OK, I am going to ask stupid Qs till I get this flywheel business sorted out. How do I tell whether its a 6v or 12v flywheel without removing the clutch & pressure plate. I have an engine with one on it already. Would a pic help?
Raf

Count the ring gear teeth. Not all of them just over a known distance.
Measure the clutch pressure plate. Usual 6 volt will be 180mm
Just measured my old one Raf (flywheel that is) and its 273mm across.

its 270 across and the clutch is 200. I guess all of them regardless are same outside diameter so what I am trying to determine now what I can is 6v/200 or 12v/200. By counting the teeth? How many teeth is what?

If its 270 Raf I would say its a 6 volt one.
Remember you have to clearance the 6 volt trans for a 12 volt Flywheel. So it must be bigger.

I thought it was just deeper/thicker not larger diameter?

Nah different diameter and the pitch on the starter ring gear is different cause the 12 v starter pinion is smaller.

Raf you can tell by just looking at the teeth. the 6V are wider apart... than the 12V... I suspose its just experience.

Yeah, read the How to Rebuilt book last night. FOund out that 12v have 132 teeth while 6v have only 109. What I have is 12v