The bezels can be unscrewed... but not really while they are in the dash as Ari asked. The goo that Chopshop mentioned is often very tenacious. When the gauge is out of the dash you can invert the gauge and use a tiny screwdriver to just BARELY lift the tabs on the back of the bezel, then use rubber pot-holders to grab the bezel and unscrew it. Once the bezel lock tabs are over the corresponding cut-outs in the case the bezel can be pulled forward and off the gauge.
Chopshop, I've been looking at that tach of yours for a while. I can't read the face and the picture link in your first post comes up as a TINY thumbnail that looks like you might be trying to show the back of the tach. Is this tach an RVI or RVC type? It will be written somewhere on the face. Also, is it positive or negative earth and what is the case diameter?