Cleaning the magnetic Smith odometer (Part 2)

Ed’s note: Before we proceed to Part 2, Mike Leadbeater commented on Part 1 of the article as follows:

Thanks for the excellent advice. My TD’s speedo, as well as being a mile (sorry) out of calibration due to fitting a Ford ‘box and 4.55 axle ratio, has a greatly oscillating arm. The mechanism, having done 74k miles, is greatly worn. I have detected a lot of end float, maybe 2mm or so, in the aluminium cup, due, I believe, to wear on the outer cup spindle bearing which is basically a pointed needle. As the cup floats back and forth, the torque it receives from the rotating magnet will vary, causing this wavering.

Do you agree?

I am considering making a repair of this bearing, which looks fiddly.

Another issue is the new cable, acting I guess as an Archimedean Screw, brings oil up from the gearbox, which then clogs the magnet and cup, causing excess drag and making the arm to wildly fluctuate.

Any suggestions on how to cure this problem?

Laurent replied as follows:

For sure, end float of 2mm is way too big. You’re right. It is impossible to have a constant magnetic coupling. You first have to fix this. There is a slotted screw just behind the dial, close to the needle shaft. You can adjust the end play with this screw. See figure 17 of Anthony’s paper. Not sure it will be enough. Best solution to prevent oil from the cable is to fit Doug Pelton’s mod for speedo pinion Also described in TTT2 N°1. You can also fit a speedo gearbox suited for your rear end ratio.

Now to Part 2…….

After the delicate operation about speedometer and odometer separation described in the last article, we will now see the odometer unit. Not as fragile, but many small parts are involved in the odometer wheels.


Remove the pin that fixes the reset command shaft to the gear. Mine is only a lock wire. Slide the entire command shaft with its end plate toward the trip wheels, releasing the gear and the return spring.

Both wheels assemblies are similar. On figure 1, spot on the hole on the yoke of the trip wheels, close to the gear. By pushing the tip of the locking clip through that hole you can unlock the clip and extract it.

Figure 1

That clip locks the shaft of the wheel assembly. Now you can extract the shaft either way but before, take pictures, take notes.

Figure 2

Here is what you get (main odometer)

Figure 3

How does it work? See the picture with all the elements on the table. The ratchet wheel (on the right), and all the brass washers are keyed to the shaft. The number wheels have no key and are thus free to move around the shaft. But they are pressed against the brass washers by the end spring (on the left). So, when the ratchet wheel is rotated by the pawl, the brass washers are driven and then rotate the number wheels. All the number wheels?  No. Each number wheel is maintained in its position by a U spring located on the casing. So, the brass washers rotate but the number wheels don’t.  All?  No. When a number wheel displays a 9 it unlocks its left-hand side neighbour by pushing the corresponding U spring down. All other wheels are maintained in their position whilst the shaft rotate 1/10th of a turn. The wheel showing 9 will show 0 and the left-hand side wheel will increase its display by one. The U spring is then released and all wheels are locked once again. The rightmost wheel is never locked. Ingenious system, isn’t it? You also guess that pushing down the desired U spring with a small screwdriver allows to position manually one wheel to any number.

Figure 4

Figure 5

On this picture, you can see the set of U springs that maintain the wheels unless the previous wheel shows a 9.

Figure 6

On this picture, you can see the unlock tooth of the wheel that progressively pushes down the U spring to unlock the next wheel when it displays a 9. The wheel of this picture is special. This is the first wheel of the main odometer which corresponds to 1/10th of a mile. Actually, it is a flat disc wheel that does not show any number but just rotates so as to unlock the mile wheel once per turn. The main odometer as the trip odometer accounts for 1/10th miles but the wheel of the main is just a flat disc.

Now you can clean everything. Be very cautious when cleaning the numbers. I would even advise to not clean them unless they are very dirty. Remove any old grease gunk and dust on the sides of the number wheels where they rub with the brass washers.


Insert all parts one by one on the shaft as you put it back into the yoke. There is a recess on one end of the shaft. This is for the locking clip. It should be oriented towards the ratchet wheel. Start with the spring for the main and with the ratchet wheel for the trip odometer. Smear a thin film of grease between each wheel and each washer.

Don’t forget grease on both sides of the ratchet wheel.

There are many parts so many ways of assembling the wrong way. The document of Anthony Rhodes available on the web shows a diagram of the assembly. Unfortunately, it does not match my odometer. More than this, the wheels do not appear correctly centred in the windows of the dial. I suppose that assembly can be different depending of the windows of the dial showing the number wheels. This is related to the odometer model.

Here is what I found on mine. This works, but seems quite strange. Look also at your pictures and your notes.

Isn’t a cup washer missing next to the spring of the trip odometer?

Isn’t a keyed brass washer missing between the 1/10M disk wheel and the spring for the main odometer?

Isn’t a keyed brass washer missing between the steel washer and the leftmost number wheel of the trip odometer?

When all wheels and washers are inserted, the shaft is protruding on both sides of the yoke. You can now insert the locking clip. With a small screw driver you can splay the tab of the clip that pinches the ratchet wheel. Finally, check that the locking tab of the clip is in the hole of the yoke as shown in Figure 1.

When both, trip and main are reassembled, it’s time to position the pawls on the ratchet wheels and to attach the springs. They can be a nightmare to install. Once more, beware those tiny springs that tend to jump out of the instrument. Work on a wide table covered with a white sheet of paper. I prefer to first attach them to the yoke and then to the pawl.

This ends this series of articles about the clever mechanism designed by Smith. Install the instrument on your car and enjoy. Enjoy but test, check and make measurement for accuracy as I will come back for calibration tips.

Ed’s note:

The calibration article is scheduled to appear in the August issue of TTT 2.