So i will be rebuilding my 87 NA motor and i will be replacing any parts that arent up to spec. However, i am wondering what weak areas or recommended upgrades i should look at? i've heard the rear iron is weak in the s4 and prone to cracking and that i should go with an s5 rear iron (or something along those lines). Any other similar advice?

 

First off, whats your 'in spec' area you are looking at. If you go by mazda's spec you will probably be getting all new irons and housings.

fyi, only some s5's had a thicker casting in one spot to make it handle more power, and on an n/a you have no reason to need it. The s4 rear iron is good to probably 400 or so hp.

If you are looking at power upgrades and don't care about emissions you can look at getting a streetport done to up some power. You can also swap in a s5 non-turbo rotating assembly (rotors, front-counterweight, flywheel) to save some weight and add a bit of compression. Swapping in Turbo exhaust sleeves vs the N/A exhaust sleeves will also add some HP but will also add to the exhaust volume.