dirty film on inside of speedo/tach glass...

just curious - why?

I have two sets of gauges - originals and rebuilt/reconditioned smiths instruments - both with a dirty film on the INSIDE glass. I replaced and repaired my original speedometer, which BTW was fairly easy - at least easy enough that I was able to do it. bought a bezel/glass kit on-line, and hand crimped the bezel per instructions in Graham Blighe's book, Magnetic Speedometer Repair. BTW, good, informative technical reading. so, back to my question - how do the instruments get the dirty film on the INSIDE? the only thing I can figure is over time, vapors from any cable lube migrating up and into the instrument housing - ??? can't see how anything gets in there from the topside with the rubber seals on both sides of the glass. anyone have a solution, or is mine a somewhat isolated condition? any preventative fixes?

couple, show and tell pics -





while I was at it, it gave me the opportunity to set and adjust the correct mileage (sum of the two odometers). one thing added - I gave the inside housing a fresh coat of flat white (hobby shop rattle can) and I found a matte sky blue (testors) that was a close match to the blue used on the inside housing. if anything, it should brighten up the night-time viewing.

joe c

Maybe some sprayed WD40 inside in an attempt to clean it and left a residue, without a time machine its a guess. The bulb hole can be left open, it's not an airtight seal underneath.

kommando said:

Maybe some sprayed WD40 inside in an attempt to clean it and left a residue, without a time machine its a guess. The bulb hole can be left open, it's not an airtight seal underneath.

Click to expand...

both sets? guess anything's possible. although, can't see how anyone could think WD-40 is a glass cleaner. I would think spraying anything on the inside would screw up something. if this is a common problem, it would be nice if the instruments were designed with a screw-on bezel.

Had a similar issue when I lubed mechanism with wd40. It is not sealed from the back and lube can migrate.

I've seen various instrument glass on different bikes get similar crud build up. I think it is due to condensation on the inside that eventually builds up a deposit, like hard water spots etc.

I got condensation in my speedo only for years. It also had a 'film' when dry. I recently had it rebuilt. I still get condensation.

Yes there's nothing that really seals the inside from outside air so condensation is a common thing. Heat from the lamp will help minimize it and may explain why it seems to occur mostly near lower edge.

Using a good cleaner and maybe glass silicon coating like RainX on underside of glass should help prevent water condensing.