The trip meter Knob is moulded onto the shaft and is also too large to pull through the case hole even after the sealing grommet has been carefully removed. To dismantle the clock for refurbishment I did the following:-
1 Remove the stainless steel sealing band from the case using 3/16" screwdriver blade. This can be levered against the instrument housing a little at a time, slowly working your way round.
2 Remove the two screws found at the back of the clock, these are sealed with green paint.
3 Using leverage from a small screwdriver, separate the two clock halves, removing the bottom casing.
4 Next to the hundreds digit there are two copper clips, remove the one nearest the trip meter reset knob.
5 Remove the circlip also nearest the trip meter reset knob.
6 Pull the reset knob outwards to allow the removal of the nylon gear wheel next to the 10th's digit.
7 Now remove the exposed circlip and also the other clip by the hundreds digit.
8 Make careful notes of where all clips, dogs etc. live! for future re-assembly.
9 Pull the entire shaft/knob out of the casing, the top case can now be removed.
10 Carefully remove the trip meter grommet from the case. A knife may help to break the seal of contact adhesive.
11 Blast clean all rusty outer casings, apply primer and final coat of satin black paint. Bake the paint on for around 30 minutes @ 75C if possible to give better chip resistance.
12 Re-assemble in reverse order. The ring is a tight fit, consider using silicone sealant to make water tight, i.e. on the stainless ring, the two rear screw heads and the trip reset grommet.
13 Re-crimp the stainless steel band using a smooth jawed vice, trying to exert an even pressure all-round.
PS. The tacho is much easier to dismantle, however, the red line is always faded and requires touching up with a matt fluorescent orange paint along with the tip of the RPM indicator needle.NOTE: After refurbishment any marks on the instrument seals will always arouse suspicion to a buyer. Try to keep your work invisible if at all possible.