I've got a couple spare flywheels here. The identifier '326' is cast in the back of two of them, that as I understand it indicates the flywheel balance is to suit an 86-88 n/a 13B engine. I have another with '304' marking, which looks from the clutch side at least, very similar. Does anyone know what series engine this is for?

Also, I have an early 4-port 13B with a lightweight steel flywheel and separate balance weight. Can this setup be used with the 86-88 series 13B?

 

can't answer your frost question because i don't know. however, for your second question -(assuming that the engine is already assembled) you'll have to get the rear counterweight for an automatic '86-'88 engine.

if the engine is apart, then you can get it dynamically balanced with the counterweights from the old 13B - provided you don't plan to use them with that engine again.

 

Some further digging turned up these two Mazdatrix sites:

http://www.mazdatrix.com/faq/flywheel.htm
http://www.mazdatrix.com/faq/rotorwgt.htm

Turns out the 304 is a GSL-SE, with rotor weight of 11.58#, while the S4 n/a engine uses a flywheel marked as 326, balanced to suit rotors weighing 10.04#.