Changed my bike from 6 to 12 volt. Do I have to change the 6 volt ammeter to 12 volts.
The ammeter is in series with the electrical load and offers virtually no resistance to current flow. There will be no appreciable voltage difference between the 2 terminals, irrespective of the system voltage. On your original 6v system you would measure 6v (nominally) at both ammeter terminals. After changinging to 12v you would measure 12v at both terminals. Assuming the same wattage the current will halve on the 12v system and so the ammeter maximum scale will not be an issue either.
Just noticed that my ammeter has + to - of 30. Is that too large a scale for the needle to move on a 250 Jubilee.
IF you did have 30 Amps going into your battery I think there might be some serious concern, similarly if your lights drew more than 10 amps (both filaments on) again there would some concern. Yes 30A is a bit on the high side but there is no harm done whatsoever.8-0-8 would be adequate, anything else is also academic.At the end of the day as the ammeter is very cheap it is undamped and swings about more with vibration than electricity. Once you have a known working charging system get use to the max swing and after that use the Ammeter as just an 'am I winning or am I losing' device.
PS As the meter measures Amps then the voltage supplying those amps is academic.