If the CB900 instruments are basically the same as the CB750 ones I have a trip reset knob, it's the type secured by a tiny crosshead screw. There are other alternatives if it does not matter too much that it's not original, I've re-machined radio knobs in the past. Then there's tube caps, toothpaste ones are white but some products have back ones. Certainly ideal for a 400f ones that I get on a regular basis to restore, even more so following a 400f feature in the May Classic Motorcycle Magazine. As for painting, satin black I assume, it's better that they are taken apart for re-paint but if they are not too bad it can be done assembled. A decent job can be made with a rattle can. The best can to use comes from Halfords, seems to provide a better Finnish.
