I am assembling a 1985 12A MT but somehow the front counterweight for this engine got into a box of unidentified front weights.
And... I don't know what a 1985 weight is supposed to look like.
I have half circles, about 90 degrees and less than 90 degrees front counter weights.

Can someone help ?
With a picture maybe ?

Thanks !

 

http://mazdatrix.com/b5.htm

in the middle of the page there will be some info about the counter weights.

 

make sure you use the right one.

I had an engine that had a 12A 1982 model with the heavy rotors, manual transmission front weight (the auto one is different for this series) and a 70s model rx-4 13b rear counterweight, the car had awful vibration problems.

When I rebuilt my motor I used the correct weights for my 1885 (light 12A) rotors and it was a world better.

I paid someone hundreds of $$ for that dodgey work too 8-)

 

I already had a look there but unlike rear counterweights and flywheels, it does not show specs for the different front counterweights.
While searching, I did find this interesting page: http://nagaracing.com/Techinfo/counterweights.htm

It looks like I can change the front weight to the 81-83 type, changing the flywheel to 81-83 at the same time.

 

Mixing the 81-82 counterweight and flywheel, with the lighter 83-85 rotors is a good way to off-balance the engine.
The 76-82 12A rotors are ~.5LB heavier than the 83-85 12A rotors. This extra weight requires extra weight in the counterweights to balance the entire assembly.
Putting together an engine that shakes can really wreck the bearings, or more.