Tach drives are not the same Atlas to early Commando. The reason is one has a 2:1 head and the other a 4:1 head. I believe the info below is sound.
Stephen Hill
Reply with quoteRe: Norton Atlas Tach Question
by BillT » Mon Jul 25, 2011 11:17 am
a 1:1 tach drive would run a 4:1 tach.
I have a 1:1 drive on my Ranger and a 2:1 tach, so my tach registers 1/2 the actual RPM.
Atlas, Mercury and Hybrid motors ran the 2:1/2:1. Early Commando ran the 1:1/4:1
Atlas tach drive should be stamped 1508/05 on the body, and Commando should read 1508/06. Matchless G80 and G12 ran the /05 box, same as the Atlas.