Brent, you are correct in stating that the XT (points ignition) coil is a 6V coil and the SR (CDI) coil is a 12V coil. These voltages are nominal voltages as the ignition source coils can put out significantly higher voltages as the revs increase.
From my workshop manuals;
The XT coil has a primary resistance of 0.75 ohms and a secondary resistance of 5,700 ohms.
The SR coil has a primary resistance of 0.98 ohms and a secondary resistance of 12,000 ohms.
These resistances don't mean a great deal in themselves as different diameter wire will have different value of resistivity and the two coils could still have similar turns ratios. It is the turns ratio that give the increase of voltage from the primary voltage (6 or 12 volts) up to the secondary voltage required to "spark" the spark plug. For example, if the coils had a 1:1000 ratio, 6 volts on the primary would give you 6,000 volts at the seconday / sparkplug and 12v would give 12,000 volts. Normally you would be looking at getting a secondary voltage some were in the order of 20,000 volts or more to give a good spark at the plug, so the turns ratio would probably be more than 1:1000, however the increase in primary voltage as the revs rise could easily give well over 20,000 volts at the top end of the rev range even with a 1:1000 turns ratio.
The main concern with using an XT (6V) coil on a SR (12V) ignition system would be the current (amps) drawn by the coil. Ohm's Law, one of the major laws of electricity, states that in a purely resistive circuit;
Current (amps) = Voltage / Resistance
So, discounting inductive reactance and a few other finer points of electrical theory, the current drawn by the XT coil would be (6 volts / 0.75 ohms) 8 amps when suppied by the XT source coil.
The current drawn by the XT coil if supplied with the 12 volts from the SR ignition source coil would be double that drawn when supplied from the XT source coil, i.e, 12 volts / 0.75 ohms = 16 amps.
This doubling of the amount of current drawn by the coil would as a minimum cause the coil to run hot, in a worst case it would cause the coil to burn out as the coil would have been wound with wire to suit the nominal maximum current draw of 8 amps.
Now all the above is just very basic theory, and like every thing in life, some times thing will work that just defy the theories that they are built upon, so you could probably fit the XT coil to the SR ignition and never have a problem with it. That is until you are somewhere down along the Tanami Track, because then Murphy's Law will overide everthing!
To get away from the electrical theory, I have a complete SR electrical system fitted to the XT500's that I ride in road trials here in SA. I am running the STD SR ignition coil even though it will not fit into the mounting brackets welded to the frame for the XT ignition coil. Like all good electricians I have held the coil in place with a couple of cable ties, and had no problems. Last year the XT was ridden in 2 x 6hr events, 1 x 8hr event, for 13hr in the 24hr and for about 1500km of road use without any problem with the cable ties, so although its not the best solution, it has worked well enough for me to cable tie the coil inplace on the years bike as well. The std XT fuel tank fits without rubbing on the coil, although it did take a bit of dicking around initially to get the coil into a position where it didn't touch the tank.
An option might be to check out some of the aftermarket universal ignition coils, you might find that you can get a coil suitable for use with a CDI that will fit into the std coil brackets. I know that the universal "points ignition" coils fit as I have one fitted to my TT500.
CJ
