Loose Tach Board

Not normally, I'd probably stick a shim or two in to get it spaced correctly and to hold it securely, that would be 0.8mm to 1.0mm.   You certainly don't want it plowing furrows in the flywheel!

Heavy rubber band helps secure the tack reader. CA (cyanoacrylate, super glue), and styrene shims to adjust the sensor distance to the tack strip.  Careful, sometimes the small wires, (red, blue, black),  attached to the bottom of the reader board will break loose. 

Rubber bands are not permanent they wear, stretch and break. Especially within closed heated spaces.

Not sure that is the best choice for a hold down device.

Mike, the PS/2 tach reader is much smaller and sets in a holder that is secured to the motor.  It's nothing like the TAS one you picture.  Rubber bands ain't gonna' get it.

Thanks John.  Good to know.  Although it seems loose, when I turned the loco upside down and shook it, the board stayed put so I'm just gonna leave it.  Should be good to go for another 1000 miles.