Hi Guys, I found your request when I was browsing A10 stuff on the net and just signed up yesterday.
A couple of points on the tacho - I wanted to do the same thing about 6 years ago and when visiting the BSA rally I met up with Roger Sharman of Cake Street Classics (01986798504) - Roger supplied me with a modified inner timing cover - originals are like rocking horse pooh a tabbed worm drive for my oil pump, cable and associated hardware but most importantly a brand new anti-clockwise 3:1 tacho.(RC1303/03)
Now before you all jump up and down and tell me you are after a mag drive unit let me continue!
As my machine is a 1956 A7SS my tacho should be driving off the mag (at the time my mag was auto advance but more later). So I acquired a right angle drive - an old timing cover in which I machined the tacho mounting area flat and tapped two holes in the bosses provided and mounted it all up - great up to the point when yes you are right, not only is my tacho 3:1 it is also anti-clock so no chance of that indicating any revs at all.
So, I researched the drive unit and found that it is made by Speedograph-Richfield (
http://www.speedograph-richfield.com) very helpful people who offered ( at a small cost ) to modify my 2:1 drive to perform the magic of converting not only direction of rotation but to provide the correct ratio to drive the 3:1 tacho - and yes it all works and the drive gearbox is available from Speedograph Richfield as a standard item.
I now have the best of both worlds - I can use 1 tacho in either a 1956 guise off the magneto or in 1960 guise off the oil pump.
What I am trying to say is - get a tacho from Roger then get the drive - from Richfield.
Finally if you haven't got a manual magneto you will need to get a special nut from George Prew to adapt the auto-advance unit or wait patiently for a manual cam ring housing to pop up on e-bay like I did.
I look forward to hearing from you.
Cheers
Gordon