Several times I've taken apart magnetic-type speedos to clean, lube and
adjust.  Some people might want to try this themselves, so I'll discuss
it here.  I apologize in advance to some of you, because this is going
to be very elementary for the sake of those who may need it.  What I did
recently on Raul's speedo was open the unit, blow out the considerable
dirt, etc., since the last time I worked on it (mid 80's), clean & lube
the geartrain to the odometer, add 600 miles to the total, which elapsed
when my speedo drive was broken in July, and reset the needle slightly
because the unit was reading a little high at highway speeds.   No major
repairs, which might better be left to a pro, but these little repairs
may interest some of you.
 
Here's a summary of how to do them:

<<<WARNING: I used to be a professional camera repairman, and have also
done watches.  I am very comfortable working on tiny,  mechanisms and,
as a friend once remarked, my tool collection contains "some of the
weirdest shit imaginable."  Therefore, telling you how to do this does
not mean that I recommend that everyone on this list SHOULD do it,
particularly since individual parts accidentally broken might be hard to
find.  Use your judgment.  On the other hand, a speedo is much larger
and beefier than a camera or a watch, and not THAT hard to work on.
Someone who can, say, clean and adjust an ammeter or a spring-wound
clock will probably do OK --
indeed, the hardest job is getting the needle off, which is not unlike
removing the hands of a clock.  If you've never touched anything smaller
than a gearbox, however, maybe you should practice on something else.>>>

Do this work on a clean desk rather than a workbench.  A magazine, pad
of paper, blotter, etc., will provide a resilient work surface.  You
also need a pad to make notes on, unless you're working on one so large
you can
write & draw on that.  Ideally, the desk should have a drawer, which
should be held open 4" or so with a clamp, block of wood, etc.  That way
if the unimaginable happens, it'll land in the drawer rather than on the
floor.

TOOLS & MATERIALS
-- set of jeweler's screwdrivers, mostly to use as probes.  When I say
"large" or "small" screwdriver, I mean of these.
-- Q-Tip.  Get the genuine article, with the rolled-paper stems rather
than the wood or plastic.  (generics have shorter fibers, which can come
off, etc.)
-- WD-40, Liquid Wrench or similar; white grease.
-- single-edge razor blade.
-- machine oil (sewing machine oil or gun oil is good.  Not engine oil,
or "smelly" oils like 3-in-1).
-- clear or black silicone; glue (contact cement or airplane glue); nail
polish or other lacquer; talcum powder.
-- black enamel paint, if needed.
-- egg carton (w/o eggs).
-- 2" to 3" plastic or metal cap  (from large aerosol can is good).
-- tweezers ("MM" style watchmaker or hobbyist tweezers preferred [i.e.,
5" long tapering to a point].  Hemostat or VERY fine needle-nose pliers
could also be used.  Medicine-cabinet tweezers are not satisfactory).
-- one or more soft, wooden probes.  Chopsticks are ideal.

STEPS (assuming speedo out of bike and on desk).

1.  Get the bezel loose.  When I first worked on my speedo, I pried it
off all the way around the back, and scarred it quite a bit.  Someday I
may get a new one, but haven't yet.  Others on this list have suggested
more sophisticated ways than I know.

2.  Take off the bezel, glass, various rubber seals, etc.  Pull gently,
because they may be stuck from age.  Lay them out in order, making notes
as to direction, etc, if you plan to reuse them.  There is a black metal
disk on the case above the dial, which may be glued on.  Pry off gently
& evenly
all around the edge with a small screwsdriver.

3.  Removing speedo mechanism [hereafter, "guts"] from speedo case:
there are 3 screws in the back.  Loosen them part-way, then use them,
and the central threaded hub [hereafter, "hub"], to push the guts up
evenly. The screws go through rubber washers [hereafter, "rubbers"], and
there are also three rubber bumpers around the outside of the guts.
These may be stuck to the case from age.  Push the screws alternately to
rock the guts back and forth, and eventually it'll unstick.  (Finger
pressure should be enough -- put that hammer down!)  The screws and
rubbers should go in the first compartment of the egg carton, to keep
them orderly.  Take notes on which side of rubbers faces in, etc.

4. Slowly move the guts up out of the case.  Eventually, you'll have to
remove the screws and push only on the hub.  Be ready to catch the guts
when it comes loose, but hold it by its main casting or the edges of the
dial, lest the needle catch on something.  From now on, you can hold the
guts upright by sitting it in the aerosol cap.  It won't sit flat on the
desk, because various things stick out of the back.  Do not put it
face-down.

REMOVING THE NEEDLE
5.  Taking the needle off is the trickiest job, a bit like taking the
hands off a watch or clock.  To make it easier, put some WD-40 into the
depression at the center of the needle hub and let it sit awhile.

NOTE:  From now on, "putting" oil, grease, glue, etc. means to put some
onto a saucer, scrap of paper, etc., then use one of the smaller
screwdrivers to transfer small amounts.  Under NO circumstances should
you spray WD-40, etc. onto the speedo.  Also, do not use undue force on
any of the following procedures.

6. Before removing the needle, you must figure out where it points at
when put back on, so you can return it to the same place.  Notice the
needle stop, which is a piece of wire.  It's spring loaded, so you can
push it in down into the dial.  Move the needle forward from the stop a
bit, then push the stop in far enough in with the tweezers so the needle
can move backwards OVER the stop; i.e., you will move the needle below
10 mph toward the area that says "MADE IN U.K." (NOTE:  From now on,
when I discuss the needle, "forward" means clockwise, "backward" means
counterclockwise.  "Up" and "down" mean toward or away from the guts).

8.  Now, the needle is not held by a stop, and will move to the position
where the spring no longer exerts a force on it.  Tap the hub gently on
the work surface to help overcome friction, and ensure the needle moves
completely, then note where the needle winds up.  Try this several
times, moving it 1/4" or so one way or the other to see if it moves to a
repeatable spot on the dial when the hub is tapped.  Carefully make
notes about
that spot , because this is where you'll aim the needle when you put it
back on, unless your speedo has been reading consistently low or high,
in which case you'll compensate.

7.  Move the needle forward over the stop, then look inside the guts
which you wiggle it forward and backwards.  You will see an aluminum
disk that turns with the needle.  If you like, spin the inside of the
hub with the chopstick.  You'll see that there's a fat disk connected
directly with the chopstick or cable, right below the thin, aluminum
disk that connects to the shaft on which the needle turns.

8. To remove the needle, turn it up to 30 mph or so so it won't hit the
stop. Then immobilize the shaft by touching the edge of the aluminum
disk with a screwdriver, and gently turn the needle back-and-forth until
it comes loose from the shaft.  Use a fairly large screwdriver on the
disk, so you can hold the blade crosswise to the edge of the disk and it
won't slip off.  NOT MUCH FORCE IS NEEDED.   Hold the needle on both
sides of its hub and as close to the center as possible so you won't
bend it.

9.  Ultimately, the needle should get loose on the shaft, and can be
pulled off.  If you think you need to use too much force, try more WD-40
or Liquid Wrench.  There are more drastic ways to get the needles off,
which I'm not mentioning here.  E-mail me if this way doesn't work.

10.  When the needle is off, put it in the egg carton.  You can cut a
thin wedge of paper, twist it to a point, and use it to clean the WD-40
from inside the hole, then put the needle into the egg carton.

REMOVING THE DIAL

11. The dial is held by two odd little fasteners at 3:00 and 9:00. These
are vaguely like the little metal fingers that hold the light sockets on
the speedo's bottom, but have solid plastic shafts pressed inside to
keep these prongs from vibrating loose.  If you look on the back of the
dial, you will see how the shaft holds the prongs  apart.  The shaft is
probably stuck tight.  Use a probe to push it toward the front of the
dial -- a chopstick works well, but may have to be whittled down (WD 40
may be needed). Once the shaft breaks loose, you can push it out either
from the front or the back.

The front is far more accessible, but be careful not to slip and scar
the dial.

12. Once the shafts are out, the dial pops off easily.  It can be
cleaned
carefully with mild soap and water, if needed, but nothing harsh.  (I
used shampoo and my fingers, blotted the bulk of the water off, then let
it dry under a desk lamp.  VERY LITTLE HEAT or you could melt the
fasteners).
You can repair flaws in the dial with black model airplane enamel, etc.
I find the shaft of a Q-tip, cut at an angle with a razor blade, makes
an excellent micro paint brush for this purpose.

INSIDE
13.  With the dial off, you can see how all the little wheels work
(unless you're opening the speedo because they don't).  Turning the
center of the hub with the chopstick turns the fat disk, which is a
magnet.  This sets up eddy currents in the aluminum disk that drags it
along, but that disk is connected to the coil spring that wants to push
it back, leading to an equilibrium at a given speed, depending on how
fast the hub turns.  Down under the disks is a brass ring gear that
turns a nylon geartrain for the odometer.  If you have a basic speedo,
you'll see another nylon gear down near 6:00 on the guts. This is for
the trip odometer (which some makers who weren't as cheap as Norton
fitted in their speedos).   I hope your speedo needs no work, but at
least blow out dust gently with a rubber ear syringe, and gently pick
off hairs, etc.  The end of the hairspring that does not move is held
with glue, and you may want to put a bit more airplane glue on there,
with extraordinary caution not to get it on the rest of the spring.
[See at end if you do need work here].

14. My odometer works fine, so I merely cleaned visible grime off the
teeth of the big nylon gear, which is turned by the big brass gear and
in turn moves the others.  Then I lightly applied white grease to the
nylon
gear's teeth, again using a Q-tip shaft cut on the diagonal.  ["lightly"
means that if you can see the grease easily when you're done, it wasn't
lightly enough].
 
Some people would not use grease at all on these gears, and I don't use
it on the others, but the big nylon one slides against the brass gear to
a considerable degree.

15. At this stage it's a quickie to change the odometer reading to
correct errors, etc.  I like a visible reminder of the miles Raul and I
have spent together, so I added 650 miles to my speedo to correct for
the period in July when my speedo drive was kaput.  I suppose this could
also be used for less than honest purposes, so if you see bikes being
sold with suspiciously low mileage (or bikes at rallies getting trophies
for suspiciously HIGH mileage), now you know why.

16.  To adjust the numbers, you remove the e-ring from the left side of
the shaft on which they turn [this is also the side where there's quite
a bit of room.  The e-ring on the other side is so tight against the
casting that it would be very difficult to reach.]

17.  If you've never heard of an "e-ring," take a look at it: a dark
metal ring that snaps into a groove on the shaft.  You can either pry
the side opposite the opening away from the shaft with a tiny
scredriver, or
press on the open side with a larger one.  (One could also grab the
thing with small pliers to pull it off -- tweezers are not strong
enough).  When an e-ring comes off, it usually flies across the room, a
good reason to wear eye protection.  In this case, however, the e-ring
should fly no further than the big magnet 1/2" away. Turn the hub center
if you can't find it.  (If you inspect it when it's off the shaft,
you'll see why it's called an e-ring).

18. With the e-ring off, the disks with the numbers on them can be slid
to the left, turned, then slid back.  The left digits are the more
readily moved, which is convenient, since few people would want to
correct an
error on the low digits.  Once the numbers are right, push the disks
back and get the e-ring back in, starting it in the slot with the
tweezers. which can be a royal bitch of a job.  There's a small washer
that will block the groove unless you get it evenly on the shaft.  Also
note that the e-ring has one smooth side and one side with an uneven,
burred edge.  I would face that edge away from the numbers to make it
easier to get it in the groove.

Obviously, if you hold the shaft vertically, gravity will help keep the
disks and washer in place while you struggle with the e-ring. (Note: if
your washer is steel, it may be attracted to the tweezers and
continually jump up out of its groove.  Tweezers and other small objects
can be demagnetized by quickly passing them into and out of the loop
formed by the copper-wire tip of a soldering gun.  Do not shut off the
gun with the object inside, which may remagnetize it).

PUTTING IT BACK TOGETHER
19.  First, push the dial back on, then slide the plastic shafts into
its fasteners with the chopstick.  You can install the needle either
before or after the guts are back in the case:  Tap the hub until the
spring-loaded disk has settled to its rest position, push the needle
firmly on the shaft in the location you previously noted, then move it
clockwise over the spring-loaded stop.

20.  To install the guts in the case, get the internal rubbers in place
the right way [you took notes, remember?].  Brushing all rubber parts
with A LITTLE talcum powder on a Q-tip helps them not to stick (don't
forget the 3
rubber bumper strips on the outside of the guts).  Check that the foam
seal on the back of the guts was not disturbed, then gently wriggle the
guts back into the case.  Note that the 3 holes are NOT symmetrical --
the guts fits only one way.    Once it's on, you can install the outer
rubbers and the screws, then glue on the black metal surround with
airplane glue, contact cement, etc.

21. I do not reassembled the bezel and glass the "proper" way
[crimping, etc.], and I do not reuse the rubber seals.  I just carefully
glue the glass inside the bezel with clear silicone, then use more
silicone to hold the bezel onto the speedo case.  (Let the glass harden
first, so you can scrape off any smeared silicone with the razor
blade).  Someone else on this list can tell you how to do it "right,"
but a leakproof joint is more
important to me than historical accuracy no one sees, and the "right"
way will not be more leakproof than silicone.

TIPS ON ADVANCED STUFF
I haven't had to do serious surgery on a Smiths, so I can't get too
specific [unless someone gives me a dead one to dissect]. The general
approach is as follows: The shaft that holds the gears is held with
rivets, which can be drilled out -- best done with a Dremel tool. The
hairspring system, however, is SO delicate that if you bent or derange
it, you may never get it right again.  Therefore, if you drill or
otherwise do serious work on the odometer, cover the spring mechanism
with a small bottle cap, etc., then wrap all of it with masking tape or
similar -- stuffing tissue in large cavities first, if it seems
appropriate -- so shavings and crud will not get in.  [This works very
well: I have on occasion actually sawed body castings of cameras without
removing far more delicate than this by protecting them with masking
tape.  If I had to disassemble the hairspring and had never done it, I'd
practice on a wind-up clock -- very similar.
 
Odometer parts seldom break, though I once had a plastic gearwheel in a
VW speedo that sheared off its shaft, and I was able to epoxy it in
place. Take very careful notes with drawings, etc. for any major
surgery, and have many, many egg cartons).

Best of luck, and keep the speedo side up.