The speedometer is really two sub assemblies, the first which registers the speed of the vehicle. The second sub assembly, which measures the miles travelled is the odometer.
Apart from the physical size of the container of the speedo, in for example the series one 80 inch landrover it is 60mm in the later 86 and 88inch landrover it is 100mm diameter.
There are a few significant differences between the Mark 11 speedometer fitted to the 80 inch and the Mark 111 fitted to the 86 and 88. Most of the disassembly featured below is relating to the Mark 11
Disassembly and inspection of the odometer is not difficult, but very fiddly, and unless you have a really good reason to do so, I would avoid taking it apart, there is little
that you can do to it, apart from cleaning the components, and provided it is all working I suggest you leave it alone.
But if your heart is set on taking it apart, the only tools you will need are a pair of tweezers a small pair of pliers and a small screwdriver all shown in the photo.
Firstly release the total retaining spring from the left hand post of the stator plate.
This can be slid out.
It is now possible to remove the total counter spindle from the right hand side, do this will lots of care, and carefully keep a record of the key washers and their positions.Gradually
side out the spindle, and soon you should have disassembled the odometer, a picture of the disassembled odometer may help you make sure all the bits are in the right places.
Mark 11 Odometer.
Starting at the left and working to the right hand side:
total retaining spring; total ratchet wheel; total distance bush; steel key washer; brass key washer counter drum; washer; drum; washer; drum; washer; drum; washer; drum; washer cam disc;
coil spring; cupped washer.
All of the above slide on the total counter spindle which has a longitudinal milled groove in it. The counter shaft has a stepped left hand end next to a small groove, it is important to
make sure that the on reassembly the spindle is put back in the right way round.
Now that we have the odometer apart we can have a close look at the total comb spring. If this is damaged badly bent or missing then the odometer will not work. It is possible to drill
out the rivets holding the comb on to the stator plate and try to straighten the comb, but it is an exercise fraught with difficulty. It is not really possible to repair them, they are
subject to corrosion, and can rust quite badly, and in addition bits of debris, moths, dead spiders etc can get under the comb and stop the odometer from working.
Once you have the whole thing apart cleaning should be very gentle, the lettering on the very early drums may have been painted directly on to the drums, the later ones have a plastic
ring which has the numbering on, take care in cleaning. Very gently is best, using cotton buds and washing up liquid. Give the spindle a clean with some wire wool to make sure it is smooth.
Make sure that all the key washers are clean, a very slight glaze of fine oil, perhaps only a suspicion of oil, is allowed on the brass washers but hardly any, as the whole device
is in compression and too much lubrication will leak on to the drum faces and can also cause operation difficulties. On the Mark 11 have a very good look at the drums in particular
the beaks or cams on the edges. On the left hand edge (with the numbers legible) there is only one cam, on the right hand edge there are ten. There are no cams on the Mark111 but a
single locating flag on each drum which fits in to a groove in the stator plate.
Take a look at the ratchet wheel it is made from brass in the Mark 11, and is much smaller and steel in the Mark 111. The Mark 11 should have the number of teeth stamped on it,
usually 60 Teeth, ratchet tooth numbers will be important later.
There are special tools and assembly devices that will help, but as most people will not have them I have tried to describe assembly, not using the special tools.
Once every thing is clean and dry we can make a start on reassembly. I start by inserting the spindle from the right hand side and build from the right to the left. Starting with
the cupped washer, then the spring etc etc.
It is very fiddly to get the parts back on to the spindle in the right order so take lots of care. I use a pair of tweezers to help.
After a while I turn the stator plate on its side, it helps me to reassemble, keeping a bit of pressure on the ‘multi layered sandwich’ as you go.
The ratchet wheel can go in the wrong way round have a look at the photos and get the ratchet teeth pointed forward.
Slide the total retaining spring back in and you should not have any bits left over, if you have, you have made a mistake.
You will find that you have assembled the odometer with the numbers all over the place. It is very easy to re set them correctly, slide a cut down business card between the drum
wheels and the comb spring, take care and when doing this, but you will find it goes in with a tiny bit of resistance, when it is in the card keeps the comb from the beaks on the
drums and allows you to turn the drums and get to the desired mileage.
I suppose the unscrupulous will now be able to ‘clock’ a speedo, but as we are dealing with cars that are usually quite old, I do not think this should be a problem. However, I believe
that to ‘clock’, that is to reset a speedometer so as to show a lower mileage than the true mileage is an offence, so, don’t do it!
The drive from the worm wheel was originally a ‘tufnol’ gear usually with 25 teeth on it which is fitted to a shaft on which has an ‘eccentric throw’ on this is fitted the ratchet pawl,
there are a number of different ratchet pawl designs and the Mark 11 and Mark 111 are different.
The various marks of speedometers
The Smiths Jaeger ‘Nemag’ types. This pattern of speedometer certainly originated well before mid 1930’s. This pattern of speedometer was fitted to very many British cars and light
commercials from the mid thirties to the late sixties.
In my very limited experience, I have learnt that the more that you research something, the more you find out, that what you thought you knew at the start, you are now not so sure is correct.
Such is the case with the on going speedo research. Looking at the Smiths drawings that I have got I can be pretty sure that the speedometer fitted to the 86 and the 88 inch Landrover
are of the Mark 111 pattern, these are fitted in the 100 mm housing. In the photograph the stator plate on the right hand side is a mark 111 the one on the left is probably a mark 11.
The obvious difference in the plate from above is that the mark 111 has an additional section of plate on the top right hand side this is where the trip mechanism would have been fitted.
Both the stator plates have the trip return spring hook but in the Mark11 stator plate the additional fixing element is not present.
Looking at the plates from underneath, the mark 111 being on the right and the mark 11 on the left we can see the major difference.
The earlier speedometer had a comb and cam mechanism for the odometer drive this is seen by the two rivets on the lower part of the plate. The Mark 111 had a different drive system
for the odometer and has a slot in the stator plate.
There are other differences, holes drilled to accept the trip mechanism and its supporting linkages. The two stator plates are broadly interchangeable. If the extra section of the
Mark 111 is removed in my experience the later Mark 111 can be fitted in the earlier 60mm diameter housing but it will require a fair bit of fettling as the stator plate of the
Mark 111 is a slightly large diameter. The fit for the 60mm speedo housing is very snug.
I have referred to the earlier speedometer as a Mark 11 and here I confess I am not too sure, I believe that there may well have been an even earlier system a Mark 1 which although
had a comb and cam odometer mechanism drive for the odometer did not have any extra drillings to allow for a trip mechanism to be fitted. I have found what I think may well be a Mark 1
speedometer which is fitted to the Austin Seven and other cars of the period. However I do not know if Smiths ever referred to it as the Mark 1.
The photograph shows the Mark 111 mazac casting on the right hand side and the earlier Mark 11 on the left hand side the differences are small but obvious, the later casting is much
squarer in appearance and has extra, both tapped and un tapped holes in the body.
The mazac castings do have some other identifying number on them raised in the castings.
On the stator plate of most of the mark 111 ‘s I have seen there are usually written in ink 403.34 or 403.37. On the earlier comb and cam stator plates the numbers 632 7, 602 7, 60 7.
I am still puzzling this lettering system out, I had hoped that the numbers referred to the gearwheel numbers or the ratchet wheel numbers, as some have suggested sadly this I have not found to be true.
What I am calling the Mark 1 speedo has some major differences to the later post war Speedos. and to some extent may well be of only passing interest to readers.
At first glance there does not seem to be many differences but the stator plate is unlike the other marks being undrilled and is held on to the mazac casting with only two screws.
The odometer is the same as the mark 11 being comb and cam. The dial is also different having the 30mph outlined in Yellow
On the dial the instrument is made by Smiths and not Jaeger as on the Mark 11 and 111. I believe the speedometer photographed is from an Austin 7.
The reverse of the speedo showing the mazac casting it is a bit lighter than the mark 11 and only has two fitting lugs for the stator plate. The screws for the keyhole washer are fitted
from the back and not the front as in the mark 11 and 111. There are two unidentified holes un-tapped but counter sunk on the right hand side, which may possibly have been for a trip.
The magnet assembly is round as opposed to more commonly in the Mark 11 and 111, a hexagonal based design. The odometer wheel has 20 teeth and is ‘tufnol’ found in the Mark 11 but more
commonly plastic in the Mark 111.
The operation of the Mark 1 odometer.
The drive to the odometer is taken from the brass worm gear which is driven from the speedometer cable. The small, originally tufnol gear, usually 25teeth is pressed on to a shaft,
one end of which has an offset peg. As the shaft revolves, a spring loaded ratchet pawl, impulses a ratchet wheel, usually brass with 65teeth. The ratchet wheel is keyed onto the grooved shaft.
The shaft transmits the drive from the left hand side of the odometer to the right hand side to the tenths wheel.
At all times as the car moves forward the shaft is revolving. The number drums do not have protruding key element and in fact would be free to revolve on the shaft.
They are held static by the beaks on their edges. The drive is transmitted from the shaft through the small brass key washers. The washers and the drums are held together
by spring pressure. When the tenths wheel gets round to past the nine mark, a beak on the right hand edge of the drum pushes the comb fingers down and allow the next left
hand wheel the chance to revolve one space. The finger then re locks the drum wheel.
The complete operation of the odometer is built up this way. Without the right hand cam beak unlocking the comb finger the left hand adjacent drum is unable to move, as soon
as the beak releases the comb finger the drum can revolve a tenth of a turn.
Problems during operation are usually down to:
1) The tufnol gear teeth stripping, usually caused as the steel ratchet shaft seizing in the mazac casting.
2) Erratic operation of the odometer may well be caused by dirt in the combs, or broken comb fingers.
3) Other mechanical damage broken springs, sheared teeth, will also cause problems.
4) Too much oil getting into the speedo head will also cause problems, preventing the brass key washers transmitting the drive to the drum wheels.
I do not recommend spraying the inside of the speedo head with any lubrication or ‘freeing off’ treatment.