Took some time last night and took apart the tach on my D17 series 4.  It seems everyone has the white back tachs for sale but I havent seen any black face gauges for sale like the one I have.  I'm not the most careful patient person so I didn't have high hopes on taking things apart and not having anything ruined but I figured it's broke anyways so might as well see how it ticks. I used a small knife and a screwdriver to bend the bezel piece backside up and got  the glass off in one piece without braking anythhing.  Then i got what was left of the needle which was just the center as each pointer had broken off over the years.  Still nothing broke.  Got it all torn down to find that one of the worm gears in the very back of the tach was worn and chipped apart, hence why the tach was turning, but the hourmeter wouldn't click over.  All that work and I must say I was proud of myself and it's junk anyways.  Does anyone sell parts for the inside of these tachs or do I have to bite the bullet and just buy a new one.  All I really need is the one worm gear and a new needle and it would be back in business.    Or do any of the vendors on here sell the tachs that have the black background, not the white background.  Thanks for any help.