A couple basic engine internals questions
 

I've been collecting parts for the 13B I'm buildling. I was looking to get S5 rotors, but they are so hard to find, I decided to settle for S4. I have two people, each selling one rotor that I'm looking to buy. Are the front and rear rotors different? Also, for manual trans cars, is the rear counterweight built into the flywheel? TIA.

Each rotor as a letter stamped on the side face of it

If I were you, I would for 2 that's got the same letter

for M/T, on stock flywheel, the counterweight is incorporated into the assembly

l-p

Series four rotor housings are different. But only for emissions reasons. If you use two F or two R rotor housings the Port Air from the ACV will NOT get to the exaust PORTS.

If you don't have to pass emissions, then you can use two F or two R and it won't matter a bit. Look for a F or a R stamped into the TOP of the rotor housing.

So it doesn't matter which one I put in front or rear, but I have to make sure the letters match?

What do you mean by "side face", the actual face of the rotor or the flat side?

Will I have a balancing problem since the two rotors are coming from different engines? I don't want to shell out 300 bucks to balance it.

There is no difference between front and rear rotors.

Each rotor has a balance letter, from A to D. In an engine, you don't want to use two rotors that are more then 2 letters apart. The number is stamped on the side of the rotor. The rear counterweight is on the flywheel.

by the side face they mean the surface that faces the irons/has the side seals on it... the stamp is about size 12 font and is a letter A - D. A is the heaviest and D is lightest, but you just want two that match or, like someone else said are within 2 letters of eachother.

you won't have problem with different engines as long as you match series and then weight stamp of the rotors.

remember to use s4 counterweight/flywheel on s4 rotors and s5 on s5 rotors. while they both are marked A - D, series 4 rotors are heavier and produce a different compression ratio than series 5.

you can tell s4 rotors from s5 rotors from the rough/cast (as opposed to machined) appearance that the indents on the combustion face have.