S5 vs s4 irons
 

I opened up my s4 engine last night to discover that my rear iron is pretty hurt. There is a guy near me selling the rear and middle irons from an s5. Both are from na's so does this mean his s5 ones will bolt right up?

The simple answer is yes, assuming your car is also NA. If you're running your car as it came from the factory though, keep in mind that the intermediate housing is different. The S5 intermediate has no EGR provisions.

Yes, my car is NA, for now. I am going to go the 6-port turbo route with this motor after I rebuild it. Just out of curiosity, what are the differences between the two?

the Front S5 iron is does not use the water pump shims, so the pad where the water pump housing mounts is machined with the shims built in.
the center S5 iron adds the air injection port under the intake ports, and deletes the EGR
the rear S5 iron looses a bolt for the top mount starter to clear the intake. water temp sensor changes threads

Thank you and sorry for all the dumb questions but what is the top mount starter? also, won't the different threads for the water temp sensor matter? how will my s4 sensor work if the treads are different? again thank you for all the input this is my first time rebuilding.

Some of the older cars (70s) had the starter mounted up on top of the engine.

For the sensor, just get the correct one for the S5 engine and make an adapter for the plug.

Ok cool, thankyou for the quick and knowledgeable response.