I don't get it.

Top is C (I already had that one), bottoms are D. As you can see, the holes patterns are all different. Why is that?

 

Those holes are for balancing. The 'C' rotor is considered the middle weight of castings, and it makes sense that it would be more uniform, or with less drilling.

Weigh them on a scale and compare the weights. Not sure if those machine marks represent stock or aftermarket balancing.

 

Probably a good idea to send them out to all be balanced before install.

 

Mazda casts each rotor, machines them, and then spin balances them. they balance the casting by drilling on the rotor sides, like you see.

then they weigh the rotors, by batch, and then grade them A-E, heaviest to lightest. they then take matching, or nearly matching pairs (it can be 1 letter off?) and put the engine together.

when rebuilding from parts there are two problems;

the dynamic balance of the rotor is ok, but Mazda made these from 1991 to now, so expecting a C weight rotor from 1991 to weigh the same as a C weight rotor from 2017 is silly, and quite frankly wrong, we know they changed the casting/machining a bit after the Rx8 came out.

and all the replacement rotors are magically a C weight. is this because they picked all the C's out of the batch, or because they didn't grade them after they stopped building the cars?

so the letter is a guide, but you're better off putting those on a gram scale and picking the two that are the closest in weight.

 

if you care about equal rotor weight i suggest you weigh your rotors on a gram scale rather than match them using the letters.

i recently found two rotors (a B and C) that were 60 grams apart! no way do they assemble in the same motor.

Howard

 

I've even seen the same letter stamp rotors be quite a bit off. Personally I won't build a performance motor with 2 different stamp rotors. I recommend balancing for anything over 400 hp.

 

rotors are like wheels, you can't just chuck 1 wheel weight on every wheel and expect great results. mazda balances the mass at each corner and apex for the best overall balanced product.

so of course all rotors won't look indentical.

 

Alright I didn't know that, I'll weigh them for sure! Thanks a lot guys!