I'm planning out my rebuild on my se right now. I know that the 89-91 13b had the lightest rotors and a little higher compression. However, my se's rotor are already set up to take 3mm apex seals. So is swapping rotors worth it, is there much to be gained if it can be done?

 

don't mix 85 and under with 86 and above rotors or rotor housings.
so only gains will be rotors hitting housings..... ouch
I think you can use a whole 89 rotating assembly, housings and rotors.
but you'll need a carb or aftermarket ECU to make more power.

 

Strange, I'm currently running the lighter and higher comp rotors out of a 90 with my 85 rotor housings. Over 9k on this rebuild so far and no problems. The problem you are talking about is when using the entire rotating assembly out of a later 2nd gen with earlier rotor housings. The difference is in the ecentric shaft not the rotors.

 

That doesn't make sense... Mazdatrix and RB both openly recommend using an '85-earlier E-shaft in the later models as an easy way to eliminate the oil bypass mechanism. Also, if the E-shafts had different eccentricity, then the rotor gears and stationary bearings would have to be sized different, and IIRC you can swap stationary bearings with no probs. Now you've got me wondering...

I was told all along that the difference was the actual shape of the rotor housing, too bad we don't have a new and old housing to compare! (Slam dowel pins in one and pop the other one on 'em and see the difference) But in retrospect that's probably why Mazda switched the coolant O-rings from rotor housing to side housing... an effort to prevent swapping housings from new shape to old or vice-versa.