Are the S4 and S5 rotors interchangeable or is there a major difference. Im asking because i would like to know if i could put S5 rotors in my S4 housings

 

Yes, they are different.

http://www.mazdatrix.com/b3.htm

 

But the housings aren't different. (other than diffusers and such)

You have to match both rotors and get the right counter balances for them.

 

You can run any 13b rotating asembly in any 13b housings. You have to have the matching front and rear counterweight for the rotors. So you cant just put s5 rotors with s4 counterweight/flywheel.

 

Ok so if I have a complete S5 rotating assembly it will work in the S4 Housings. If so what letter weight would they be?

 

The letters on the rotor do not signify series or absolute weight. They specify weight range within a given series. For example, 84-85, 86-88, and 89-95 are 3 different series and weight ranges. If you had one of each of those rotors, they could all have an A through E on them. That doesnt distinguish them from one another, you have to do that visually or on a scale. It does identify that an A weight 89-95 rotor is heavier than an E weight 89-91 rotor. E is lightest within a series, A is heaviest.

The rotors were mass produced so weight was not exact. To help with factory balancing, they weighed each one, assigned "ranges" with letters to represent them, and classed each rotor as one of those letters...the engine was to be built with matching letters, or letters within 1 of each other (C and C or B and C for example). This way the weights of the 2 rotors in the engine were always close to each other, though not exactly the same.