Lightweight Tachometer

I used a Triumph rev-counter gearbox, the sort the screws in to the crankcase, drive side, driven by the exhaust cam.  I used one of these because I had one!

Fitted it to the timing cover, positioned to be driven by the exhaust cam.

The Smiths style head is Indian and hopelessly inaccurate.  Its about right at 30 - 40 mph in top gear, useless at other speeds, if I've done the sums properly.

I took a Smiths chronometric off another bike to get some realistic numbers, speed vs revs, seemed okay.

Yet another of my daft daydreams! Yes I've toyed with the notion in the past, so I'd had a rummage in my Norton detritus. 
Correct me if I'm (probably!) wrong, but l/w camshaft nuts are larger than Dommie ones, so their drive nuts for a tacho drive won't work. So what drive nut does fit a l/w? 

Nick H, you are right about the "screwdriver blade".  On the Triumph 750's it engages a slot in the "tacho drive plug" which, in turn, screws in to the end of the exhaust cam.  Or, I think, on late models it engages in a slot directly in the cam.

I made a special nut to screw on to the timing end of the cam, see photo.

Another thing I had to make was the nut/bearing that fixes the box in place.  On the Triumph its a left-hand thread, needed to be replaced with a right-hand thread on the Navigator.
I used a cable from a T140.

Why not use an electronic one ?
I have one of these fitted to a Commando with electronic ignition. No nasty whirling cables with potential oil leaks.
https://www.smiths-instruments.co.uk/products/motorcycles-norton-nvt-logo-tachometer/

Andover Norton sell the speedo version (which I also have) so it might be worth asking if they can supply the tacho as well.

... was my thought as well. There are some very cheap ones around but some say they only work on 4 cylinder engines.

Those electronic Smiths replica tachos are very nice but expensive, ~£250?  Accurate and easy to set up.  I may "bite the bullet" and buy one for my next project.

I have one of those cheap electronic tachos off eBay, has anyone got one to work?   I have a couple, both given to me because they could not be persuaded to work. I tried both, neither worked, seems to be the usual story with them.

Having said that, I fitted one to an Enfield.  It worked, sort off.  It read double engine speed.  An electronics expert sorted that for me.

I mentioned in an earlier post that the Indian tacho was hopelessly inaccurate.  Sorting through a box of bits I have found a Smiths head but, unfortunately, wrong rotation.   At one time it was possible to buy a tiny gearbox that fitted on to the input of the head that corrected rotation and/or ratio.  Are they still available?