Date: Sat, 9 Mar 2002 15:36:59 -0500
From: Ron Thompson <rtetetet1@FUSE.NET>
Subject: Re: Speedo Repair
Ian don't be afraid of those Smith's gauges. They are easy to fix really.
The link you posted goes no where for me but this one I attached will give
you a brief overview.
I have used the screw driver method to get the bezel open but it is slow and
usually trashes the bezel. I am working on a tool for my lathe to do this. I
find the biggest problem to be dirt, dried grease and occasionally a nick in
the aluminum disc that causes erratic slow speed readings. I recently
repaired a speedo that the odometer had vibrated the axels out and chewed
the case to the point that it could not be captured any more. I mare delrin
bearing blocks that were held in by the rubber shock mounts.
The calibrating adjustment is easy to do, just make sure you lock the
adjusting screw down after you are finished. But take the unit as far apart
as you dare and clean it out of all old dirt and grease first.
NOTE: for those of you who have gauges that just seem stuck, before you
break them open, try using a small wire to flick the disc or needle through
the light hole first. I've opened a bunch that were just fine except that
some grease had stuck the needle. Had to do the whole glass and bezel
process just to unstick the needle. Yes I did the whole clean up thing too
The trick to calibrating is to have a rotating source that you can use that
you know the RPM. If you have other gauges that you can compare to, a drill
works fine with an old stub of cable in it. If you can't find a source with
given RPM that you need, if you know the amount you instruments are off, you
can use a drill at high to see where it reads before you adjust it, then
back it off to where you need to be.
If you are looking to get dead on, it will take some math. The speedo math
is like this. But first you need these.
Rotations of the rear wheel per mile. ( R)
You'll have to find the actual circumference of your rear wheel. divide
5280' by the circumference.
How many rotations of the wheel to rotate the speedo drive on rotation. (V)
The speedo reduction ratio. i.e. 3:1 or 4:1
R / V = how many rotations the cable makes per mile (SD)
So SD / speedo ratio (3 if 3 to 1 or 4 if 4 to 1) will equal the RPM per
mile per hour the speedo needs to turn to give you the proper speed reading.
So if your rear wheel has to turn 769.7 times per mile. (19" wheel with
tire) and you have 10:1 ratio speedo drive it would look like this;
789.7/10 = 76.97 so your cable spins 79.67 times per mile.
If you have a 4:1 ratio speedo, 79.67 / 4 = 19.242 RPM per mile per hour.
So if you drill turns 1000 RPM / 19.242 = 51.96 MPH indicated speed on
Now for the bezels. I buy new ones from one of my favorite Brit dealers,
last ones came from Art X. I chuck the gauge case in my lathe, stack the
shade ring, gasket, glass and bezel on the case and use a wood circle to
hold it in place with the tail stock center. With an old bearing tipped
router bit in the tool post, I roll crimp the bezel around the lip of the
case. By gradually rolling the lip with the bearing it goes down nicely.
If you have to do yours by hand, wood block and hammer, go slow. You are
forming metal and it don't like major shape changes without deforming.
Don't use solvent based cleaners in the speedo or they eat the paint on the
ODO and the plastic parts too. Silicone spray lube, the stuff that dries
works well. I use silicone grease on the internals. (*Super Lube*) Don't use
Alcohol on Chronometric units, the ODO is a decal.
Be extra careful of the face of the gauge. It is flat paint and will scratch
or take oil stains easily. If you have to pry the needle off with a tool,
you must put something under to tool on the face plate to keep from marring
Paint the inside of the gauge white before you re-assemble, helps the gauge
light better. You can also buy fluorescent orange paint at art supply stores
to perk up the color of the needle. I have been playing with putting a plexi
ring under the shade ring to light the face better. Especially that neon
clear stuff. LEDs too.
Do this on a clean table with plenty of light.
Have fun, move slow
----- Original Message -----
From: "Ian Bardsley" <Ian_Bardsley@TELUS.NET>
Sent: Saturday, March 09, 2002 11:29 AM
Subject: Speedo Repair
> Having exhausted this years Commando repair budget on an unplannedfront-end
re-build, I am now contemplating taking a run at fixing the speedo and tach
myself. (..... I can hear you doubters snickering). Both instruments
> demonstrate the same fault - they read high, which I think could be due to
> cable lube getting onto the magnet/drum mechanism.
> Anway, I dug out Mike Taglieri's 2 part epistle on speedo repair as a
> reference. I also trolled the web and came across this interesting speedo
> repair manual:
> It covers Smiths and Jaeger magnetic instruments in a generic way. There
> numerous full colour pictures and detailed instructions.
> There a couple of useful post in the archive on bezel replacement.
> Does anyone have any other useful information before I dig-in.
> Ian 75 MK III