I know I did something with my Spec 7 regarding mix and match but could not remember!

Excerpt from From Mazda Competition Manual:

Counterweights

Counterweights balance the rotors. If you are changing rotor weight, you will also need to change or modify the counterweights. If you are using a lightened flywheel, to keep everything in balance, you will need to use a rear counterweight for an automatic transmission flexplate.

Stock front counterweights used on RX7s after 1980 are smaller and their weight is more concentrated than earlier types. They have a 100° angle instead of the 180° angle found on earlier counterweights, which helps reduce rotating mass weight.

Using the lightest flywheel available will allow your race engine to "rev" more freely and reach its power band more quickly. However, SCCA Improved Touring class and many other classes do not allow changing to a lightened flywheel. It is legal to update or backdate parts from other years of the same model. The 1981-82 RX7 flywheel (Part #N201-11-700) is the lightest stock flywheel available for the 12A, weighing 24 lbs. Early 1974-80 12A flywheels are 32 lbs. and 1983-85 12A flywheels are 27 lbs.

If changing to the 1981-82 12A flywheel (Part #N201-11-700) be aware of these necessary parts/ modifications:

For 1979-80 RX7 or a pre-1979 rotary model: Purchase a 1981-82 RX7 manual transmission front counterweight (Part #N201 -11 -511 ). Rotary models prior to 1979 - be advised that changing to an RX7 flywheel may not be legal.
For a 1983-85 RX7; purchase a 1981-82 RX7 manual transmission front counterweight (Part #N201-11-511). The combination of 1981-82 flywheel and 1983-85 rotors is recommended because they are the lightest stock components available.
The complete rotational component group (eccentric shaft, rotors, flywheel, counterweights must be rebalanced when changing any of the components.

 

For me i FINALLY gotta hold of a 12A engine to drop into my FB. It turns out it's an 1985 12A. But referencing the article you just posted.... let me get something straight. I can use my "new" 1985 12A and use the flywheel from my 1982 Blown engine and install it along with the 82 counterweight and that would create a better option because its a lighter combo. Am i reading this correctly?

Because not only would this save me some cash but it seems like this is the lightest way to go when talking about stock parts. However i have been advised to use my "new" 85 12a short block and the 85 flywheel from it and just buy a new 85 clutch kit and mount it to my 82 tranny. Which way would you guys go?

 

Hes saying you would have to rebalance the rotating assy if you did use the 82 fly with the 85 internals (the whole engine in your case).

 

I installed a '82 in my '85 GS, and was wrongly advised to use the '85 flywheel. Result was bad vibes, you could tell right away that something wasn't right. Jeff20b set me straight on the situation, so out came the motor again so I could swap flywheels.

 

ok i have a problem i have an 82 auto and i got a 83 motor and 5 speed from the junk yard now i know that i cant use the 83 flywheel with the 82 engine. so sence i dont have much time to go back and find a 82 and atempt to take that f***ing 2and 1/8 nut off . thats if i can even find one. So if i wanted to use my 83 flywheel with my 82 i need to chang what? is it both front and back counter waits or just the back one?

 

You DO NOT need to rebalance internals if you match flywheel with counterweights and are not exceeding about 8500 rpm; otherwise all those people with aftermarket lightweight flywheels would have torn their engines apart by now!

It is the sanctioned racing rules that may cause you to stick with the Mazda parts bin updating /backdating from there as our ’85 has 85 ish engine, 83 rotors, 82 flywheel

I also have an ’83 with RB LW flywheel now run for 20 years that way (legal under Solo SP)
And a ‘94 with RB flywheel and auto rear counter weight original front CW; both of which I have never entered the engine.
Total flywheel miles 195k