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The speedometers on these cars are, despite popular opinion, very simple devices. They consist of a magnetic disk, which is spun by the speedo cable, sitting inside an aluminium cup, which has the speeod needle attached to one of its pivots. That's all there is to it, apart from a hairspring which returns the needle to 0 when you're not moving!In the photo, the magnet and cup are enclosed in the aluminium housing to the right. The shaft through the centre carries the cup at it's right, the needle at it's left (in front of the dial) and the hairspring just behind the dial (visible as a faint brass coloured line between the dial and it's bracket)
As long as you work carefully, and keep everything clean, there's no reason at all why you can't service one to cure sticking or "bouncy" readings (by far the most common problems). Both are usually caused by the tiny amounts of oil in the cup's pivots going sticky with age - front pivot will tend to make the needle stick, rear pivot (which is in the centre of the spinning magnet) will tend to make the reading "bounce" as the sticky pivot tries to drive the needle higher than the true speed.
Calibration needs equipment outside the normal DIY workshop. It's effected by changing the length or strength of the hairspring but, if the work is done carefully, original factory calibration shouldn't be affected by this work.
The first (and only) problem in this job is actually gaining access to the insides. As with many of this type of instrument, the front is sealed with a rolled-over, chrome plated, bezel. This has to be removed by carefully working round the rolled edge with a screwdriver until it's loose enough to lift off. Take care with this - the base metal is brass and if you're too enthusiastic you risk cracking it. On bezels like this I tend to open about 3/4 of the rolling and try to leave the rest. If it will come off like that then refitting securely is a lot easier!
Once the bezel, glass, and internal bezel is removed you can undo the two screws on the back and, carefully supporting by the dial edge, lift the casing away. While undoing the screws, hold the cable connector to stop the workings dropping - if they land on the pointer they may damage the pivot.
