That's right. There is, as stated above, a hardness difference between 91 and 93, the older ones required a bit more OMP lubrication, 4 oil injectors instead of 2. You wouldnt want to use one FD and one FC housing because of this difference. There is no way to lube one more than the other, and it's even slightly possible that you'd get more wear in one chamber than the other (on apex seals) because of the hardness difference.

That said, I don't think there would be any REAL WORLD problems with using one and one. Most fd's arent daily driven and most dont even make it 50k at a time on an engine. This is the kind of thing that makes a difference at 100k, but not really by 50k or less. REgardless, I would shy away from it unless it was really necessary, i.e. the car had to be on the road asap, or the owner had no money, etc. Either use 2 of one or 2 of the other. This is true of the differences in 86-88 and 89-91 as well...I just dont like to mix series if at all possible.

You can swap any 13b rotating assy for any other one. We'll assume you're keeping your eccentric shaft, those rarely wear out or tear up unless they saw an oil system failure or were abused. Other than that, the rotating assy consists of both rotors, the front counterweight that goes under the front cover, and the rear which can be a stock manual flywheel with integrated counterweight, an automatic rear counterweight for use with an aftermarket flywheel, or an automatic rear counterweight for use with the auto tranny. The only time you dont have to use all these parts from the same year/rotating assy is if you are having the parts balanced by a pro...in which case they PUT the parts into balance by shaving metal where necessary.

Obviously you would NEVER want to run different rotors...they need to match in compression ratio and weight. Here's the differences back to 84, that's the oldest that I've worked with.

-84-5 GSLSE 13b nonturbo, 9.4:1 CR, 11.5lb each, uses 3mm apex seals and thicker side seals than all 86+

-86-88 FC 13b nonturbo, 9.4:1 CR, 10.5lb each, uses 2mm apex seals and thinner side seals (same for all 86+, 2mm seals and thin sideseals)

-87-88 FC 13B turbo, 8.5:1 CR, 10.5lb each

Note that after 88, the rotors got lighter, but they also got weaker. Detonation can dent in the face of the 89+ rotors, so some prefer the older ones for their strengths.

-89-91 FC 13B nonturbo, 9.7:1 CR, 9.5lb each

-89-91 FC 13B turbo, 9.0:1 CR, 9.5lb each (this is the same as an REW, RE, or 20B motor)

Note that all rotors up to 93 used the same rotorbearings, with a shallow oil passage in the center. The later bearings had a thicker passage to retain a bit more oil, so this should be considered if building a performance engine, you may want to use the later bearings even if the originals were useable.

-93-95 13B-REW, -RE, 20B, 9.0:1 CR, 9.5lb

As far as rotor weightcodes/letters, it's simple. They are lettered A through E, each representing a different weight code. You want to stay within one letter of each other, such as C and D. IF you can get 2 of the same letter, that's the best balance you can obtain without getting a professional to rebalance. It could be said that rotors are balanced from the factory because of this.

For instance, if you were dealing with FD rotors, they're suposed to weigh right at 9.5lb with no bearing. An FD rotor with a weight code A might actually weigh 9.70lb, while a Fd rotor with a weight code E might weigh 9.30lb. Those are not actual numbers, but you get the idea. Mazda knew their manufacturing process wasn't perfect, so rotors had different weights. Their solution was to weigh them and mark them a certain range, so that the engines would be closely balanced to one another, for the best smoothness and power possible from a mass produced engine.

Oh, and if you've ever seen the circles drilled out on the sides of rotors randomly, that's how they balance each tip of the rotor. During their process, one tip may have settled with more material than another, so sometimes they had to drill material out so that one side wasn't heavier/more massive than the others.