Michael, I sure wish I had those carbide-tipped, dual-cutting arms when I was scaring the living crap outta myself when, years ago, I was cutting holes in 0.125 sheet aluminum to fit gauges in an aircraft panel. The bigger the hole, the scarier it got. Wobbly it got.
You would have still scared yourself Monte.
what you need is a thing called a ''panel punch".
I can't help myself at auctions and garage sales, a long time ago I bought a set of knock-out punches that look like the photo below in Imperial increments from 1/4" to 6" They have since made holes in cars, boats and aircraft for me to fit meters, gauges and instruments and they do so painlessly.
Any kind of circular cutter into thin metal becomes a "measure with a micrometer, mark with chalk, cut with a chainsaw" type deal.
And sorry for the thread drift as this does not solve the OP's problem of how to support his tacho whilst he operates.
For what it is worth, whilst there is a shortage of R65 speedos on account of them fitting early G/S machines, the same is not true of tachos - in fact there is something of a glut of tachos discarded by the misguided "cafe" builders. i would try for a working 2nd hand unit.
