[Graham]

my point was

1/ 
can SCX cars in class A use the SCX RX motor ? or would it have to be changed to a 20k 'S' can ?

2/
if any car is on the list of eligible cars for a class 
then it cannot be used in any OTHER class ?

the 917 being only an example  

hope thats clearer

Class A is a FC130 (S-Bell) motor only class, no RX, long Can, short can  slimline or Flat motors allowed. But 3d chassis are allowed, so you can run a SCX or Ninco model with a FC130 motor fitted or Ninco with a chassis adaptor or 3d chassis. You do not necessarily have to fit a 20k rpm motor, lower rpm FC130 motors are still permitted.
 SCX cars fitted with RX motors have never performed particularly well in our proxy competitions anyway, due mostly to the fact that they take a long time to warm up and reach a good operating temperature. Additionally SCX is no longer using these motors for current or future models.

2) Yes your right, I have chosen the classes so that the same car does not qualify in any two classes, for the reasons  stated in the Class Introductions in the rules. Each class has its own  "car types" described. As administrator I have decided where the cars are best placed for competitive reasons. For example, I think the only obvious Car which fits both Class B and Class D criteria might be the Ford GT40 which ran 1st time in Le Mans in 1964. But I have chosen to ignore that fact and place it in Class D to place it with more competitive slot cars.