My tach had the black cover that goes on the indicator needle sitting at the bottom of the dial obstructing the temperature indicator since I bought it, not to mention the clear part needed cleaning on the inside.
I finally decided to fix it or get another tach. It was not designed to come apart without messing up the little metal band the secures the front cover.
I didn't want the band to look any different afterwards so when I got to the point when taking the tach off that it could be turned upside down the trusty dremel with a cutoff disk came into play. The rear chrome cover was pulled back along with the rubber o ring. On the bottom of the dial I cut crosswise, only the lip that the metal band secures together and of course the metal band itself. It took a small flathead scewdriver to get it started coming off, starting from the cut then prying along the side all the way around. This was done slowly so as not to distort its shape. When the band came off I had to use the screwdriver between the cover and main body to get it started separating. It came apart! After using a small amount of plastic model glue to reattach the black cover to the needle and cleaning the inside of cover I put it back together. It took a little time to get the band started back on the lip around the body but I believe it would hold itself on by itself. I insured the cut in the band would be on the bottom when reinstalled. A small amount of plastic model glue under each end of cut metal band and when it was dry, after re-installation a dab of silver model paint. It looks good and you don't notice it at all. I am a happy camper!