Okay, I just want to get a definitive answer. I'm pretty sure I know, but just to be absolutely sure...
Counterweights only balance the rotors. Rear weight balances front rotor and vice versa.
So, if you want to use a lightweight flywheel, the ONLY reason you need an additional counterweight is because the stock one is built onto the flywheel, thus unusable with the new flywheel. Is this the case?
So, there's no reason to change actual weight of either counterweight if you're doing a 5 speed swap, or flywheel upgrade?

 

If your converting an automatic to a 5spd, then you already have the correct counter weights, just bolt on the light FW. If your wanting to use a stock FW, then you need the correct one for your engine group. If your switching to a light FW on an already 5spd car, then you need the rear CW for that year specific engine group.

 

Yeap, for a '79-82 has a different rotating assembly then the '-83-85 12a's. Interchanging the flywheels between the two will result in engine imbalance and cause bearing failures in the tranny forsure and more vibration the higher its rev'd.

I would prefer to have the '-83-85 engine because the rotors are lighter weight. Just be sure to use the correct front counterweight(inside the front cover) and the correct rear counter weight for your engine year that you get with the light steel or aluminum flywheel you buy.

 

If you're going to buy a lightweight steel flywheel, make sure you tell the folks you buy it from that you don't want the counterweight. Total from Mazdatrix is about $75 cheaper if you don't need it.

Btw the flywheel is the same part for all of the non-turbo models up to '91 [at least up to 88, I know], so if you ever upgrade to a 13b engine all you need is the automatic counterweight from that year.

edit: assuming that you're doing a 5-speed swap.

 

oops, edit
Okay, nevermind. I got it now.
I don't know WHERE I got this crazy notion about swapping out the front counterweight with flywheels. For some reason, that's always stayed with me. It's gone now, tho.
Anyways, thanks fellas.