This came off of my spare '87 13Bt block. It has holes drilled in the sides. I have no idea if that's a stock feature or what. It wouldn't make much sense, but I need to know in case I were to try using it... Drilling doesn't sound like such a bad idea, but not knowing if it was balanced and if so how it was balanced leaves me at a loss. The depth of the holes varies, too, looking like the deeper ones are drilled where the weight lip is present.

 

ussually the holes are from the factory, its what mazda did to balance the flywheel

 

Some of these are like 1/2" deep, and they're all about 1/2" wide. Would they go that far sometimes? It would just surprise me some that their mold and quality of steel wasn't more precise, but perhaps this method was cheaper somehow.

 

kompressorlogic has it right. They are for balancing the stock cast flywheel to get what is effectively a properly yet still out of balance flywheel.

Out of balance? Yes. That is why you need a counterweight from an automatic when using an after market flywheel. Those will also have drilling to get to the proper weight to offset the weight of the rotor. In other words, the counterweight is cast into the stock flywheel and the drilling is just an adjustment to make it right and they do it to the automatic counterweights too.

 

I understood that part of it, but didn't realize they'd need to drill to precision for the flywheel and the auto counterweight.

Good info, thanks guys!

 

another interesting one is look at the side of the engine rotors, they have holes drilled in the sides too of various depth to keep them in balance

 

I knew that, and that bit made more sense to me as the rotor castings are much more complex. I don't suppose there's anything else on the car they have to do that to, though, is there?