Electra starter motor - screw connecting starter motor to epicyclic gearbox

...fits nicely.

...or not 2BA, that is the question.....

2BA,
If you want to send me your bolts/screws I can clean them for you and confirm.

I believe this is the correct screw?

Measurements on photo but metric pitch guage 0.8mm fits perfectly. 

From dia too, I go with Alan. 

Could it be 10 UNF (0.190" dia x 32 tpi)?

Where did Norton source the motor?   If its Lucas maybe they were using American threads by the time the Electra was introduced to the market.

Ps. Sorry to muddy the waters.

Mr Crocker-go away. The Motor started life on the Reliant Robin. Lucas were never known for changing threads to make life easy. One story is that they invented the BA series for small electrical parts, and the dimensions of the BA is nearer Metric than any other system.

... Lucas invented BA (stands for British Association). They were quite clever in that there was a specific relationship between diameter and thread pitch which is why they seem a bit odd sometimes. According to my Machinery handbook the thread was initially proposed by the BA in 1884 and adopted in 1903. The same learned volume says that they're based on the Thury thread which was a watchmaker's standard and that pitch = (0.9) to the power of n where n is the designating number. One of the few metric British standards.The trouble with BA is that most sizes don't have a simple pitch eg 4BA is 0.6600mm

2BA pitch is 0.8100mm so Peter's measurement is pretty much spot on.

The thread angle is 47.5 degrees so very different to both Whitworth and Cycle.

So we can't blame Uncle Joe.

By the way, that book is a lifesaver on occasion and copies can still be picked up from autojumbles and charity shops.

Must get a copy. Funny the more you learn the more you realise you do NOT know.  But at least we can't blame American Fred!

The gearbox that is attached to the end of the starter motor, was that specifically Electra or from something else?
The reason I ask is that a friend has the bare bones of an Electra engine and would like to gather the parts for the starter.
Now knowing that a Reliant Robin starter motor (thank you Mr Osborn), might be a useful acquisition for the project.
 

Not recommended. The original Reliant starter was an Inertia starter, the Electra is not such. So although my idea that the starter came from the Reliant world, I think it might just be the windings/armature that are common (bear in mind these both came from the Lucas factory, under mass production rules!) ie you do not start a new product when you can adapt one already in manufacture. (The Lucas 3 phase alternator should/might have had 3 GREEN or YELLOW wires, but they still had the same colours as the RM19 single phase alternator).
The gear box-as far as I know was special Electra. Any one else seen it elsewhere?