I've got an unknown rear cw here and my results were inconclusive using the Mazdatrix measurement method seen here: Identifying Rear Counterweights

"No real distinguishing features" like S5

The measurement result was between S5 and S4 of around .511" to .512"

The weight in grams of mine is: 1845

I've weighed my others from known years / rotor weights and got expected results ranging from heavy to light, as follows:
74-78 13B: 1909 & 1900
79-82 12A: 1861
83-85 12A: 1819

I don't have access to my known S4 right now. Otherwise I'd weigh and include it here. It does have a groove on the lower part of the rear face, as mentioned on Mazdatrix.

Can anyone help identify my mystery counterweight? Thank you.

 

Not sure if the results are conclusive, but I just weighed a S5 counterweight (J-spec) @ 1831g.

 

Thank you!!!

Makes me wonder what the weight range really is on these. I weighed two old 13B ones and get a spread of only 9 grams. But I don't know if that is the heavy side or the light side of the acceptable range to begin with.

 

Both weight and the lip seem fairly inconclusive ways to tell the counter weights apart to me. How hard would a stamp be Mazda?

I measured my S6 auto counterweight (should be same as S5) and it weighed 1897 Grams on my China electronic pull scale. No claims of accuracy.

The lip width did fall closer to the S5 & S6 range as best as I could measure (0.488"). I found it easiest to use a micrometer with the anvil on the edge with the ramp cut and you can feel when the spindle starts to slip down the bevel on the outside edge.

 

If your range is 3000g, it's likely your scale is accurate within a gram easily, so I think if you're getting > +/- 1 gram you're within the scales accuracy. I read on this thread .. https://www.rx7club.com/rotary-car-p...inside-496649/ .. that pistonsuk mentioned 2 letter differences in weight which was ~30grams allowable.. if he's right then it looks like you're within tolerances.

 

The scale I use can go up to 7000 grams, which is enough to weigh an old school rotor, and is probably accurate enough for the counterweights.

It is also accurate over time. I first weighed some of these a few years ago. The numbers came back the same just now.

Blue TII, it sounds like you could use your cw on an old school 13B, being that close to 1900 grams.

But here is another issue people might not realize at first. How much weight does it offset while in motion? There were two possible front counterweights for the 79-82 rotors. One was a half circle and weighed 1284g while the other is narrower with a thicker casting at the edge weighing 1033. How can two very different weights work on the same rotors? Because as it spins, its lesser mass more toward the outer edge can take the place of greater mass more spread out. Does that make sense? Makes me think twice about wanting to use a 12A rear cw on a 13B and vice versa. I mean 79-82 rotors are similar in weight to S4 rotors, after all. Likewise 83-85 12A are similar to S5 rotors. But their front counterweights are quite different in shape and weight.

 

I think I figured out what is up with this mystery rear cw. I think it is an S4. The reason the front side bevel was inconclusive is because it lacks the groove cut on the other side, and Mazda needed to make up for the extra required mass some how. If it had the groove cut, it would need that material on the other side and more closely resemble what Mazdatrix said.

Oh it only has one drilled balance hole.

Does anyone else have any S4 or S5 counterweights they could weigh for me?