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Re: trip odometer

To: Montgomery Morris <>, <>
Subject: Re: trip odometer
Date: Mon, 10 Sep 2001 02:31:29 -0500
At 11:08 PM 9/9/01 -0700, Montgomery Morris wrote:
>My trip odometer .... just quit last night.  Speedo and odometer still
work. ....

First, see picture here:
The rest of this memo below is repeated on that web page.

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 in turn 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), and 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.  Unfortumately 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 disasemble one of the instruments to
retrieve the remaining good phenolic gear for use to repair the other

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.

You can learn more about the internal workings of your speedometer here:
This tutorial is specifically for the MGA speedometer, but aside from the
drive gear ratios it also applies to the MGB speedometer.


Barney Gaylord
1958 MGA with an attitude

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