<VV> Speedometer question -- odometer failure

[Jim Simpson]

Typically it's the second worm gear and/or the worm and magnet (parts 4 and
8 respectively in the Clark's blowup).  If you're lucky, it will be the
second worm gear -- they're still available.

I wrote up overhauling a speedometer for CORSA a few years ago; I think it
was in 2010.  It's not hard to work on if you are careful.  If you want to
work on it yourself, I'd recommend taking a look at the article.

In my case, both parts had to be replaced and since the worm & magnet part
is no longer available, I had to go to a specialist shop (Bob's Speedometer
in Michigan) to have it replaced.  They still had some in stock.  Not
cheap, but they did a great job in overhauling the whole unit, cleaning it
all, and recalibrating the speedometer.

After 50+ years, the grease has dried out and the gears wear out.  It's not
something people normally think about lubricating, but given that the parts
are disappearing, it's worth while servicing.  I'd recommend that even if
you find a used one, have it cleaned and lubricated.

The reason your speedometer is still working is that the speedometer needle
is moved by the spinning magnet whereas the odometer is driven by the gear
that picks off the magnet's shaft.  They're essentially independent except
for the gear contact.  Once the gear wears out, the odometer stops even
though the magnet still spins.

Jim Simpson
Group Corvair