Are there any differences between S5 TII and S6 housings? Is it possible to intermix them in a motor? I don't have one of each sitting here, so I can't inspect them myself...

I think that basically they are the same.. I do know that the are interchangeable between S5 and S6...

Big difference is the coolant passage on the FC housings. The S6 doesn't have those. I've heard anecdotal information that the S6 housing has a better chrome and that seems to prove out in that most of the S6 engines I've torn down come apart looking much better than their FC counterparts. But again, that is strictly anecdotal. Not sure about the timing of the port but one would assume you're opening that up anyway so that shouldn't matter.

fd rotor housings also have coolant ridges machined into the passages near the spark plugs and slight larger exhaust port.

these differences can be remedied by porting and doing a racing beat style coolant mod anyways https://www.nopistons.com/forums/pub...IR#>/smile.gif which you should have done even if you have fd housings cuz they cut a lot more ridges.!

the fd has a "better" graphite coating on the chrome to assist break in, and we noticed the port matched the exhaust sleeve a little better.