Have 3 good 13b rotors here, all are from fc's, iirc. I was told to measure the tubs. They could be 8.5, 9.0, 9.4, or 9.7:1 if they're fc rotors so its tricky.

thanks!

 

i can tell you any year just send me a pic, i might have some trouble with some earlier 70s rotors but anything above that i can tell you what it is and about what it weighs depending on its letter designation.

Josh

 

iirc, zenki are cast and kouki are machined. Not sure how o tell the diff between na and turbo tho. What letters mean what?

 

na rotors have a smaller "tub" for higher compression and turbo rotors have a deeper tub for lower compression. there is a letter on every rotor on the gear side and it is supposed to be mazdas way of designating which rotors will make up a set, just match up the letters, i always weigh them though because sometimes there will be a significant variance in two rotors of the same letter.

 

yeh, i know about the tubs...but seeing that it could be any of 4 tubs. i have no idea where to start.

 

Most are lettered B-E though the letter does designate them to be a "certain wieght" it is imparative when useing mixed rotors that you actually wiegh them. When pairing rotors their wieght cannot be off more that 3/4 of an ounce, or else you'll have a miswieghted engine. Also pre 89 rotors were a cast and are heavier that and pro 89 which were milled. Also on the 89+ rotors you can see the milled lines on all the rotor faces, including the tubs.

 

If you have acess to a micrometer, take 2 or 3 measurements along the long axis of the tub. The deeper one should have a lower compression ratio.

Can you measure the volume of the tubs in a similar fashion to measuring piston cut volumes on a boinger?

Put a tight fitting sheet of formed plexiglass with a burettee attached over the tub, hold it to the rotor with grease, and then see how much water the tub holds. The tub with more volume will have a lower compression ratio.

IMPORTANT WARNING: I am a newbie, what I suggest here is based on academic research (reading car mags) and a bit of general knowhow. That is to say I'm making an educated guess. I am not responsible for anything, ever, especially not your car.