Help why so many different flywheels?
 

I have acumulates a pile of rx7 stuff and i have noticed several different fly wheels some stamped 21,31,41 the higher the number the heavier the flywheel.Not sure the reson or application,also i have noticed if you dont align the pressure plat with the flywheel in a certain position you will get a slight but very noticable vibration.

Different years used different weight rotors and stuff, so the flywheels are probably weighted for that...

Flywheels
 

Felix is spot on! The flywheel on the rotary contains the counter balance weight. Different weight rotors, different balance weights. For example, the 86-88 n/a flywheel has 326 on the back face towards the engine. N326 is the first 4 digits of the parts numbers for just that model. If a part is used for more that one model, the first year the part was make is usually the p.n. used. Rob

Flywheel /vibration?
 

So if i have the wrong size/weight flywheel for my engine ,may that cause the vibration i am feeling.And also does anybody know if the pressure plate has to go on in a certian spot on the flywheel?

The wrong flywheel and/or wrong front counterweight will cause a vibration.

I think you only have to orient the pressure plate a certain way if you are reusing it on the same flywheel.

The FAQ section of the Mazdatrix website has a chart of flywheels, going by weight, year, and markings. Also it's pictorial!

I noticed on the rotor weight chart they have that some 13Bs and some 12As have the same rotor weight, which makes me wonder if they'd have the same imbalance. Maybe not because a 13B rotor stores more oil than a 12A rotor, which affects balance... but it's interesting to think about. (Like directly swapping a Turbo flywheel onto a 12A so you can use a normal, non-solid-hub clutch when running a T2 trans, instead of a special adapter clutch that is only available with a solid hub)