Maybe I wasnt searching the right terms.

basically, whats the differences between an S4 and S5 front iron? can a S6 iron be used?

I have a good S5 middle and rear iron, so I assumed I should stick with a S5 front iron as well.

 

S4 and S5 can be interchanged.

S6 is completely different and cannot be swapped in place and only belongs on the REW S6 engine(for all irons).

you can even use non turbo FC center irons in a turbo engine, though not ideal and requires some modification.

 

Thank you for the quick reply!

 

Be sure to use the gasket and o-ring set that match the front iron you use. Also the turbo engines have slightly different port runner widths, although the port shape on the iron face itself is identical.

 

S4 and S5 series specific (S4 N/A to S4 Turbo, and S5 N/A to S5 Turbo) front cover differences from N/A to turbo only difference is that on the N/A front cover the oil feed to the turbo has not been machined open. Correct?

 

not exactly, the S5 has the boss on the cover for the oil drain, the S4 really doesn't have anything cast into it for the drain. so the S5 is easy to modify to add the drain where the series 4 is easier to just install an AN fitting through a single threaded hole through the cover.