Venhill actually recommend electrical solder or plumbers solder.
The only problem with multicore is that the flux may not be aggressive enough, especially when modifying or repairing a used cable.
On a slightly different note, I worked on a dealers on the sixties and seventies. They had special nipple soldering tongs. They looked like curling tongs but with carbon brushes at the end that faced each other.  The tongs were connected to a 12v battery. The nipple was held between the brushes, and when the button was pressed, the dead short across the battery heated up the nipple to solder it! A bit brutal on the battery, but they worked well. I can't remember who made them, but I think they came from  the motorcycle trade suppliers, MCA.

But it's a 4 stroke, so half the number you calculate.

I don't know Ducati, but certainly Japanese bikes favour a relay. It depends who make the system for BSA.
One thing we are agreed upon is that the signal used to switch the lights on and operate the tachometer originate from the ECU.
I notice that there is an owners manual for download. This shows 9 fuses. Are all of these OK? Remember that the lights fuse may not necessarily control the ECU signal to turn them on!

I'm not too sure. The tachometer recieves it's signal from the ECU. It depends upon the system that BSA use. In general it can be either from a dedicated wire or via a shared network communication such as CAN BUS or a more basic system called K-line. The latter two share data between the ABS unit, tachometer,  engine ECU and any plug in diagnostic system.

I think that the headlight should come on when the ignition is turned on, the engine cranked and engine rpm exceeds 1000rpm for the first time. This is to save battery power when starting. This system is used on other brands as well.
This means that there is a headlamp relay controlled by the engine ECU.  Is this relay being switched on?

My gray faced magnetic are 2BA. My Chronometric ones are 0BA

2BA thread size. Same TPI as 3/16 BSF and cycle thread but different thread angle. 

The big Lucas Master catalogue lists a CZ27 ammeter. Part number 364455. It has 8-0-8 on the dial and is 1 5/8" diameter and has a metal body. They turn up occasionally but are rather expensive. The part number was referred at some point in the fifties to 36084. This is listed as CZU27 and only has  - or + on the scale but is still 8 amp. They both would have windows in for back lighting.

I would be lost without my lathe. I needed the part you were looking for plus the lever and cable stay. A bit of time in the shed and here we go!

It is 2BA size. They are easy to find on Ebay

The speedo should be a standard Smiths/Jagger 80mph pin drive chroometric unit with (according to the parts book) a 29" cable. The speedos do come up occasionally but you need deep pockets!
You can use the later square drive speedo with the appropriate later cable. You can camouflage these later cables to look like the earlier fatter ones by running them through a piece of black rubber tubing. I went this route on my pre war Norton until I found the correct speedo 18 months ago.
At the wheel end, the wheel mounted gear is 70 teeth. This is pressed onto the hub but also has 2 drive dogs. The speedometer gearbox has a 23t gear.

With pdf file

Please find attached a drawing of the plate before any bends have been put in. I have tried to mark the positions of the bends.
The return bend has a 3/16 inner radius. I.e. is has been wrapped around a 3/8 bar. The plate thickness is 3/32 or 2mm.
The picture illustrates the profile.
I hope this helps.

Ok. Give me a couple of days to sort the genuine one out and measure it.

I struggled to find this so after careful study I fabricated mine. I have since found a new old stock one. After seeing the genuine article, I was really pleased how close my copy was. I will change it for rhe genuine article at some point. I can supply some dimensions if required.