The speedometer on my 76 750 fogs up routinely after sitting in the sun and then being exposed to cooler air when riding. There seems to be a film on the inside of the glass that I'd like to clean. The two halves of the unit seem to be held together by a pressed ring that I apparently have to open up to get apart. Before I go for it, is there something I don't know about taking these gauges apart? Just trying to avoid the usual dope slap...