So one of the first things I had to do after purchasing the bike (from craigslist) was to fix some things that sorta needed it.  Like the tachometer.  These aren’t made to be taken apart and put back together for some reason.  Note the silver holding ring on the bottom left – it has to be pried off.  Or cut off.  Even if pried off there’s no guarantee you’ll get it back on the same way.  Once apart though the tach was easy to fix.  Mine was missing one of the screws that holds the face to the gear drive behind it so the tach just jumped all over the place.  Fortunately I’ve collected a fair number of computer related screws over the years and was able to find a couple that fit.  Easy fix.  Except putting it back together.  After reassembling it properly (except for noted holding ring) I put the holding ring back on & then silver foil taped it in place.  This has been working since April 16, 2011.