I need help here -- I am not using the mechanical tach drive.  I
need to cap it off somehow.  The vacuum pump (Tempest) is ridiculously
close.  I have measurements for machining a special cap but clearly
everyone with an electronic tach (i.e. almost all of us) have capped
this off.  How did you do it?

Rob,

 

I scrounged the "cap" from a discarded tach cable - of course, it had the hole for the cable in it for which I fashioned an aluminum disk.