works, then you get the instrument very cheap, and you proceed to disassemble one of the instruments to retrieve the remaining good phenolic gear for use to repair the other instrument.
This is all jolly good fun when you succeed and finally have a working odometer again. Keep in mind that different models of speedometer will have different number of teeth on the odo drive gear parts, so you need to find another unit with the same model number on the face of the instrument.
And this from another friend with a different route to repair:
Toothless Odometer Drives Again
The TD speedometer also uses the phenolic gears. The odometer on my TD was also not working. I repaired a phenolic gear that was missing TWO teeth. I applied some epoxy to the areas of the missing teeth. After it dried, I cut new teeth. I did this using a THIN hack saw blade and some jewelers files. The hack saw blade had the set taken out of the teeth by hammering it on an anvil. You may also need to grind the edges so the blade is narrower at the teeth as in a V. I did this repair 34 years ago, and it is still working. I may have also switched the repaired gear from the odometer to the trip meter. It's been a long time, but I believe this gear is just pressed onto a square shaft.