Posted for Lehanover@aol.com:

I doubt if there are any dealerships anywhere with balancing equipment. And some with mechanics that have never seen the inside of a rotary, outside of a class room. On balance weights from auto trans engines, the front and rear weights have matching numbers. If the numbers are the same, they have been balanced together by Mazda. I have not noticed any numbers on the rotors but if they are from the same engine as from new, they are matched and have been statically and dynamically (spin) balanced. The primary balance is the two rotors. The counterweights with much less mass than the rotors, just take out the rocking couple caused by the distance between the rotors. If your engine was from a manual trans car, you can come up with a rear weight, install it on a shaft and static balance it. There may be a service by Racing Beat or others to do this for you. Or you can buy a auto trans set from Mazda or an aftermarket supplier. In the 12A there is a large variation in rotor weights from year to year. I have mixed rotor weights and counter weights without a noticeable problem. The driver might be able to feel a poor balance and it might cost a horse or two but so far (since 1978) no one has noticed.
I am in favor of a good balance for airplane use. Time would go by very slowly behind
a buzzing monster. Lynn E. Hanover