There are differences in the overall length of the nut as well, though the M20 only uses one of them...I'll post the relevant dimensions later today (wed.)...Ian
email (option): ian@wright52.plus.com
Here are the dimensions for the magneto drive gear retaining nut...
It is a shouldered nut with a plain section below the hexagon...
The hexagon size is .600" across the flats (5/16" Whitworth)...
Overall length...640"
Length of plain 'shoulder'...400"
Drilled and tapped 3/8"BSF through...
Length dimensions plus or minus .010"...
Hexagon tolerance plus .000, minus .010"...Ian
email (option): ian@wright52.plus.com
OOPS!
..I forgot to note that the outside diameter of the shouldered section is .540"...Ian
email (option): ian@wright52.plus.com
I'm quite interested in Lucas MO1 data plates. Does that make me a magneto nut ?
I have quite a few of these data plates, I can send you photos of them if you want. But not right now as I am relaxing here on sunny Crete at the moment.
I have only seen one single cylinder BMW so far, not one BSA.
Henk
email (option): ahum@quicknet.nl
Nick, I'm definitely an English size (despite persistent attempts to turn me metric). 
Henk, I was thinking about starting a thread asking for copies of plates. There are a lot of identification numbers appear on plates (some of which are included in LV7 lists)but which do not appear in the post-war Lucas parts listings.
In order to follow this through properly, I probably need as many copies of LV7 lists as possible and of course they are only attached to quite a small proportion of Spare Parts Lists.
When are you coming home anyway ? It's going to be 25° here on Sunday
Do you know the actual working condition of your mag? If it's an old un-restored mag, the chances are it will need a rewind and overhaul anyway. In which case it would pay to find another armature and get the whole thing restored by a professional. I guess most magneto guys will have spare parts in stock also.
On the other hand I feel more than confident that my own engineer could restore the original thread.
The magneto is the heart of a bike and if it's not in tip top condition, you could spend a lot of time trying to kick start your engine. 
Ron
email (option): ronpier@talk21.com
The mag has to be stripped down to do it, but you can save an armature that has a thread that is beyond use...
Once the armature is out, cut off the threaded portion. Next mount the armature in a lathe, face the cut end and drill and tap to 3/8" BSF...
Finally fit a stud or cut down bolt of the correct length. Use some 'retaining fit' Loctite on the stud/bolt when fitting......Ian
email (option): ian@wright52.plus.com
Once the armature is out, cut off the threaded portion. Next mount the armature in a lathe, face the cut end and drill and tap to 3/8" BSF...
Finally fit a stud or cut down bolt of the correct length. Use some 'retaining fit' Loctite on the stud/bolt when fitting......Ian
That's one way! But the risk is that the stud might unscrew itself despite the loctite.
I would expect that my guy (Hayward) would tig weld over the old stud and re-machine it back to the correct thread in his lathe. He always tells me there's nothing he can't do
Ron
email (option): ronpier@talk21.com
I've supplied a few mags that have had the thread repair done and with the application of retaining fit Loctite I can confirm the studs don't move in service...
However, there's always more than one way to do a job....
Without knowing the material spec. of the armature I would hesitate to weld it personally. There might also be problems with introducing that much heat to the windings via the shaft...
Never say never though...It may work fine...Ian
email (option): ian@wright52.plus.com