I need to replace one of my rotors on my engine that im building and i was wondering if i needed to get the assembly balanced because of the new rotor?

 

As long as the rotor follows Mazda's rules (i.e. the letter of the rotor is in the proper range based on the letter of your current good rotor), then no.

 

thanks for the replay. where is the mark that shows what rotor it is?

 

Yes. Each rotor is marked with a letter from A to E. The idea is that you are only suppose to rotors that are one letter away. So if you have a C rotor, your other rotor can be a B, C or D rotor.

Now, these rotor letters "sort of" correspond to the weights of the rotors. However, they aren't exact and people have found that even rotors of the same letters are off slightly between each other.

 

Check this thread for a picture:

https://www.rx7club.com/showpost.php...30&postcount=8

 

ok thank you very much for all your information.

 

I had rotors of the same letter code in my engine and the assembly was out of balance enough to loosen pressure plate bolts. We tore the whole engine down and had the entire rotating assembly balanced. Turns out my assembly was 45 grams out of balance (Mazda standards are +-50grams). After the balance I am within 3grams. Piston engine rotating assemblies have a +-10gram tollerance, and the machinest said that Mazda standard was WAY WAY high.

If I was you, and had the whole assembly out of the car anyway, why not pay the extra $300 or so it will cost to balance it? It could save you some big time headaches down the road...

My out of balance rotating assembly was a big reason why my engine was shearing pilot bearings and eventually lead to the need of a complete rebuild.