If replacing just the stator, make sure it's the same size as the rotor. I know Triumph changed from the smaller diameter in the early Sixties, I'm guessing most Brit manufacturers did the same as the larger one became available from Lucas. The larger size is approximately 4mm bigger in diameter than the early one. I have just looked at the factory workshop manual and see that both RM15 and RM18 were fitted to the Ariels, I assume from Brian's post that these were both the larger diameter ?
According to the Lucas bulletin which can be viewed as a pdf on the 'British Only' website, the RM18 superseded the RM13 for smaller capacity machines and the RM15 was for larger bikes.
The Lucas 12volt ones currently (terrible pun !) available from most stockists are the RM21 and 23 models, (plus the three phase 24). Personally I wouldn't bother going for a new 6 volt job, might as well go for 12 volt if you are spending money.
Can anyone give a definitive answer as to which RM model is best ?
When I get around to mine I will probably fit a new alternator, so would be very interested to know which model is best.
Just re-read above posts and see that Brian has the RM18. According to that pdf it was meant for smaller capacity bikes with low ratio top gears.
I guess the big danger nowadays is to fit the supposedly best performing alternator and then finding the bike doesn't rev fast enough to charge the battery, or it produces so much power (with high watt bulbs, heated grips etc) that it saps engine power.