I have had a real challenge trying to figure out what counter weight goes with which engine. So, since I have four different counter weights I thought I would take some measurements and make some observations, then share that with you. The counter weights I have are from an 80 12A, 85 12A, 85 13B, and an 86 13B. They should be representative of most applications that we use in our first gens, except a series 5. I have used each of the four counter weights to represent the counter weight used for that series of engines. For example, the 80 12A (and its measurements) should represent all the counter weights used on 12As from 72 through 82. Note, I am only using one sample to represent several counter weights. I believe this assumption is accurate, but I am sure you will let me know if it is not an accurate assumption.

I have attached photos and a chart with measurements (I hope the chart up load well). The lobe measure is the distance between the inner hole (that fits on the eccentric shaft) to the furthest outside edge of the counter weight lobe. The lobe thickness is the measure of the thickest part of the lobe (I measured the thickness of the body of the counter weights and found that they only varied .007 between the four). The weights were taken on a postal scale, which goes only one decimal point. The weights of your parts may vary slightly since the factory dynamically balances the rotating assembly to some degree. I have attached a photo of the bottom edge of all of the lobes so you can see the various holed drilled into the counter weight at the factory.

Model Weight Lobe Measure Lobe thickness
12A 72 - 82 4 lb 3.5 oz 3.124 inch .646 inch
12A 83 - 85 4 lb .5 oz 3.057 inch .649 inch
13B 72 - 85 4 lb 3.5 oz 3.256 inch .597 inch
13B 86 - 88 4 lb 1.5 oz 3.197 inch .544 inch


My Conclusions
•The 12A counter weights are thicker at the lobe and have a smaller lobe measure than the 13Bs (see the side photo of the counter weights - the 12As are on the left and 13Bs are on the right).

•Weight alone is not enough to determine which counter weight you have. For example the early 12A and early 13B counter weight weigh the same, but the 13B has a .0132 (about an 1/8 of an inch) larger lobe measure. This may seem small, but and 8,000 RPM this is a great deal of difference.

•The only casting mark that is helpful in determining which counter weight you have is the 1029A marking of the early (72 – 85) 13Bs.


Good luck and if you get additional measurements please post them.

 

Thank you very much for this one. Should sticky archive this one in a hurry..

 

These are all rear counterweights for stock automatic or aftermarket flywheel applications, correct?

 

measure in grams not ounces, and do note that some of them are not balanced from the factory anymore

also front counterwegith matters...

 

And please use mm instead of inches.

 

Good comments,
as far as the weight, I used what I could find. There was not scale in grams that was handy. Jeff, good point, this really should be in MM, I will add that.

 

The morale of this lesson Kids is that crack kills, askhis brain cells

 

I believe he meant that they may not be to factory specs anymore. I neither agree nor disagree with that statement, primarily as I don't know where these weights attach to. Which segues me nicely to my own stupid question: where do these go? I've been reading up on TII swaps, and seen mention of needing to change the counterweight depending on which transmission will be used. Sorry if this is too off-topic, but I figured this thread might as well answer that question too, if it's going to be stickied.

-dave

 

at least its a worthwhile question.

you have two counterweights, one on the front, and one on the rear (manual transmission flywheel (stock only) has it cast into the flywheel. In otherwords running a zero balance flywheel w/out the counterweight, will throw your engine out of whack. all aftermarket flywheels for the rotary engine bolt to the rear auto tranny counterweight. the important thing is that the counterweight matchs the exact same specs for the engine you have IE: 89 TII rotors AND crankshaft requires a auto counter weight from the same balance weight engine hence mazda broke it into about 11 different models that I am currently aware of depending upon rotor weight/ecentric shaft model/size or number of rotors.

incedentally (sp) they still balance at the factory, just like all reciprocating engines, the bearings would not last without them being offset and ballanced (nor the journal shaft nor anything else)(think clearances) sorry for being a smart *** in the earlier post, but people need to think anything that rotates at any appreciable rpm needs to be balanced if you expect its bearings to live think balanced tires ?

And I still claim that crack kills: regardless of form......drug, plumbers crack, cracked block, cracked apex, cracked skull, cracked egg, i think you get the photo

 

Great stuff, glad I caught it before it left page one. We will definately archive this. Any chance on a front CW write up?

 

I will measure the counter weight s in MM this week.
No plans on a front counter weight write up from me.
Glad it is helpful.

 

It's been well over a week.

Anyhow... So, I need a counterweight to install a light weight flywheel on my car. When searching for a counterweight, I should really be searching for TWO counterweights? Both a front AND rear conterweight, or just the one? Match year of engine, what else?

 

What the hell? My car didnt have one??? Or are those 4 automatics only ?

 

4 shore.

 

very useful ... deserves to be sticky.