Here is a pic of the back of a stock flywheel (this happens to be a 205mm from an 84-5 GSLSE 13b, but the mounting looks the same for all stockers) for reference as to how it attatches to the rotary. Note that it is a single hole with a keyway to hold it from spinning on the shaft, and a single large nut (indy car wheel style) to hold it in place…this is different from almost every other engine in the world, most of which use a 6 or 8 bolt pattern on the end of the crankshaft.

Also note the counterweighting, which is cast into the outside edge, favoring one side more than the other.

Let’s say you have a manual transmission car, and want to install an aftermarket flywheel, to save weight. Buying the flywheel is simple. Though there are many companies that offer flywheels, and several weights available from each company, there are only 2 diameters of flywheel, the small 205mm and the larger 225mm. Aftermarket flywheels DO NOT have the counterweight cast into them or attatched in any way. They are a flat unit with a 6 bolt pattern. Obviously this won’t bolt to your engine, you’re missing something.

All aftermarket flywheels are made this way. They REQUIRE use of the appropriate automatic counterweight for your year of engine. Any engine within the same series (86-88, 89-95, etc.) can use the same rear counterweight, regardless of whether it originally was an automatic or a manual, or whether it used a 205 or 225mm flywheel. So, if you have an 89 manual nonturbo, you can use an 89 nonturbo automatic counterweight. IF you had an 89 manual turbo, you could still use that 89 nonturbo automatic counterweight. You could not, however, use an 87 counterweight.

Here is a pic of the auto counterweight. These usually sell used for $25-60, and new for $100-120.

So you bolt this to your engine, and then install your flywheel to that (or you can bolt your flywheel to the counterweight, then bolt it all to the engine, doesn’t matter). It looks like this when installed together.

The reason flywheel manufacturers do this is threefold (in my estimation, anyway).

First, they can claim that their flywheel weighs less than half what the stock one does. Of course it does…it has no counterweight attatched, and the stock one does. The counterweight weighs 4lb or so…making a 9.5lb aluminum flywheel weigh 13.5lb. This is compared to a stock flywheel that, depending on year and model, weighed between 20 and 25lb.

Next, it’s MUCH cheaper to make a flat piece of metal, than it is to machine out the complex rotary mounting pad (the center section that’s fat), which is tapered inside with a keyway. To get around this unique issue, they just continue making the regular 6 bolt pattern flat flywheels that are similar to what they sell for every other car in the world, and let you and mazda handle the mounting issue.

Finally, it’s MUCH cheaper for them to manufacture 2 flywheels for the rotary (205 and 225mm) than 6 or 10 (the different stock variations from year to year). Since demand for old rotary parts is relatively low compared to a large market like a honda civic, it would be unrealistic to expect them to make a bolt on flywheel for every year and model specifically. Letting you use your respective automatic counterweight from mazda gets them around that issue.

IF you have an automatic engine, you don’t have to do anything…just buy the flywheel, and bolt it to your existing automatic counterweight.