Originally Posted by bpeckm
I simply used some black silicone caulk to fill the hole in the rubber grommet. Later front-covers (after the electronic tachs came in ('+78?)) don't have the notch for the rubber grommet, if you would want to go that way. I also used some high-melting-point grease, like wheel-bearing grease, to plug the hole inside the front cover where the tach cable had been.
Thanks, bpeckm! Good solution. I will order a new grommet, because the old one is splitting into pieces.